82 KiB
Introduction to the command line
We'll be using a few new words to reference the command line such as shell, bash and terminal through out the course. They all pretty much mean the same thing but with some small, and not so important, differences between them. Essentially a command line is a textual interface for humans to operate a computer. What is very important to understand is that textual commands and graphical actions operate on the same computer. For example, if you create a file via the command line, it will show up in you file explorer and vice versa. The graphical and textual interfaces are just different representations of the same machine.
Now open up a terminal and you'll see the following.
waldek@hellodebian:~$
This is what we call a prompt. It's not much but it's our window into the computer. As with most things in life you can question it's who, where, what and when. This information is actually embedded in the prompt. Let's break it down.
waldekis who I am on this computerhellodebianis the what, as in what computer I'm operating on~is where I am located on this computer
What about the when then?
Let's type in date and see what happens.
waldek@hellodebian:~$ date
Fri 18 Feb 2022 03:46:59 PM CET
waldek@hellodebian:~$
There we see our when! But this miniscule operation illustrates us the fundamental operation of a command line!
- we have a prompt where we can run a program
- the program runs and outputs it's information on the terminal
- once the program finishes we can run an other program
I'm deliberately saying program here but is date really a program?
It's a bit basic no?
Well, it is a program and most commands you'll type into your terminal are actually programs.
We can illustrate this as follows.
waldek@hellodebian:~$ vlc
VLC media player 3.0.16 Vetinari (revision 3.0.13-8-g41878ff4f2)
[0000559c46ee95b0] main libvlc: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
[0000559c46f89790] main playlist: playlist is empty
Vlc is now running and the terminal is blocked meaning we can't run other programs or commands in it.
If you try to run the date command again, or ls or htop, it won't work!
Try it out if you don't believe me.
But what happens if you close vlc?
The commands you typed get executed!
This is an illustration of the sequential nature of a command line.
Now, I don't think we have sufficiently proven that date is a full blown program so let's dig a bit deeper.
waldek@hellodebian:~$ which vlc
/usr/bin/vlc
waldek@hellodebian:~$ which date
/usr/bin/date
waldek@hellodebian:~$ which which
/usr/bin/which
There is quite bit to unpack in the example above.
First, what on earth is which?
Well, it's also a program and it's sole purpose in life is to tell you where a program is located on your system.
Because which by itself does not make a lot of sense it needs what we call an argument.
Here the argument is the name of the program we want to know it's location of.
The existence of arguments is the second big thing we discovered here.
The third new thing we can observe here is what we call paths, meaning locations on the system.
For example, vlc is a binary program located in a folder called bin which is located in a folder called usr which is at the root of your system.
If this sounds complicated, don't worry, we'll go into detail a bit later.
Now that we know where some of our programs are located, let's find out what they are.
The methodology is the same as with which but we'll use an other program called file who's purpose in life is to tell more about the content of a certain file.
Logically, file needs an argument and this argument is the path to the file you want to inspect.
waldek@hellodebian:~$ file /usr/bin/vlc
/usr/bin/vlc: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=51c40f8234213415771b3a344cab25a140543f8a, for GNU/Linux 3.2.0, stripped
waldek@hellodebian:~$ file /usr/bin/date
/usr/bin/date: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=b740f054aaef6a85aff024858c914c7eae70a6a5, for GNU/Linux 3.2.0, stripped
waldek@hellodebian:~$ file /usr/bin/which
/usr/bin/which: POSIX shell script, ASCII text executable
waldek@hellodebian:~$ file $(which which) # this is some command line kung fu...
/usr/bin/which: POSIX shell script, ASCII text executable
waldek@hellodebian:~$
Here we learn that both vlc and date are executables, compiled for an x86-64 system.
I would say they are both created equally no?
Both are actual programs.
But what about which?
It's also an executable but not compiled, it's a POSIX shell script.
So not all programs are created equally?
Compiled vs interpreted
Executing a file, or program, means you take this file and tell the computer it needs to execute the actions that are stored in the file. Compiled programs contain actual instructions the computer understands out of the box. This means that programs that are compiled are always compiled for a specify architecture which in our case is x86-64. On a Raspberry Pi this would be armhf or arm64.
Interpreted programs are not compiled to machine code but when we run them each line is passed to an appropriate interpreter and executed line by line.
The first line of a script is often the path to the interpreter that understands the code that will follow.
Popular interpreted languages are bash, sh, python, php, ...
We can take it one more step forward and peak into the content of the files.
A nice little program to do this is called head who's purpose in life is to show the first few lines of a file.
First for vlc.
waldek@hellodebian:~$ head /usr/bin/vlc
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If you need to use real-time priorities and/or privileged TCP ports
you can use %s-wrapper (make sure it is Set-UID root and
And now of which.
waldek@hellodebian:~$ head /usr/bin/which
#! /bin/sh
set -ef
if test -n "$KSH_VERSION"; then
puts() {
print -r -- "$*"
}
else
puts() {
printf '%s\n' "$*"
waldek@hellodebian:~$
Notice how the vlc file is mostly not human readable but we can still make out some keywords?
The which file however is perfectly readable!
The first line of the which file is #! /bin/sh which is the path to the interpreter.
The #! is called a shebang.
We can dig into this to learn more about this mysterious sh.
waldek@hellodebian:~$ file /bin/sh
/bin/sh: symbolic link to dash
waldek@hellodebian:~$ which dash
/usr/bin/dash
waldek@hellodebian:~$ file /usr/bin/dash
/usr/bin/dash: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=cb6911fd56559717336c938bef1ce479b0a85b35, for GNU/Linux 3.2.0, stripped
waldek@hellodebian:~$
Surprise surprise! The interpreter is a compiled program! When you think about it it does make sense because at the end of the day the computer only understands machine instructions. So when you execute a script each line of code is converted to machine instructions on the spot. This makes scripts easier to write but slower at execution. The above is a generalization so take it with a grain of salt please!
Note on file extensions
Linux does not rely as much on file extensions as windows does. We could even say it often does not care about them. Due to the decentralized nature of Linux development, some programs do care about them while others don't. It's a bit of hit and miss but I advise you to read this post on AskUbuntu for a more detailed explanation.
More about paths
The prompt in our terminal tells us who we are, on which machine, and where we are located.
This where is symbolized with the ~ character.
But where is this where?
waldek@hellodebian:~$ pwd
/home/waldek
waldek@hellodebian:~$
The pwd program prints our working directory, meaning where we are located on our system.
A user's home directory is symbolized with this tilde character.
We can move around our system with the cd command which is an abbreviation of change directory.
cd by itself seems to to nothing but this is not the case.
It's a shortcut to go to your user's home directory.
Let's try this out.
waldek@hellodebian:~$ cd
waldek@hellodebian:~$ cd /usr/bin
waldek@hellodebian:/usr/bin$ cd
waldek@hellodebian:~$ cd -
/usr/bin
waldek@hellodebian:/usr/bin$ cd
waldek@hellodebian:~$
The first line appears to do nothing because we are already in our home directory.
Next we move to the directory which contains the programs we discovered before and we see our prompt change.
It now shows where we are located!
The next line takes us back to our home and from there we can move back to where we were before with the cd - command.
This last command is also a handy shortcut!
Moving around is nice but once we arrive at our destination it would be practical to be able to list the files in that directory no?
This can be done with the ls program.
For example.
waldek@hellodebian:~$ cd /usr/bin/
waldek@hellodebian:/usr/bin$ ls
'[' getconf nmtui-hostname systemd-escape
7z getent nohup systemd-hwdb
7za getfacl notify-send systemd-id128
7zr getkeycodes nproc systemd-inhibit
aa-enabled getopt nroff systemd-machine-id-setup
aa-exec gettext nsenter systemd-mount
aconnect gettext.sh nslookup systemd-notify
add-apt-repository gio nstat systemd-path
addpart gio-querymodules nsupdate systemd-resolve
addr2line gjs ntfs-3g systemd-run
alsabat gjs-console ntfs-3g.probe systemd-socket-activate
alsaloop gkbd-keyboard-display ntfscat systemd-stdio-bridge
alsamixer glib-compile-schemas ntfscluster systemd-sysusers
alsatplg gmake ntfscmp systemd-tmpfiles
alsaucm gnome-calculator ntfsdecrypt systemd-tty-ask-password-agent
# ------------------------------------
# --- cropped the list for clarity ---
# ------------------------------------
gdk-pixbuf-csource nisdomainname system-config-printer-applet zforce
gdk-pixbuf-pixdata nl systemctl zgrep
gdk-pixbuf-thumbnailer nm systemd zipdetails
gdm-control nm-applet systemd-analyze zipgrep
gdmflexiserver nmcli systemd-ask-password zipinfo
gdm-screenshot nm-connection-editor systemd-cat zless
gedit nm-online systemd-cgls zmore
gencat nmtui systemd-cgtop znew
geqn nmtui-connect systemd-delta
GET nmtui-edit systemd-detect-virt
waldek@hellodebian:/usr/bin$
That's a lot of programs!
Can you find cd in there?
Spoiler alert, you wont...
Maybe it's located somewhere else?
Let's have a look.
waldek@hellodebian:/usr/bin$ which cd
waldek@hellodebian:/usr/bin$
cd is not a program but a builtin command.
From a practical point of view there is no difference but I do like to mention it for completeness.
Later down the line it will help you to contextualize the differences between shells a bit better.
But built into what?
Below you can see a snippet of a part of the bash-builtin manual.
For now just accept the specific shell we're using is called bash.
BASH-BUILTINS(7) Miscellaneous Information Manual BASH-BUILTINS(7)
NAME
bash-builtins - bash built-in commands, see bash(1)
SYNOPSIS
bash defines the following built-in commands: :, ., [, alias, bg, bind, break, builtin, case, cd, command, compgen, complete, continue, de‐
clare, dirs, disown, echo, enable, eval, exec, exit, export, fc, fg, getopts, hash, help, history, if, jobs, kill, let, local, logout, popd,
printf, pushd, pwd, read, readonly, return, set, shift, shopt, source, suspend, test, times, trap, type, typeset, ulimit, umask, unalias,
unset, until, wait, while.
Absolute and relative paths
All roads lead to Rome.
There are multiple ways to go to the same location on your system.
You can always go either in an absolute way, or in multiple relative ways.
The root of your system is / and to list what we can find there we can use ls.
waldek@hellodebian:~$ cd /
waldek@hellodebian:/$ ls
bin dev home initrd.img.old lib32 libx32 media opt root sbin sys usr vmlinuz
boot etc initrd.img lib lib64 lost+found mnt proc run srv tmp var vmlinuz.old
waldek@hellodebian:/$
From here we can go back home in multiple ways.
waldek@hellodebian:/$ cd
waldek@hellodebian:~$ cd -
/
waldek@hellodebian:/$ cd home/waldek/
waldek@hellodebian:~$ cd -
/
waldek@hellodebian:/$ cd /home/waldek/
waldek@hellodebian:~$
The first one is the handy shortcut we learned, and we go back to the root of our system with the cd - shortcut.
The following two manipulations look very similar but there is a subtle difference.
The first one home/waldek is a relative path, and the second one /home/waldek is an absolute one.
Relative paths depend on where you are located, absolute ones always point to the same location.
An example.
waldek@hellodebian:~$ cd /usr/bin/
waldek@hellodebian:/usr/bin$ cd /home/waldek/
waldek@hellodebian:~$ cd -
/usr/bin
waldek@hellodebian:/usr/bin$ cd home/waldek
-bash: cd: home/waldek: No such file or directory
waldek@hellodebian:/usr/bin$
The second command fails because from where I'm standing, there is no folder called home/waldek!
I can however still go to my home in a relative way but I need to go back a few directories first.
waldek@hellodebian:/usr/bin$ cd ../../home/waldek/
waldek@hellodebian:~$ pwd
/home/waldek
waldek@hellodebian:~$
The .. means go back one directory so in our example we go back two directories, which brings us to the / of our system and from there we go up to home and then waldek.
root and / and /root are not the same thing
The word root has multiple meanings and can be a bit confusing at the start.
Every Linux machine will have a root user and a / of the system.
The root user is an account essential to the proper working of a Linux system, whereas / is a location, the root of the filesystem.
The /root is also a location but is the home of the user root, located at the / of the filesystem.
If this sounds confusing read the above paragraph slowly until you see the differences.
I do want to note that the name of the user root can change but it's ID will always be 0.
We'll go into detail about users and ID's later down the line.
Exercise
Explore your system a bit using cd and ls.
You'll probably encounter some weird messages along the way.
Keep note of them so we can discuss it together.
A pit stop to review what we've learned so far
Below is a list of programs and command we've seen so far. We've seen that some take arguments, some don't. This list will grow as we go so I advise you to take some notes of your own.
| command | description |
|---|---|
| systemctl | manipulate running services |
| apt | the Debian package manager |
| htop | a command line task manager we installed |
| vlc | a video player we installed |
| date | display the time of day |
| which | show the path to an executable |
| file | print more information about a file's content |
| head | show the first lines of a file |
| pwd | print working directory |
| cd | change directory |
| ls | list content of a directory |
Arguments and flags
The ls program is one of the most used commands.
It's like a very basic file explorer in a graphical environment and just like it's graphical counter part you can use it to display it's data in different ways.
Think of the details versus thumbnail views in your favorite explorer.
But how do you do this?
We can add flags or options to the program to modify it's behavior.
Have a look at the output below.
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos
waldek@hellodebian:~$ ls -l
total 32
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Desktop
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Documents
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Downloads
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Music
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Pictures
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Public
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Templates
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Videos
waldek@hellodebian:~$ ls -a
. .bashrc Desktop .gnupg .profile Templates .vboxclient-seamless.pid
.. .cache .dmrc .local Public .vboxclient-clipboard.pid Videos
.bash_history .config Documents Music .python_history .vboxclient-display-svga-x11.pid .Xauthority
.bash_logout .dbus Downloads Pictures .ssh .vboxclient-draganddrop.pid .xsession-errors
The -l and -a additions are what we call flags or options.
Aren't they just extra arguments?
There is no real rule to the naming of this concept but this is a rather interesting take.
We can combine arguments if we want, and some programs even allow concatenation of arguments. Have a look below to understand what I mean by that.
waldek@hellodebian:~$ ls -a -l
total 112
drwxr-xr-x 16 waldek waldek 4096 Feb 21 18:12 .
drwxr-xr-x 3 root root 4096 Feb 18 12:49 ..
-rw------- 1 waldek waldek 2772 Feb 22 10:45 .bash_history
-rw-r--r-- 1 waldek waldek 220 Feb 18 12:49 .bash_logout
-rw-r--r-- 1 waldek waldek 3526 Feb 18 12:49 .bashrc
drwx------ 16 waldek waldek 4096 Feb 18 14:24 .cache
drwx------ 20 waldek waldek 4096 Feb 18 14:23 .config
drwx------ 3 waldek waldek 4096 Feb 18 12:52 .dbus
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Desktop
-rw-r--r-- 1 waldek waldek 35 Feb 18 13:33 .dmrc
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Documents
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Downloads
drwx------ 2 waldek waldek 4096 Feb 18 15:07 .gnupg
drwxr-xr-x 3 waldek waldek 4096 Feb 18 12:50 .local
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Music
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Pictures
-rw-r--r-- 1 waldek waldek 807 Feb 18 12:49 .profile
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Public
-rw------- 1 waldek waldek 11 Feb 21 18:12 .python_history
drwx------ 2 waldek waldek 4096 Feb 21 12:45 .ssh
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Templates
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-clipboard.pid
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-display-svga-x11.pid
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-draganddrop.pid
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-seamless.pid
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Videos
-rw------- 1 waldek waldek 56 Feb 18 13:33 .Xauthority
-rw------- 1 waldek waldek 2701 Feb 18 13:33 .xsession-errors
waldek@hellodebian:~$ ls -la
total 112
drwxr-xr-x 16 waldek waldek 4096 Feb 21 18:12 .
drwxr-xr-x 3 root root 4096 Feb 18 12:49 ..
-rw------- 1 waldek waldek 2772 Feb 22 10:45 .bash_history
-rw-r--r-- 1 waldek waldek 220 Feb 18 12:49 .bash_logout
-rw-r--r-- 1 waldek waldek 3526 Feb 18 12:49 .bashrc
drwx------ 16 waldek waldek 4096 Feb 18 14:24 .cache
drwx------ 20 waldek waldek 4096 Feb 18 14:23 .config
drwx------ 3 waldek waldek 4096 Feb 18 12:52 .dbus
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Desktop
-rw-r--r-- 1 waldek waldek 35 Feb 18 13:33 .dmrc
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Documents
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Downloads
drwx------ 2 waldek waldek 4096 Feb 18 15:07 .gnupg
drwxr-xr-x 3 waldek waldek 4096 Feb 18 12:50 .local
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Music
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Pictures
-rw-r--r-- 1 waldek waldek 807 Feb 18 12:49 .profile
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Public
-rw------- 1 waldek waldek 11 Feb 21 18:12 .python_history
drwx------ 2 waldek waldek 4096 Feb 21 12:45 .ssh
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Templates
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-clipboard.pid
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-display-svga-x11.pid
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-draganddrop.pid
-rw-r----- 1 waldek waldek 5 Feb 18 15:06 .vboxclient-seamless.pid
drwxr-xr-x 2 waldek waldek 4096 Feb 18 12:50 Videos
-rw------- 1 waldek waldek 56 Feb 18 13:33 .Xauthority
-rw------- 1 waldek waldek 2701 Feb 18 13:33 .xsession-errors
waldek@hellodebian:~$
Getting help
Options
OK, so we have a lot of programs we can use on the command line, plus we can change how they work by adding obscure characters.
How on earth can we discover what a program can and can't do?
Introducing the most important flag: --help.
Most programs come with builtin help to explain you how to use the program.
Often, but not always, this help can be displayed on the terminal by adding the --help flag.
Let's investigate this.
waldek@hellodebian:~$ ls --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.
Mandatory arguments to long options are mandatory for short options too.
-a, --all do not ignore entries starting with .
-A, --almost-all do not list implied . and ..
--author with -l, print the author of each file
-b, --escape print C-style escapes for nongraphic characters
--block-size=SIZE with -l, scale sizes by SIZE when printing them;
e.g., '--block-size=M'; see SIZE format below
-B, --ignore-backups do not list implied entries ending with ~
-c with -lt: sort by, and show, ctime (time of last
modification of file status information);
with -l: show ctime and sort by name;
otherwise: sort by ctime, newest first
-C list entries by columns
--color[=WHEN] colorize the output; WHEN can be 'always' (default
if omitted), 'auto', or 'never'; more info below
-d, --directory list directories themselves, not their contents
-D, --dired generate output designed for Emacs' dired mode
-f do not sort, enable -aU, disable -ls --color
-F, --classify append indicator (one of */=>@|) to entries
--file-type likewise, except do not append '*'
--format=WORD across -x, commas -m, horizontal -x, long -l,
single-column -1, verbose -l, vertical -C
--full-time like -l --time-style=full-iso
-g like -l, but do not list owner
--group-directories-first
group directories before files;
can be augmented with a --sort option, but any
use of --sort=none (-U) disables grouping
-G, --no-group in a long listing, don't print group names
-h, --human-readable with -l and -s, print sizes like 1K 234M 2G etc.
--si likewise, but use powers of 1000 not 1024
-H, --dereference-command-line
follow symbolic links listed on the command line
--dereference-command-line-symlink-to-dir
follow each command line symbolic link
that points to a directory
--hide=PATTERN do not list implied entries matching shell PATTERN
(overridden by -a or -A)
--hyperlink[=WHEN] hyperlink file names; WHEN can be 'always'
(default if omitted), 'auto', or 'never'
--indicator-style=WORD append indicator with style WORD to entry names:
none (default), slash (-p),
file-type (--file-type), classify (-F)
-i, --inode print the index number of each file
-I, --ignore=PATTERN do not list implied entries matching shell PATTERN
-k, --kibibytes default to 1024-byte blocks for disk usage;
used only with -s and per directory totals
-l use a long listing format
-L, --dereference when showing file information for a symbolic
link, show information for the file the link
references rather than for the link itself
-m fill width with a comma separated list of entries
-n, --numeric-uid-gid like -l, but list numeric user and group IDs
-N, --literal print entry names without quoting
-o like -l, but do not list group information
-p, --indicator-style=slash
append / indicator to directories
-q, --hide-control-chars print ? instead of nongraphic characters
--show-control-chars show nongraphic characters as-is (the default,
unless program is 'ls' and output is a terminal)
-Q, --quote-name enclose entry names in double quotes
--quoting-style=WORD use quoting style WORD for entry names:
literal, locale, shell, shell-always,
shell-escape, shell-escape-always, c, escape
(overrides QUOTING_STYLE environment variable)
-r, --reverse reverse order while sorting
-R, --recursive list subdirectories recursively
-s, --size print the allocated size of each file, in blocks
-S sort by file size, largest first
--sort=WORD sort by WORD instead of name: none (-U), size (-S),
time (-t), version (-v), extension (-X)
--time=WORD change the default of using modification times;
access time (-u): atime, access, use;
change time (-c): ctime, status;
birth time: birth, creation;
with -l, WORD determines which time to show;
with --sort=time, sort by WORD (newest first)
--time-style=TIME_STYLE time/date format with -l; see TIME_STYLE below
-t sort by time, newest first; see --time
-T, --tabsize=COLS assume tab stops at each COLS instead of 8
-u with -lt: sort by, and show, access time;
with -l: show access time and sort by name;
otherwise: sort by access time, newest first
-U do not sort; list entries in directory order
-v natural sort of (version) numbers within text
-w, --width=COLS set output width to COLS. 0 means no limit
-x list entries by lines instead of by columns
-X sort alphabetically by entry extension
-Z, --context print any security context of each file
-1 list one file per line. Avoid '\n' with -q or -b
--help display this help and exit
--version output version information and exit
The SIZE argument is an integer and optional unit (example: 10K is 10*1024).
Units are K,M,G,T,P,E,Z,Y (powers of 1024) or KB,MB,... (powers of 1000).
Binary prefixes can be used, too: KiB=K, MiB=M, and so on.
The TIME_STYLE argument can be full-iso, long-iso, iso, locale, or +FORMAT.
FORMAT is interpreted like in date(1). If FORMAT is FORMAT1<newline>FORMAT2,
then FORMAT1 applies to non-recent files and FORMAT2 to recent files.
TIME_STYLE prefixed with 'posix-' takes effect only outside the POSIX locale.
Also the TIME_STYLE environment variable sets the default style to use.
Using color to distinguish file types is disabled both by default and
with --color=never. With --color=auto, ls emits color codes only when
standard output is connected to a terminal. The LS_COLORS environment
variable can change the settings. Use the dircolors command to set it.
Exit status:
0 if OK,
1 if minor problems (e.g., cannot access subdirectory),
2 if serious trouble (e.g., cannot access command-line argument).
GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Full documentation <https://www.gnu.org/software/coreutils/ls>
or available locally via: info '(coreutils) ls invocation'
waldek@hellodebian:~$
The output above teaches us that the options -la change the output so it becomes:
-a, --all do not ignore entries starting with .-l use a long listing format
To not ignore entries starting with . we can use two different flags, -a or --all.
The former is called short arguments and the latter long ones.
The long ones are more verbose so it's often easier to understand what a line does just by reading it.
You can read up on the history of arguments.
Note that ls only has one way of showing the help by adding --help whereas the help for htop can be shown with both -h or --help.
This inconsistency is sadly a byproduct of the decentralized nature of Linux.
To add to this mess some programs use single dash long form (-help) but those programs are rather rare.
Manuals
The --help flag is tremendously useful as it gives you an overview of how you can modify a programs behaviour, kind of like settings, but it doesn't always explain you what a program is for.
The is where the manuals come into play.
I see the --help flag as a cheat sheet to quickly discover options and the man pages as a reference.
We can read the manual by calling the man program with an argument.
This argument is the name of the program you want to read the manual for.
Same idea as with which.
waldek@hellodebian:~$ man ls
You're now in a program called less, which is a pager.
You can scroll the manual with the arrows of vi based navigation (which we'll get into later).
If you hit h you'll see the builtin documentation of less and press q to quit (twice if you're in the help section).
This brings you back to your prompt.
It's worth reading the wikipedia entry of the man command to understand it's history and operation.
The output below illustrates the tree of a terminal that's reading a manual.
In a bash shell you invoke man ls which in turn runs the pager program to display the content.
If you're intrigued by this pstree program go and read the manual!
waldek@hellodebian:~$ pstree 10845 -a
bash
└─man ls
└─pager
waldek@hellodebian:~$
Not every command has a manual.
For example, cd does not have one.
This is kind of logical because cd is a builtin.
To learn more about builtin commands you can use the help program.
On it's own it gives you all the builtin command you can invoke and with an argument it outputs the help about that specific command.
waldek@hellodebian:~$ help
GNU bash, version 5.1.4(1)-release (x86_64-pc-linux-gnu)
These shell commands are defined internally. Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find out more about the shell in general.
Use `man -k' or `info' to find out more about commands not in this list.
A star (*) next to a name means that the command is disabled.
job_spec [&] history [-c] [-d offset] [n] or history -anrw [filename] or history -ps>
(( expression )) if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else>
. filename [arguments] jobs [-lnprs] [jobspec ...] or jobs -x command [args]
: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [>
[ arg... ] let arg [arg ...]
[[ expression ]] local [option] name[=value] ...
alias [-p] [name[=value] ... ] logout [n]
bg [job_spec ...] mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C c>
bind [-lpsvPSVX] [-m keymap] [-f filename] [-q name] [-u name] [-r keyse> popd [-n] [+N | -N]
break [n] printf [-v var] format [arguments]
builtin [shell-builtin [arg ...]] pushd [-n] [+N | -N | dir]
caller [expr] pwd [-LP]
case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p>
cd [-L|[-P [-e]] [-@]] [dir] readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C>
command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] or readonly -p
compgen [-abcdefgjksuv] [-o option] [-A action] [-G globpat] [-W wordlis> return [n]
complete [-abcdefgjksuv] [-pr] [-DEI] [-o option] [-A action] [-G globpa> select NAME [in WORDS ... ;] do COMMANDS; done
compopt [-o|+o option] [-DEI] [name ...] set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
continue [n] shift [n]
coproc [NAME] command [redirections] shopt [-pqsu] [-o] [optname ...]
declare [-aAfFgiIlnrtux] [-p] [name[=value] ...] source filename [arguments]
dirs [-clpv] [+N] [-N] suspend [-f]
disown [-h] [-ar] [jobspec ... | pid ...] test [expr]
echo [-neE] [arg ...] time [-p] pipeline
enable [-a] [-dnps] [-f filename] [name ...] times
eval [arg ...] trap [-lp] [[arg] signal_spec ...]
exec [-cl] [-a name] [command [argument ...]] [redirection ...] true
exit [n] type [-afptP] name [name ...]
export [-fn] [name[=value] ...] or export -p typeset [-aAfFgiIlnrtux] [-p] name[=value] ...
false ulimit [-SHabcdefiklmnpqrstuvxPT] [limit]
fc [-e ename] [-lnr] [first] [last] or fc -s [pat=rep] [command] umask [-p] [-S] [mode]
fg [job_spec] unalias [-a] name [name ...]
for NAME [in WORDS ... ] ; do COMMANDS; done unset [-f] [-v] [-n] [name ...]
for (( exp1; exp2; exp3 )); do COMMANDS; done until COMMANDS; do COMMANDS; done
function name { COMMANDS ; } or name () { COMMANDS ; } variables - Names and meanings of some shell variables
getopts optstring name [arg ...] wait [-fn] [-p var] [id ...]
hash [-lr] [-p pathname] [-dt] [name ...] while COMMANDS; do COMMANDS; done
help [-dms] [pattern ...] { COMMANDS ; }
waldek@hellodebian:~$ help cd
cd: cd [-L|[-P [-e]] [-@]] [dir]
Change the shell working directory.
Change the current directory to DIR. The default DIR is the value of the
HOME shell variable.
The variable CDPATH defines the search path for the directory containing
DIR. Alternative directory names in CDPATH are separated by a colon (:).
A null directory name is the same as the current directory. If DIR begins
with a slash (/), then CDPATH is not used.
If the directory is not found, and the shell option `cdable_vars' is set,
the word is assumed to be a variable name. If that variable has a value,
its value is used for DIR.
Options:
-L force symbolic links to be followed: resolve symbolic
links in DIR after processing instances of `..'
-P use the physical directory structure without following
symbolic links: resolve symbolic links in DIR before
processing instances of `..'
-e if the -P option is supplied, and the current working
directory cannot be determined successfully, exit with
a non-zero status
-@ on systems that support it, present a file with extended
attributes as a directory containing the file attributes
The default is to follow symbolic links, as if `-L' were specified.
`..' is processed by removing the immediately previous pathname component
back to a slash or the beginning of DIR.
Exit Status:
Returns 0 if the directory is changed, and if $PWD is set successfully when
-P is used; non-zero otherwise.
waldek@hellodebian:~$
Last but not least there are a few extra programs to give you quick and condensed information about a program, whatis and apropos.
waldek@hellodebian:~$ whatis bash
bash (1) - GNU Bourne-Again SHell
waldek@hellodebian:~$ apropos bash
bash (1) - GNU Bourne-Again SHell
bash-builtins (7) - bash built-in commands, see bash(1)
bashbug (1) - report a bug in bash
builtins (7) - bash built-in commands, see bash(1)
dh_bash-completion (1) - install bash completions for package
rbash (1) - restricted bash, see bash(1)
waldek@hellodebian:~$
Exercise
Read some manual pages on the commands we've seen until now. Apply some options you read about in the manual to experiment with said programs.
Creating, modifying and deleting
We've seen how to navigate our filesystem and how to list files and directories at certain locations. Now let's learn how to add, modify and remove files and directories.
Directories
The main tool to create directories is mkdir.
It's usage is pretty simple, it will create a directory at the location you specify as argument.
This location can be absolute or relative.
By default it will not create parent directories if needed but we can force it to do so with the -p flag.
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos
waldek@hellodebian:~$ mkdir website
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos website
waldek@hellodebian:~$ ls website/
waldek@hellodebian:~$ mkdir website/page_one/assets
mkdir: cannot create directory ‘website/page_one/assets’: No such file or directory
waldek@hellodebian:~$ mkdir -p website/page_one/assets
waldek@hellodebian:~$ ls -R website/
website/:
page_one
website/page_one:
assets
website/page_one/assets:
waldek@hellodebian:~$
The counterpart to mkdir is rmdir which can be used to remove empty directories.
As it can only remove empty directories it's not used that much but it's worth knowing it exists.
waldek@hellodebian:~$ rmdir website/
rmdir: failed to remove 'website/': Directory not empty
waldek@hellodebian:~$ rmdir website/page_one/
rmdir: failed to remove 'website/page_one/': Directory not empty
waldek@hellodebian:~$ rmdir website/page_one/assets/
waldek@hellodebian:~$ rmdir website/page_one/
waldek@hellodebian:~$ rmdir website/
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos
waldek@hellodebian:~$
To remove directories that have subdirectories or files in them we need to use an other program, rm but with a specific flag -r.
It works as such.
waldek@hellodebian:~$ mkdir -p website/page_one/assets
waldek@hellodebian:~$ rm website/
rm: cannot remove 'website/': Is a directory
waldek@hellodebian:~$ rm -r website/
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos
waldek@hellodebian:~$
There is no trash or recycle bin on the command line! Removed is removed so think before you hit enter.
Files
We can create empty files with the touch command.
It's worth pointing out that using touch to create files is not it's main purpose.
Reading the manual the fact that it creates files is kind of an extra feature.
Non the less, it's widely used to create files.
TOUCH(1) User Commands TOUCH(1)
NAME
touch - change file timestamps
SYNOPSIS
touch [OPTION]... FILE...
DESCRIPTION
Update the access and modification times of each FILE to the current time.
A FILE argument that does not exist is created empty, unless -c or -h is supplied.
A FILE argument string of - is handled specially and causes touch to change the times of the file associated with standard output.
Mandatory arguments to long options are mandatory for short options too.
In practice it works as follows.
We can create files with touch and delete them with rm.
For deleting files we don't need any extra flags as opposed to directories.
waldek@hellodebian:~$ mkdir -p website/page_one/assets
waldek@hellodebian:~$ ls website/
page_one
waldek@hellodebian:~$ touch website/index.html
waldek@hellodebian:~$ ls website/
index.html page_one
waldek@hellodebian:~$ file website/index.html
website/index.html: empty
waldek@hellodebian:~$ rm website/index.html
waldek@hellodebian:~$ ls website/
page_one
waldek@hellodebian:~$ file website/index.html
website/index.html: cannot open `website/index.html' (No such file or directory)
waldek@hellodebian:~$
What if you created the file in the wrong place?
With mv we can move files from one location to an other.
When you think about it, renaming is actually the same as moving so there is no specific program to rename files or folders.
waldek@hellodebian:~$ touch index.html
waldek@hellodebian:~$ ls
Desktop Documents Downloads index.html Music Pictures Public Templates Videos website
waldek@hellodebian:~$ mv index.html website/indexxxx.html
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos website
waldek@hellodebian:~$ ls website/
indexxxx.html page_one
waldek@hellodebian:~$ mv website/indexxxx.html website/index.html
waldek@hellodebian:~$ ls website/
index.html page_one
waldek@hellodebian:~$
We can also move entire directories from one location to another.
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos website
waldek@hellodebian:~$ mv website/ Documents/
waldek@hellodebian:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos
waldek@hellodebian:~$ ls -R Documents/
Documents/:
website
Documents/website:
index.html page_one
Documents/website/page_one:
assets
Documents/website/page_one/assets:
waldek@hellodebian:~$
Copying is done with cp and behaves very similar to mv.
For example.
waldek@hellodebian:~/Documents$ ls
website
waldek@hellodebian:~/Documents$ mv website/ website_1
waldek@hellodebian:~/Documents$ ls
website_1
waldek@hellodebian:~/Documents$ cp website_1/ website_2
cp: -r not specified; omitting directory 'website_1/'
waldek@hellodebian:~/Documents$ cp -r website_1/ website_2
waldek@hellodebian:~/Documents$ ls
website_1 website_2
waldek@hellodebian:~/Documents$ ls -R
.:
website_1 website_2
./website_1:
index.html page_one
./website_1/page_one:
assets
./website_1/page_one/assets:
./website_2:
index.html page_one
./website_2/page_one:
assets
./website_2/page_one/assets:
waldek@hellodebian:~/Documents$
Links
TODO - symlinks
Editors
nano
The easiest to use command line editor that comes installed with most Linux distributions is called nano.
Below is a screenshot of an empty text file, opened up in nano.
In this window we can write anything we want.
I added two lines of text, but how do I save and quit this program?
At the bottom of the screen there is a menu.
^X is the button to exit nano but how do we invoke it?
Well, by pressing CRTL-x.
By logical consequence to search the text file we can use CTRL-w.
When you try to exit, dialog will pop up to ask you to save.
If you answer Y, you'll be prompted for a filename.
Here you can just hit ENTER.
Exercise
Create some text files with nano.
Move them around with mv, delete them with rm and copy them with cp.
Can you edit the /etc/passwd file?
vi and vim
On of the most popular text editors is by far vim.
It is an improvement on an older editor called vi, vim(proved).
Personally it is my favorite editor, or even program, on Linux and I can highly advise you to become somewhat comfortable with it.
The learning curve is quite steep but so very much worth it!
The reason why it is a bit complicated at first is because vim is a modal text editor.
The paragraph below is taken from the wikipedia page of vi.
vi is a modal editor: it operates in either insert mode (where typed text becomes part of the document) or command mode (where keystrokes are interpreted as commands that control the edit session). For example, typing i while in command mode switches the editor to insert mode, but typing i again at this point places an "i" character in the document. From insert mode, pressing ESC switches the editor back to command mode. A perceived advantage of vi's separation of text entry and command modes is that both text editing and command operations can be performed without requiring the removal of the user's hands from the home row. As non-modal editors usually have to reserve all keys with letters and symbols for the printing of characters, any special commands for actions other than adding text to the buffer must be assigned to keys that do not produce characters, such as function keys, or combinations of modifier keys such as Ctrl, and Alt with regular keys. Vi has the property that most ordinary keys are connected to some kind of command for positioning, altering text, searching and so forth, either singly or in key combinations. Many commands can be touch typed without the use of Ctrl or Alt. Other types of editors generally require the user to move their hands from the home row when touch typing:
Exercise
In order to use vim we need to install it.
Most Linux distributions come with some form of vi or vim installed.
On a modern Debian system this is:
waldek@debian:~$ vi
vi view vim.tiny
waldek@debian:~$ vi
vim.tiny is a version of vim compiled with only the basic features.
We can see what this means by adding the --version argument.
waldek@debian:~$ vim.tiny --version
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Oct 01 2021 01:51:08)
Included patches: 1-2434
Extra patches: 8.2.3402, 8.2.3403, 8.2.3409, 8.2.3428
Modified by team+vim@tracker.debian.org
Compiled by team+vim@tracker.debian.org
Small version without GUI. Features included (+) or not (-):
+acl -farsi +mouse_sgr -tag_old_static
-arabic -file_in_path -mouse_sysmouse -tag_any_white
+autocmd -find_in_path -mouse_urxvt -tcl
-autochdir -float +mouse_xterm -termguicolors
-autoservername -folding +multi_byte -terminal
-balloon_eval -footer -multi_lang +terminfo
-balloon_eval_term +fork() -mzscheme +termresponse
-browse -gettext -netbeans_intg -textobjects
+builtin_terms -hangul_input +num64 -textprop
-byte_offset +iconv -packages -timers
-channel +insert_expand -path_extra -title
-cindent -ipv6 -perl -toolbar
-clientserver -job -persistent_undo +user_commands
-clipboard +jumplist -popupwin -vartabs
+cmdline_compl -keymap -printer +vertsplit
+cmdline_hist -lambda -profile +virtualedit
-cmdline_info -langmap -python +visual
+comments -libcall -python3 +visualextra
-conceal -linebreak -quickfix -viminfo
-cryptv -lispindent -reltime +vreplace
-cscope +listcmds -rightleft +wildignore
+cursorbind +localmap -ruby -wildmenu
-cursorshape -lua +scrollbind +windows
-dialog -menu -signs +writebackup
-diff -mksession -smartindent -X11
-digraphs +modify_fname -sound +xfontset
-dnd +mouse -spell -xim
-ebcdic -mouseshape -startuptime -xpm
-emacs_tags -mouse_dec -statusline -xsmp
-eval -mouse_gpm -sun_workshop -xterm_clipboard
+ex_extra -mouse_jsbterm -syntax -xterm_save
-extra_search -mouse_netterm +tag_binary
system vimrc file: "$VIM/vimrc"
user vimrc file: "$HOME/.vimrc"
2nd user vimrc file: "~/.vim/vimrc"
user exrc file: "$HOME/.exrc"
defaults file: "$VIMRUNTIME/defaults.vim"
fall-back for $VIM: "/usr/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -Wdate-time -g -O2 -ffile-prefix-map=/build/vim-DtwDbo/vim-8.2.2434=. -fstack-protector-strong -Wformat -Werror=format-security -DTINY_VIMRC -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
Linking: gcc -Wl,-z,relro -Wl,-z,now -Wl,--as-needed -o vim -lSM -lICE -lXpm -lXt -lX11 -lXdmcp -lSM -lICE -lm -ltinfo -lselinux -lacl -lattr -ldl
waldek@debian:~$
We can install a more complete version which is called vim-nox.
waldek@debian:~$ apt search vim-nox
Sorting... Done
Full Text Search... Done
vim-nox/stable 2:8.2.2434-3+deb11u1 amd64
Vi IMproved - enhanced vi editor - with scripting languages support
vim-tiny/stable,now 2:8.2.2434-3+deb11u1 amd64 [installed]
Vi IMproved - enhanced vi editor - compact version
waldek@debian:~$
I highly advise you to install the more complete version. Once done, have a look at the new programs installed with tab complete.
waldek@debian:~$ vim
vim vimdiff vim.nox vim.tiny vimtutor
waldek@debian:~$ which vim
/usr/bin/vim
waldek@debian:~$ ls -l /usr/bin/vim
lrwxrwxrwx 1 root root 21 May 13 11:20 /usr/bin/vim -> /etc/alternatives/vim
The only way to learn vim is to use it.
Luckily vim-nox comes with a dedicated program to learn it called vimtutor.
It's actually a text file that you open up in vim where you just need to read and execute what you've read.
Enjoy!
Once you have completed the vimtutor I urge you to have a look at one of the many cheat sheets online.
Opening or peaking into files
We can use vim or nano to open text files and write to them, but there are a few programs worth knowing that can just show the content of a text file.
You'll use these all the time!
Before we've seen that head shows the first few lines of a file.
To see the last lines of a file we use tail.
Get it, head and tail?
The linux world is full of little inside jokes like this.
Anyway, the two most used programs to read file content are cat and less.
cat prints the content of a file straight to the terminal, where as less is what we call a pager.
If you need to search or read a long file you'll find that a pager is more practical.
Reading the man less page we see the following.
LESS(1) General Commands Manual LESS(1)
NAME
less - opposite of more
Again, the joke here is that less can actually do more than more.
Exercise
Open up some files with cat and less.
If you need some inspiration, try a few files in the /etc directory.
Finding stuff
Finding files
The easy way
On most distributions locate will not be installed by default but I advise you to install it, especially in the beginning of your linux career.
The syntax is super simple and it fetches results from a database instead of the hard drive so it's quite quick and doesn't have permission problems.
To install and use it, you should proceed as follows.
waldek@debian:~$ sudo apt install locate
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
locate is already the newest version (4.8.0-1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
waldek@debian:~$ locate bin/zip
/usr/bin/zipdetails
waldek@debian:~$ locate zip | grep bin # this is a weird thing...
/usr/bin/bunzip2
/usr/bin/bzip2
/usr/bin/bzip2recover
/usr/bin/gunzip
/usr/bin/gzip
/usr/bin/streamzip
/usr/bin/zipdetails
/usr/lib/klibc/bin/gunzip
/usr/lib/klibc/bin/gzip
waldek@debian:~$
You'll probably be a bit confused by the | part of the commands above.
Don't worry, give it a few hours and it will hopefully become fully transparent!
Next, there is a very high probability that the commands above don't immediately work for you.
This is because locate functions with a database which needs to be updated from time to time.
This is handled automatically by a cron job.
We'll go into detail on how to schedule jobs ourselves but for now just remember you can trigger a refresh of the database yourself with the following command.
waldek@debian:~$ locate nano.html
waldek@debian:~$ sudo updatedb
waldek@debian:~$ locate nano.html
/usr/share/doc/nano/nano.html
waldek@debian:~$
The hard way
A program you'll find on most Linux distributions to search for files is find.
It performs a real time search a crossed the entire file system and prints out the path of the file found.
A simple example can be seen below.
waldek@debian:~$ find /usr -iname "nano.html"
/usr/share/doc/nano/nano.html
waldek@debian:~$
The real power lies in it's various command line arguments.
We can for example search for all files which are bigger than 100MB, owned by a specific user or of a certain permission.
All of these conditions can be combined to improve the specificity of your query.
There are a few exercises below so I won't give too many spoiler.
My best advise is to read man find and experiment.
Exercise
Inside a clean minimal installation can you please:
- find all files that are 3286 bytes
- find all files not owned by
rootand$USER(yourself) - find all symbolic links
- find all files that contain ssh in the filename and are smaller that 10 bytes
For the eager ones:
- find all files in
/usrthat contain the word hippo - find all files in
/usrthat contain the word hippop or hiphop - find all Belgian email addresses in
/usr
Searching inside files
locate and find are used to search for files.
To search for content inside files we can use an other very powerful tool called grep.
It is often combined with regular expressions, or regex, for search for patterns.
The syntax of regex's is a bit challenging but, like vim, it is very much worth the effort.
Even more, you can use regex's inside vim to search and replace text!
This being said, you can use grep without regular expressions as well and this is what we'll do first.
Most distributions will come with some dictionaries installed.
We can find the American one with the locate command.
The third one in line is the one we need, but have a look at the other files if you're intrigued.
waldek@debian:~$ locate american
/usr/lib/ispell/american.aff
/usr/lib/ispell/american.hash
/usr/share/dict/american-english
/usr/share/doc/iamerican
/usr/share/doc/wamerican
/usr/share/doc/wamerican/NEWS.Debian.gz
/usr/share/doc/wamerican/README.Debian
/usr/share/doc/wamerican/changelog.Debian.gz
/usr/share/doc/wamerican/copyright
/usr/share/doc/wamerican/wamerican.scowl-word-lists-used
/usr/share/ispell/american.med+.mwl.gz
/usr/share/ispell/american.mwl.gz
/usr/share/man/man5/american-english.5.gz
/var/lib/dictionaries-common/ispell/iamerican
/var/lib/dictionaries-common/wordlist/wamerican
/var/lib/dpkg/info/iamerican.config
/var/lib/dpkg/info/iamerican.list
/var/lib/dpkg/info/iamerican.md5sums
/var/lib/dpkg/info/iamerican.postinst
/var/lib/dpkg/info/iamerican.postrm
/var/lib/dpkg/info/iamerican.preinst
/var/lib/dpkg/info/iamerican.templates
/var/lib/dpkg/info/wamerican.config
/var/lib/dpkg/info/wamerican.list
/var/lib/dpkg/info/wamerican.md5sums
/var/lib/dpkg/info/wamerican.postinst
/var/lib/dpkg/info/wamerican.postrm
/var/lib/dpkg/info/wamerican.templates
/var/lib/ispell/american.compat
/var/lib/ispell/american.hash
/var/lib/ispell/american.remove
We can use grep to find all occurrences of the word hippop with the following syntax.
waldek@debian:~$ grep "hippop" /usr/share/dict/american-english
hippopotami
hippopotamus
hippopotamus's
hippopotamuses
waldek@debian:~$
We can search for multiple patterns with the | character but this character has a special meaning in the shell so we need to escape it with a \ or we can use extended grep with either egrep or grep -E.
The exact meaning of | will come clear in a bit, but in this context it means or.
waldek@debian:~$ grep "hippo\|tamu" /usr/share/dict/american-english
Metamucil
Metamucil's
hippo
hippo's
hippopotami
hippopotamus
hippopotamus's
hippopotamuses
hippos
whippoorwill
whippoorwill's
whippoorwills
waldek@debian:~$ grep -E "hippo|tamu" /usr/share/dict/american-english
Metamucil
Metamucil's
hippo
hippo's
hippopotami
hippopotamus
hippopotamus's
hippopotamuses
hippos
whippoorwill
whippoorwill's
whippoorwills
waldek@debian:~$
Adding the --color argument to grep will make the matched patterns jump out with a color, depending on the color scheme of your terminal.
Wild cards and regular expressions
Both are quite related and for simplicities sake you can view regular expressions as wild cards on steroids. A more detailed explanation can be found here.
Wild cards
The syntax for wild cards is rater simple and also goes by the name of globbing of filename expansion. The most important rules to remember are the following.
| character | meaning |
|---|---|
| ? | one character, one times |
| * | any character, any number of times |
| [a-z] | matches all lowercase letters |
| [0-9] | matches all numbers |
| [ac9] | matches a or c or 9 |
Exercise
You can download and extract this to try and match some patterns with wildcards. For example.
waldek@helloworld:~$ ls *bo?ma*
aboymadozscmguidzrfh.txt bopmamzrcoqjmkjmjdow.txt obotmaorizmnrnycrjld.txt
waldek@helloworld:~$ ls cat*
catdmkskxbpgbijcsrbw.txt catlauqronedqtgyjrlj.txt catlocxdfvuqucyhrtti.txt catxvlgrwtvhgnohwqsc.txt
waldek@helloworld:~$ ls *boom*
mmnjgibpbvqkpboomgpu.txt phxbprfdyzboomqedgwu.txt yqrjsjdizlwjenlboomk.txt
waldek@helloworld:~$ ls *bo?ma*
aboymadozscmguidzrfh.txt bopmamzrcoqjmkjmjdow.txt obotmaorizmnrnycrjld.txt
waldek@helloworld:~$
I understand that the examples don't make any practical sense but it's a quick and easy way to learn some basic wildcards You can download a second file that contains a lot of files with lowercase letters and numbers. There you can try and match some patterns to make your list smaller and smaller. For example.
waldek@helloworld:~$ ls [6-9][0-3][ab]?x*
63aaxc38fp7t5celiiqj 70b2xt9iu9my6amgdeyu 93ayxw6c4tf1du3y4pgu 93bsxleuz0t8gogga301
70abx7v1dzhun41fywdz 92bvx59br7goqqbo4h5o 93azx9zfm5mpf9ukm412
waldek@helloworld:~$
Exercise
I hid a lot of words in this file. How many can you find? What do they all have in common? Can you count the occurrences?
Regular expressions
TODO
Pipes and redirects
I mentioned before that bash only knows characters.
It was kind of a lie.
bash only knows bytes at it's input and output.
If these bytes can be interpreted as characters you'll see readable output on your terminal, if not you'll see some gibberish.
An example.
waldek@helloworld:~$ echo "$(dd if=/dev/urandom bs=64 count=1 status=none)"
<0A>B<EFBFBD><42>-<2D>j<EFBFBD><6A><EFBFBD><EFBFBD><EFBFBD><EFBFBD>s<EFBFBD>Ɠ8<C693><38>J<EFBFBD>!
<20>ד<EFBFBD>7d<37>/<2F>X<EFBFBD><58><EFBFBD><EFBFBD>!@{<7B>{R<><52><EFBFBD><EFBFBD><EFBFBD><EFBFBD>J):<3A>sB<73><42>+c<>1<EFBFBD>
waldek@helloworld:~$
Recognise this gibberish?
It's very similar to the output we got when reading a binary file no?
The command above generates 64 random bytes and print them to the screen.
We can do the same for a string and can use pv to get some statistics.
waldek@helloworld:~$ whatis pv
pv (1) - monitor the progress of data through a pipe
waldek@helloworld:~$ echo "hello world" | pv
hello world
12.0 B 0:00:00 [ 116KiB/s ] [ <=> ]
waldek@helloworld:~$
Don't worry about the | symbol, that's up next!
Redirecting
| name | stream ID | description |
|---|---|---|
| STDIN | 0 | standard input, by default the keyboard |
| STDOUT | 1 | standard output, by default your terminal |
| STDERR | 2 | standard error, by default your terminal |
We can however redirect the input and output's!
This is done with the > character.
An example...
waldek@debian:~$ ls -l
total 0
waldek@debian:~$ date
Tue 10 May 2022 12:23:13 PM CEST
waldek@debian:~$ date > date_now.txt
waldek@debian:~$ ls -l
total 4
-rw-r--r-- 1 waldek waldek 33 May 10 12:23 date_now.txt
waldek@debian:~$ cat date_now.txt
Tue 10 May 2022 12:23:25 PM CEST
waldek@debian:~$
It's very important to realize that the file you redirect too will be overwritten!
So if you run the above date > date_now.txt again, the file will show a more recent date.
We can also append to an existing file by ising the >>.
This redirection will add the date to the end of the file if the file exists, otherwise it will just create one.
waldek@debian:~$ date >> date_now.txt
waldek@debian:~$ date >> date_now.txt
waldek@debian:~$ date >> date_now.txt
waldek@debian:~$ cat date_now.txt
Tue 10 May 2022 12:23:25 PM CEST
Tue 10 May 2022 12:27:23 PM CEST
Tue 10 May 2022 12:27:24 PM CEST
Tue 10 May 2022 12:27:25 PM CEST
waldek@debian:~$
We can also choose which of the two outputs, STDOUT or STDERR, we want to redirect.
This is done by adding the stream ID to the redirection.
Observe the following output where I search for my username in each file located at /etc.
waldek@debian:~$ grep -R waldek /etc/
/etc/group-:cdrom:x:24:waldek
/etc/group-:floppy:x:25:waldek
/etc/group-:sudo:x:27:waldek
/etc/group-:audio:x:29:waldek
/etc/group-:dip:x:30:waldek
/etc/group-:video:x:44:waldek
/etc/group-:plugdev:x:46:waldek
/etc/group-:netdev:x:108:waldek
/etc/group-:waldek:x:1000:
grep: /etc/sv/ssh/log/supervise: No such file or directory
grep: /etc/sv/ssh/supervise: No such file or directory
/etc/passwd-:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
grep: /etc/gshadow: Permission denied
/etc/subuid:waldek:100000:65536
grep: /etc/security/opasswd: Permission denied
grep: /etc/gshadow-: Permission denied
grep: /etc/sudoers.d/README: Permission denied
grep: /etc/.pwd.lock: Permission denied
grep: /etc/runit/runsvdir/default/ssh/log/supervise: No such file or directory
grep: /etc/runit/runsvdir/default/ssh/supervise: No such file or directory
grep: /etc/ssh/ssh_host_ecdsa_key: Permission denied
grep: /etc/ssh/ssh_host_ed25519_key: Permission denied
grep: /etc/ssh/ssh_host_rsa_key: Permission denied
grep: /etc/shadow-: Permission denied
grep: /etc/ssl/private: Permission denied
/etc/group:cdrom:x:24:waldek
/etc/group:floppy:x:25:waldek
/etc/group:sudo:x:27:waldek
/etc/group:audio:x:29:waldek
/etc/group:dip:x:30:waldek
/etc/group:video:x:44:waldek
/etc/group:plugdev:x:46:waldek
/etc/group:netdev:x:108:waldek
/etc/group:waldek:x:1000:
grep: /etc/shadow: Permission denied
grep: /etc/sudoers: Permission denied
/etc/subgid:waldek:100000:65536
/etc/passwd:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
waldek@debian:~$
We can see quite a few Permission denied lines which are errors.
Because STDERR is connected to our terminal by default, we also see them printed on the screen.
Let's play around with the above command a bit.
First a simple redirection.
waldek@debian:~$ grep -R waldek /etc/ > files_with_my_name
grep: /etc/sv/ssh/log/supervise: No such file or directory
grep: /etc/sv/ssh/supervise: No such file or directory
grep: /etc/gshadow: Permission denied
grep: /etc/security/opasswd: Permission denied
grep: /etc/gshadow-: Permission denied
grep: /etc/sudoers.d/README: Permission denied
grep: /etc/.pwd.lock: Permission denied
grep: /etc/runit/runsvdir/default/ssh/log/supervise: No such file or directory
grep: /etc/runit/runsvdir/default/ssh/supervise: No such file or directory
grep: /etc/ssh/ssh_host_ecdsa_key: Permission denied
grep: /etc/ssh/ssh_host_ed25519_key: Permission denied
grep: /etc/ssh/ssh_host_rsa_key: Permission denied
grep: /etc/shadow-: Permission denied
grep: /etc/ssl/private: Permission denied
grep: /etc/shadow: Permission denied
grep: /etc/sudoers: Permission denied
We only see the errors now! Where did our actual output go?
waldek@debian:~$ ls -l
total 4
-rw-r--r-- 1 waldek waldek 722 May 10 12:34 files_with_my_name
waldek@debian:~$ cat files_with_my_name
/etc/group-:cdrom:x:24:waldek
/etc/group-:floppy:x:25:waldek
/etc/group-:sudo:x:27:waldek
/etc/group-:audio:x:29:waldek
/etc/group-:dip:x:30:waldek
/etc/group-:video:x:44:waldek
/etc/group-:plugdev:x:46:waldek
/etc/group-:netdev:x:108:waldek
/etc/group-:waldek:x:1000:
/etc/passwd-:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
/etc/subuid:waldek:100000:65536
/etc/group:cdrom:x:24:waldek
/etc/group:floppy:x:25:waldek
/etc/group:sudo:x:27:waldek
/etc/group:audio:x:29:waldek
/etc/group:dip:x:30:waldek
/etc/group:video:x:44:waldek
/etc/group:plugdev:x:46:waldek
/etc/group:netdev:x:108:waldek
/etc/group:waldek:x:1000:
/etc/subgid:waldek:100000:65536
/etc/passwd:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
waldek@debian:~$
If we want to do the reverse we can redirect the STDERR by using 2>.
An example...
waldek@debian:~$ grep -R waldek /etc/ 2> files_with_my_name
/etc/group-:cdrom:x:24:waldek
/etc/group-:floppy:x:25:waldek
/etc/group-:sudo:x:27:waldek
/etc/group-:audio:x:29:waldek
/etc/group-:dip:x:30:waldek
/etc/group-:video:x:44:waldek
/etc/group-:plugdev:x:46:waldek
/etc/group-:netdev:x:108:waldek
/etc/group-:waldek:x:1000:
/etc/passwd-:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
/etc/subuid:waldek:100000:65536
/etc/group:cdrom:x:24:waldek
/etc/group:floppy:x:25:waldek
/etc/group:sudo:x:27:waldek
/etc/group:audio:x:29:waldek
/etc/group:dip:x:30:waldek
/etc/group:video:x:44:waldek
/etc/group:plugdev:x:46:waldek
/etc/group:netdev:x:108:waldek
/etc/group:waldek:x:1000:
/etc/subgid:waldek:100000:65536
/etc/passwd:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
waldek@debian:~$
And the file content.
waldek@debian:~$ cat files_with_my_name
grep: /etc/sv/ssh/log/supervise: No such file or directory
grep: /etc/sv/ssh/supervise: No such file or directory
grep: /etc/gshadow: Permission denied
grep: /etc/security/opasswd: Permission denied
grep: /etc/gshadow-: Permission denied
grep: /etc/sudoers.d/README: Permission denied
grep: /etc/.pwd.lock: Permission denied
grep: /etc/runit/runsvdir/default/ssh/log/supervise: No such file or directory
grep: /etc/runit/runsvdir/default/ssh/supervise: No such file or directory
grep: /etc/ssh/ssh_host_ecdsa_key: Permission denied
grep: /etc/ssh/ssh_host_ed25519_key: Permission denied
grep: /etc/ssh/ssh_host_rsa_key: Permission denied
grep: /etc/shadow-: Permission denied
grep: /etc/ssl/private: Permission denied
grep: /etc/shadow: Permission denied
grep: /etc/sudoers: Permission denied
waldek@debian:~$
Now the errors are not that interesting too us so we can send this output to a black hole or trash location.
This is done by sending STDERR to /dev/null.
You'll see this redirection in a lot of places!
Try it out when you're searching for files with find and set the start of the search at the root of your system /.
waldek@debian:~$ grep -R waldek /etc/ 2> /dev/null
/etc/group-:cdrom:x:24:waldek
/etc/group-:floppy:x:25:waldek
/etc/group-:sudo:x:27:waldek
/etc/group-:audio:x:29:waldek
/etc/group-:dip:x:30:waldek
/etc/group-:video:x:44:waldek
/etc/group-:plugdev:x:46:waldek
/etc/group-:netdev:x:108:waldek
/etc/group-:waldek:x:1000:
/etc/passwd-:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
/etc/subuid:waldek:100000:65536
/etc/group:cdrom:x:24:waldek
/etc/group:floppy:x:25:waldek
/etc/group:sudo:x:27:waldek
/etc/group:audio:x:29:waldek
/etc/group:dip:x:30:waldek
/etc/group:video:x:44:waldek
/etc/group:plugdev:x:46:waldek
/etc/group:netdev:x:108:waldek
/etc/group:waldek:x:1000:
/etc/subgid:waldek:100000:65536
/etc/passwd:waldek:x:1000:1000:waldek,,,:/home/waldek:/bin/bash
waldek@debian:~$ cat /dev/null
waldek@debian:~$
If you want to redirect both outputs to a file you can use the &> syntax.
The file will then contain everything that would be printed out on the terminal.
waldek@debian:~$ grep -R waldek /etc/ &> files_with_my_name
waldek@debian:~$
Input redirection
This is a bit more exotic but widely used in scripting. I personally don't use it that often on the command line. But we can demonstrate it's behaviour with a simple TCP server.
In a first terminal I run the following command. If you try this out you'll see it hang after hitting enter, this is normal, the server is now listening for incoming connections
waldek@helloworld:~$ cat message
hello world!
I'm a text file...
waldek@helloworld:~$ cat < message
hello world!
I'm a text file...
waldek@helloworld:~$ nc -l -p 9999 < message && echo "done serving the file"
done serving the file
waldek@helloworld:~$
In a second terminal I now connect to this server.
I receive the content of the text file here and when I hit ctrl-c I break the connection.
The first terminal will now continue and say done serving the file.
waldek@helloworld:~$ nc localhost 9999
hello world!
I'm a text file...
^C
waldek@helloworld:~$
Piping
A very powerful fundamental concept of the command line is are pipes which are created with the | character.
Their purpose is to send the STDOUT of one process to the STDIN of a following.
They might seem similar to redirection but they are fundamentally different.
A redirect sends to or from a file while a pipe sends to and from a process.
As always, an example should clarify it better.
waldek@debian:~$ ls -l /etc/* | grep "d.conf"
-rw-r--r-- 1 root root 6169 Feb 27 2021 /etc/sudo_logsrvd.conf
-rw-r--r-- 1 root root 3289 Mar 13 2021 sshd_config
drwxr-xr-x 2 root root 4096 Mar 13 2021 sshd_config.d
-rw-r--r-- 1 root root 1052 Mar 20 20:55 journald.conf
-rw-r--r-- 1 root root 1145 Mar 20 20:55 logind.conf
-rw-r--r-- 1 root root 609 Feb 2 2021 networkd.conf
-rw-r--r-- 1 root root 943 Mar 20 20:55 resolved.conf
-rw-r--r-- 1 root root 677 Mar 20 20:55 timesyncd.conf
waldek@debian:~$
In the example above we send the output of ls -l /etc/* to grep "d.conf".
We go from a lot of output to less.
Pipes are a very powerful tool to do text manipulations!
Text manipulation
Below is a list of some of the essential tools to perform text manipulation. I highly recommend reading the man pages for each of these programs. Some of them have a myriad of options, others are more limited but they all have their purpose. This might a good time to read up a bit on the fundamental philosophy of Linux and Unix, especial the idea to create small programs that do one thing well.
| command | description |
|---|---|
| cut | remove sections from each line of files |
| sort | sort lines of text files |
| uniq | report or omit repeated lines |
| grep | print lines that match patterns |
| tr | translate or delete characters |
| sed | stream editor for filtering and transforming text |
| nl | number lines of files |
| wc | print newline, word, and byte counts for each file |
| seq | print a sequence of numbers |
| diff | compare files |
| cat | concatenate files and print on the standard output |
| tac | concatenate and print files in reverse |
| comm | compare two sorted files line by line |
| shuf | generate random permutations |
| paste | merge lines of file |
| join | join lines of two files on a common field |
| patch | apply a diff file to an original |
| aspell | interactive spell checker |
Below are some examples to illustrate the power of pipes and text manipulation programs. You can copy/paste the command in your terminal and remove each pipe, one by one, to discover it's effect. Most of these pipes are not actually useful but I hope they illustrate the flexibility of the concept.
- sort by size
waldek@debian:~$ ls -la | sort --key 5 -g -r | grep -v -E "total|root" | nl 1 drwxr-xr-x 2 waldek waldek 4096 May 10 15:10 . 2 -rw-r--r-- 1 waldek waldek 3526 May 9 11:59 .bashrc 3 -rw-r--r-- 1 waldek waldek 807 May 9 11:59 .profile 4 -rw-r--r-- 1 waldek waldek 722 May 10 12:45 files_with_my_name 5 -rw-r--r-- 1 waldek waldek 220 May 9 11:59 .bash_logout 6 -rw-r--r-- 1 waldek waldek 74 May 9 16:06 .selected_editor 7 -rw------- 1 waldek waldek 49 May 10 15:10 .lesshst waldek@debian:~$ - number of users with
bashas login shellwaldek@debian:~$ cat /etc/passwd | grep bash | cut -d ":" -f 1 | wc -l 2 waldek@debian:~$ - files in
/etcthat contain my usernamewaldek@debian:~$ grep -R $USER /etc 2> /dev/null | cut -d ":" -f 1 | sort | uniq /etc/group /etc/group- /etc/passwd /etc/passwd- /etc/subgid /etc/subuid waldek@debian:~$ - files in
/etcthat contain the world password but listed by number of occurrences and only the three most populated ones are shownwaldek@debian:~$ grep -R password /etc 2> /dev/null | cut -d ":" -f 1 | sort | uniq -c | sort -g | tail -3 7 /etc/pam.d/common-password 9 /etc/debconf.conf 12 /etc/login.defs waldek@debian:~$
Ideas
- find unique phone numbers
- find valid mobile phone numbers based on
- find valid IP addresses
- find all ugly filenames and replace with better names (replace with _)
- regex crossword puzzle