linux_introduction/advanced/learning_bash_scripting.md

bash

A bash script is a sequence of command that are executed one by one. Most of the time we just execute one command and wait for the result to then make a decision and execute an other command. We can however create a sequence of commands on the command line.

(baseline-) ➜  ~ echo hello world
hello world
(baseline-) ➜  ~ date            
Wed 16 Mar 2022 07:06:02 PM CET
(baseline-) ➜  ~ cal
     March 2022       
Su Mo Tu We Th Fr Sa  
       1  2  3  4  5  
 6  7  8  9 10 11 12  
13 14 15 16 17 18 19  
20 21 22 23 24 25 26  
27 28 29 30 31        
                      
(baseline-) ➜  ~ echo hello world date cal
hello world date cal
(baseline-) ➜  ~ echo hello world; date; cal
hello world
Wed 16 Mar 2022 07:06:19 PM CET
     March 2022       
Su Mo Tu We Th Fr Sa  
       1  2  3  4  5  
 6  7  8  9 10 11 12  
13 14 15 16 17 18 19  
20 21 22 23 24 25 26  
27 28 29 30 31        

A very visually similar result, but completely different operation, can be obtained by replacing the ; with & or &&. & will launch a new process and send it to the background, && will evaluate the return status of your process and only continue if the status was 0, or in other words successful.

(baseline-) ➜  ~ echo hello world & date & cal
[1] 3075524
hello world
[2] 3075525
[1]  - 3075524 done       echo hello world
Wed 16 Mar 2022 07:06:33 PM CET
[2]  + 3075525 done       date
     March 2022       
Su Mo Tu We Th Fr Sa  
       1  2  3  4  5  
 6  7  8  9 10 11 12  
13 14 15 16 17 18 19  
20 21 22 23 24 25 26  
27 28 29 30 31        
                      
(baseline-) ➜  ~ echo hello world && date && cal
hello world
Wed 16 Mar 2022 07:06:40 PM CET
     March 2022       
Su Mo Tu We Th Fr Sa  
       1  2  3  4  5  
 6  7  8  9 10 11 12  
13 14 15 16 17 18 19  
20 21 22 23 24 25 26  
27 28 29 30 31        
                      
(baseline-) ➜  ~ 

This should make you think we can create quite complicated logical flows in bash and you're right! But first things first, let's create our first script!

(baseline-) ➜  bash file test.sh 
test.sh: Bourne-Again shell script, ASCII text executable
(baseline-) ➜  bash ls -l test.sh 
-rwxr-xr-x 1 waldek waldek 39 Mar 16 19:13 test.sh
(baseline-) ➜  bash cat test.sh 
#!/bin/bash

echo hello world
cal
date
(baseline-) ➜  bash ./test.sh 
hello world
     March 2022       
Su Mo Tu We Th Fr Sa  
       1  2  3  4  5  
 6  7  8  9 10 11 12  
13 14 15 16 17 18 19  
20 21 22 23 24 25 26  
27 28 29 30 31        
                      
Wed 16 Mar 2022 07:27:22 PM CET
(baseline-) ➜  bash bash test.sh 
hello world
     March 2022       
Su Mo Tu We Th Fr Sa  
       1  2  3  4  5  
 6  7  8  9 10 11 12  
13 14 15 16 17 18 19  
20 21 22 23 24 25 26  
27 28 29 30 31        
                      
Wed 16 Mar 2022 07:27:25 PM CET
(baseline-) ➜  bash 

If you observe the output above you can conclude multiple things.

• the test.sh file is a simple text file
• it has executable permissions for the owner, group and others
• we can execute the sequence of commands in the script in two ways
  • ./test.sh
  • bash test.sh
• the content is a series of commands but with a weird first line

That first line is called a shebang and is a way of explaining which interpreter understands the lines that follow. If you venture out into the python universe you'll encounter the same norm but with a different path to an interpretor, often /bin/python3 or /usr/bin/env python3. The later is a sort of shortcut that points to the local python installation, even if it's not in a standard location. A shebang is not necessary for a script to function but is highly advised.

The env program is actually very interesting! Let's try it by itself.

➜  ~ env
PAGER=less
LANGUAGE=en_US:en
GNOME_TERMINAL_SCREEN=/org/gnome/Terminal/screen/022a5a78_fe95_445c_9b33_f6dcb35cfd27
LANG=en_US.UTF-8
DISPLAY=:0
SWAYSOCK=/run/user/1000/sway-ipc.1000.1565.sock
WAYLAND_DISPLAY=wayland-0
AUTOSWITCH_VERSION=3.4.0
HUSHLOGIN=FALSE
USER=waldek
OLDPWD=/home/waldek/bin/bash
HOME=/home/waldek
MOZ_ENABLE_WAYLAND=1
VIRTUAL_ENV=/home/waldek/.virtualenvs/baseline-
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-acMe3wyyoo,guid=ad1233c6039551be866f78ae61f911b2
XDG_VTNR=1
XDG_SEAT=seat0
I3SOCK=/run/user/1000/sway-ipc.1000.1565.sock
LESS=-R
_=/usr/bin/env
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=00:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.webp=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.m4a=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.oga=00;36:*.opus=00;36:*.spx=00;36:*.xspf=00;36:
AUTOSWITCH_DEFAULTENV=baseline
VTE_VERSION=6203
LSCOLORS=Gxfxcxdxbxegedabagacad
ZSH=/home/waldek/.oh-my-zsh
AUTOSWITCH_FILE=.venv
MAIL=/var/mail/waldek
LOGNAME=waldek
PS1=(baseline-) %(?:%{%}➜ :%{%}➜ ) %{$fg[cyan]%}%c%{$reset_color%} $(git_prompt_info)
GDK_BACKEND=wayland
PATH=/home/waldek/.virtualenvs/baseline-/bin:/home/waldek/.cargo/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/home/waldek/.local/bin:/home/waldek/bin/python/:/home/waldek/.local/bin:/home/waldek/bin/python/
XDG_RUNTIME_DIR=/run/user/1000
QT_QPA_PLATFORM=wayland
XDG_SESSION_ID=2
XDG_SESSION_TYPE=wayland
FPATH=/home/waldek/.oh-my-zsh/functions:/home/waldek/.oh-my-zsh/completions:/home/waldek/.oh-my-zsh/cache/completions:/home/waldek/.oh-my-zsh/custom/plugins/autoswitch_virtualenv:/home/waldek/.oh-my-zsh/plugins/git:/home/waldek/.oh-my-zsh/functions:/home/waldek/.oh-my-zsh/completions:/home/waldek/.oh-my-zsh/cache/completions:/usr/local/share/zsh/site-functions:/usr/share/zsh/vendor-functions:/usr/share/zsh/vendor-completions:/usr/share/zsh/functions/Calendar:/usr/share/zsh/functions/Chpwd:/usr/share/zsh/functions/Completion:/usr/share/zsh/functions/Completion/AIX:/usr/share/zsh/functions/Completion/BSD:/usr/share/zsh/functions/Completion/Base:/usr/share/zsh/functions/Completion/Cygwin:/usr/share/zsh/functions/Completion/Darwin:/usr/share/zsh/functions/Completion/Debian:/usr/share/zsh/functions/Completion/Linux:/usr/share/zsh/functions/Completion/Mandriva:/usr/share/zsh/functions/Completion/Redhat:/usr/share/zsh/functions/Completion/Solaris:/usr/share/zsh/functions/Completion/Unix:/usr/share/zsh/functions/Completion/X:/usr/share/zsh/functions/Completion/Zsh:/usr/share/zsh/functions/Completion/openSUSE:/usr/share/zsh/functions/Exceptions:/usr/share/zsh/functions/MIME:/usr/share/zsh/functions/Math:/usr/share/zsh/functions/Misc:/usr/share/zsh/functions/Newuser:/usr/share/zsh/functions/Prompts:/usr/share/zsh/functions/TCP:/usr/share/zsh/functions/VCS_Info:/usr/share/zsh/functions/VCS_Info/Backends:/usr/share/zsh/functions/Zftp:/usr/share/zsh/functions/Zle:/usr/share/zsh:/usr/share/zsh
SHELL=/usr/bin/zsh
AUTOSWITCH_SILENT=1
EDITOR=vim
MOTD_SHOWN=pam
SHLVL=2
GNOME_TERMINAL_SERVICE=:1.10
COLORTERM=truecolor
XCURSOR_SIZE=24
TERM=xterm-256color
PWD=/home/waldek
XDG_SESSION_CLASS=user
➜  ~ 

What is all of this? Well, the are the current environment variables defined in your shell. You'll probably recognize some of them. Let's play around with what we have defined.

➜  ~ echo SHELL
SHELL
➜  ~ echo $SHELL
/usr/bin/zsh
➜  ~ echo $USER
waldek
➜  ~ echo $HOME
/home/waldek
➜  ~ echo $
zsh: do you wish to see all 223 possibilities (56 lines)? 

Yes, tab complete works! Your variables will be slightly different but that's not an issue at all.

➜  ~ echo $
!                              EUID                           MAILPATH                       SWAYSOCK                     
#                              fg                             manpath                        TERM                         
*                              FG                             MANPATH                        termcap                      
-                              fg_bold                        module_path                    terminfo                     
?                              fg_no_bold                     MODULE_PATH                    TIMEFMT                      
@                              fignore                        modules                        TMPPREFIX                    
_                              FIGNORE                        MOTD_SHOWN                     TRY_BLOCK_ERROR              
$                              fpath                          MOZ_ENABLE_WAYLAND             TRY_BLOCK_INTERRUPT          
0                              FPATH                          nameddirs                      TTY                          
aliases                        funcfiletrace                  NULLCMD                        TTYIDLE                      
ARGC                           FUNCNEST                       OLDPWD                         UID                          
argv                           funcsourcetrace                OPTARG                         USER                         
AUTOSWITCH_DEFAULTENV          funcstack                      OPTIND                         userdirs                     
AUTOSWITCH_FILE                functions                      options                        usergroups                   
AUTOSWITCH_SILENT              functions_source               OSTYPE                         USERNAME                     
AUTOSWITCH_VERSION             functrace                      PAGER                          VENDOR                       
bg                             FX                             parameters                     VIRTUAL_ENV                  
BG                             galiases                       patchars                       VTE_VERSION                  
bg_bold                        GDK_BACKEND                    _patcomps                      watch                        
bg_no_bold                     GID                            path                           WATCH                        
bold_color                     GNOME_TERMINAL_SCREEN          PATH                           WATCHFMT                     
builtins                       GNOME_TERMINAL_SERVICE         pipestatus                     WAYLAND_DISPLAY              
cdpath                         histchars                      plugins                        widgets                      
CDPATH                         HISTCHARS                      _postpatcomps                  WORDCHARS                    
color                          HISTCMD                        PPID                           XCURSOR_SIZE                 
COLORTERM                      HISTFILE                       precmd_functions               XDG_RUNTIME_DIR              
colour                         history                        preexec_functions              XDG_SEAT                     
COLUMNS                        historywords                   prompt                         XDG_SESSION_CLASS            
commands                       HISTSIZE                       PROMPT                         XDG_SESSION_ID               
_compautos                     HOME                           PROMPT2                        XDG_SESSION_TYPE             
_comp_dumpfile                 HOST                           PROMPT3                        XDG_VTNR                     
COMPLETION_WAITING_DOTS        HUSHLOGIN                      PROMPT4                        zle_bracketed_paste          
_comp_options                  I3SOCK                         PS1                            ZLS_COLORS                   
comppostfuncs                  jobdirs                        PS2                            ZSH                          
compprefuncs                   jobstates                      PS3                            ZSH_ARGZERO                  
_comps                         jobtexts                       PS4                            ZSH_CACHE_DIR                
_comp_setup                    key                            psvar                          ZSH_COMPDUMP                 
CPUTYPE                        KEYBOARD_HACK                  PSVAR                          ZSH_CUSTOM                   
d                              keymaps                        PWD                            zsh_eval_context             
DBUS_SESSION_BUS_ADDRESS       KEYTIMEOUT                     QT_QPA_PLATFORM                ZSH_EVAL_CONTEXT             
debian_missing_features        LANG                           RANDOM                         ZSH_NAME                     
dirstack                       langinfo                       READNULLCMD                    ZSH_PATCHLEVEL               
dis_aliases                    LANGUAGE                       reset_color                    zsh_scheduled_events         
dis_builtins                   _lastcomp                      reswords                       ZSH_SUBSHELL                 
dis_functions                  LESS                           saliases                       ZSH_THEME                    
dis_functions_source           LINENO                         __savecursor                   ZSH_THEME_GIT_PROMPT_CLEAN   
dis_galiases                   LINES                          SAVEHIST                       ZSH_THEME_GIT_PROMPT_DIRTY   
dis_patchars                   LISTMAX                        SCREEN_NO                      ZSH_THEME_GIT_PROMPT_PREFIX  
DISPLAY                        LOGCHECK                       __searching                    ZSH_THEME_GIT_PROMPT_SUFFIX  
dis_reswords                   LOGNAME                        _services                      ZSH_THEME_RUBY_PROMPT_PREFIX 
dis_saliases                   LS_COLORS                      SHELL                          ZSH_THEME_RUBY_PROMPT_SUFFIX 
EDITOR                         LSCOLORS                       SHLVL                          ZSH_THEME_RVM_PROMPT_OPTIONS 
EGID                           MACHTYPE                       SHORT_HOST                     ZSH_THEME_TERM_TAB_TITLE_IDLE
EPOCHREALTIME                  MAIL                           signals                        ZSH_THEME_TERM_TITLE_IDLE    
EPOCHSECONDS                   MAILCHECK                      SPROMPT                        ZSH_VERSION                  
epochtime                      mailpath                       status                                                      
➜  ~ echo $

We can create our own variables as follows. Notice how an undefined variable does not throw an error. This is very typical for shell scripting, python on the other hand would crash over an undefined variable.

waldek@helloworld:~$ echo $USER
waldek
waldek@helloworld:~$ echo $name

waldek@helloworld:~$ name="wouter gordts"
waldek@helloworld:~$ echo $name
wouter gordts
waldek@helloworld:~$ 

If you open up a new shell this $name variable will not be defined because variables are local to each instance of bash that is running. This can be observed as follows. We can export variables to children with the export keyword.

waldek@helloworld:~$ name="wouter gordts"
waldek@helloworld:~$ echo $name
wouter gordts
waldek@helloworld:~$ bash
waldek@helloworld:~$ echo $name

waldek@helloworld:~$ exit
exit
waldek@helloworld:~$ export name
waldek@helloworld:~$ bash
waldek@helloworld:~$ echo $name
wouter gordts
waldek@helloworld:~$ exit
exit
waldek@helloworld:~$ 

Using variables to store the output of command

Bash only really knows characters, both for sending and receiving. We can temporarily store the output of a command using variables. The syntax is a bit tricky at first but quickly becomes quite natural. We can try this out on the command line. Next we'll write a small script to leverage the power of variables and pipes.

waldek@metal:~$ grep $USER /etc/passwd
waldek:x:1000:1000:waldek,,,:/home/local/waldek:/bin/zsh
waldek@metal:~$ my_name=$(grep $USER /etc/passwd)
waldek@metal:~$ echo $my_name 
waldek:x:1000:1000:waldek,,,:/home/local/waldek:/bin/zsh
waldek@metal:~$ 

A little bit more complicated.

waldek@metal:~$ count=$(grep "/home" /etc/passwd | wc -l)
waldek@metal:~$ msg="there are $count users on this machine"
waldek@metal:~$ echo $msg
there are 4 users on this machine
waldek@metal:~$ 

Now a small script.

#!/bin/bash

FULLNAME="wouter gordts"
CITY="Brussels"

echo "this script was written by $FULLNAME in $CITY"

IP=$(ip a | grep -v "127.0.0.1" | grep -o -E "[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\/[[:digit:]]{1,3}")
TIME=$(date +%X)
DAY=$(date +%A)
YEAR=$(date +%Y)

echo "this computer has $IP as IP address"
echo "it is $TIME and we are a $DAY in $YEAR"

Which if we run it gives us the following output.

waldek@metal:~$ bash test.sh 
this script was written by wouter gordts in Brussels
this computer has 10.1.12.53/24 as IP address
it is 02:17:02 PM and we are a Tuesday in 2022
waldek@metal:~$ 

Coding challenge - Output system stats

Write a program that prints information about your computer such as:

• the hostname
• the FQDN
• number of cpus
• type of cpu
• amount of RAM

Write a program that prints information about a given user such as:

• name
• UID
• their default shell
• groups they are a member of
• number of files in their home directory
• amount of disk space they use

Getting input into the script

With read

Observe the output of the following program. It's not really complicated but it will demonstrate we can do arithmetic in bash scripts as well!

waldek@metal:~$ bash test.sh 
In which year where you born?
1986
your are probably around 36...
waldek@metal:~$ 

The output above was generated with the following code. The two things to notice are the read year line and the $(( $this_year - $year )). The former offers the possibility to prompt the user for input, the latter performs a mathematical calculation with two numbers.

#!/bin/bash

echo "In which year where you born?"
read year
this_year=$(date +%Y)
echo "your are probably around $(( $this_year - $year ))..."
waldek@metal:~$ 

Coding challenge - Secret input

Can you create me a secret password prompt? Something like this.

waldek@metal:~$ bash test.sh 
what is your secret password? 
hmmm, I don't know how to compare helloworld to supersecret
waldek@metal:~$ 

Spoiler warning!

#!/bin/bash

my_pass="supersecret"

read -s -p "what is your secret password? " pass
echo
echo "hmmm, I don't know how to compare $pass to $my_pass"

read multiple variables

TODO

With command line arguments

We can create a similar behaviour but with command line arguments. By doing so we don't have to answer any questions the script poses at runtime. If we create a script that will run for a long time, it doesn't require any interaction mid way! All necessary information is supplied by at execution time.

waldek@metal:~$ bash test.sh 1986
your are probably around 36...
waldek@metal:~$

#!/bin/bash

year=$1
this_year=$(date +%Y)
echo "your are probably around $(( $this_year - $year ))..."

The magic behind is the $1 variable. This variable represents the first argument on the command line. Knowing this, what would $4 mean? Indeed, the fourth argument...

From a file

TODO

From a pipe

TODO

Coding Challenge - output the exact output below

waldek@metal:~$ bash test.sh hello world 1986 35 foo bar linux rulez...
hello waldek, my name is test.sh
you supplied 8 arguments on the command line...
here are all of them on one line: hello world 1986 35 foo bar linux rulez...
waldek@metal:~$ 

Spoiler warning!

#!/bin/bash

echo "hello $USER, my name is $0"
echo "you supplied $# arguments on the command line..."
echo "here are all of them on one line: $@"

More math!

The let keyword

TODO

The expr keyword

TODO

Double parenthesis

We've already seen the basic syntax before but here are some more examples.

waldek@metal:~$ a=11
waldek@metal:~$ b=202
waldek@metal:~$ echo $(( $a + $b ))
213
waldek@metal:~$ echo $(( $a - $b ))
-191
waldek@metal:~$ echo $(( $a * $b ))
2222
waldek@metal:~$ echo $(( $a / $b ))
0
waldek@metal:~$ echo $(( $a % $b ))
11

Incrementing variables can also be done with the double parenthesis syntax. We can't use the $ to reference the variable though. This is a classic example of bash's finicky behaviour.

waldek@metal:~$ echo $(( b++ ))
202
waldek@metal:~$ echo $(( b++ ))
203
waldek@metal:~$ echo $(( b++ ))
204
waldek@metal:~$ echo $(( b++ ))
205
waldek@metal:~$ echo $(( b++ ))
206
waldek@metal:~$ 

Variable length

As bash only knows characters it has a built in feature to determine a variable's length. You can print the length of a variable, or use it to calculate something, with the following syntax.

waldek@metal:~$ test="hello world! bash is pretty sweet..."
waldek@metal:~$ echo ${#test} 
36
waldek@metal:~$ echo $(( ${#test} + 1986 ))
2022
waldek@metal:~$ 

Ryan's tutorials

If Statements - How to make decisions within your Bash script.

The small password checker we made before could use some conditional logic. We can easily implement this in bash with the following syntax.

#!/bin/bash

my_pass="supersecret"

read -s -p "what is your secret password? " pass
echo

if [ $pass == $my_pass ]; then
	echo "access granted!"
fi

The [ $pass == $my_pass ] is the actual evaluation and will always evaluate to either true or false. If the statement is true, the following code get's executed. If not, currently nothing happens. We can introduce a second keyword else to handle this.

#!/bin/bash

my_pass="supersecret"

read -s -p "what is your secret password? " pass
echo

if [ $pass == $my_pass ]; then
	echo "access granted!"
else
	echo "access denied..."
fi

The script above gives us the following behaviour.

waldek@helloworld:~$ bash test.sh 
what is your secret password? 
access denied...
waldek@helloworld:~$ bash test.sh 
what is your secret password? 
access granted!
waldek@helloworld:~$ 

We can add a bit more complexity to our possible branches with the elif keyword. This keyword allows us to construct a second and third branch of execution. Consider the sentence below.

If you are younger than 27 you are still young so if you're older than 27 you're considered old, but if you are 27 on the dot your life might be at risk!

This sentence can be converted to a conditional logic block as follows.

#!/bin/bash

read -p "how old are you? " age
echo

if [ "$age" -lt "27" ]; then
	echo "you are so young! enjoy it"
elif [ "$age" -gt "27" ]; then
	echo "you're sooo old!"
elif [ "$age" -eq "27" ]; then
	echo "your life might be at risk..."
else
	echo "I'm not sure I understand you."
fi

There is a little problem here though! We can input anything we want, not only numbers, and this creates some error messages. Bash is a bit special, compared to a language like python3, because it doesn't crash on an error. It just keeps going.

waldek@helloworld:~$ bash test.sh
how old are you? helloworld

test.sh: line 6: [: helloworld: integer expression expected
test.sh: line 8: [: helloworld: integer expression expected
test.sh: line 10: [: helloworld: integer expression expected
I'm not sure I understand you.
waldek@helloworld:~$ 

We can check if the input is really a number and redirect the error to /dev/null. If the number is not a real number we can't continue so we exit the script.

#!/bin/bash

read -p "how old are you? " age
echo

if ! [ "$age" -eq "$age" ] 2> /dev/null
then
    echo "Sorry integers only"
	exit 1
fi

if [ "$age" -lt "27" ]; then
	echo "you are so young! enjoy it"
elif [ "$age" -gt "27" ]; then
	echo "you're sooo old!"
elif [ "$age" -eq "27" ]; then
	echo "your life might be at risk..."
fi

How does it work behind the scenes?

exit status

Every command you execute on the command line has an exit code. You can read up a bit on what they are but the most important things to know are:

• the code is always a number
• on our systems it's a uint8 which means a value between 0 and 255
• the convention is that 0 means success, everything else is an error

In your bash shell, the variable $? always references the last exit status code. We can discover it's behaviour as follows.

waldek@debian:~$ ls does_exist 
does_exist
waldek@debian:~$ echo $?
0
waldek@debian:~$ ls does_not_exist
ls: cannot access 'does_not_exist': No such file or directory
waldek@debian:~$ echo $?
2
waldek@debian:~$ echo $?
0
waldek@debian:~$ 

The first echo $? prints the exit code of ls does_exist. The second prints the exit code of the failed command ls does_not_exist. The third prints the exit code of the echo $? that failed!

test

The presence of exit codes means we can evaluate their value and make decisions based on the outcome. The main workhorse for this is a builtin called test.

waldek@debian:~$ whatis test
test (1)             - check file types and compare values
waldek@debian:~$

I highly recommend you take some time to read the man test. Because the convention of exit codes is no news, good news there are two tiny programs that just serve to output true and false, where true is 0 and false is 1. A little demonstration.

waldek@debian:~$ which true
/usr/bin/true
waldek@debian:~$ which false
/usr/bin/false
waldek@debian:~$ man true
waldek@debian:~$ whatis true
true (1)             - do nothing, successfully
waldek@debian:~$ whatis false
false (1)            - do nothing, unsuccessfully
waldek@debian:~$ true
waldek@debian:~$ echo $?
0
waldek@debian:~$ false
waldek@debian:~$ echo $?
1
waldek@debian:~$ test true == true
waldek@debian:~$ echo $?
0
waldek@debian:~$ test true == false
waldek@debian:~$ echo $?
1
waldek@debian:~$ 

Again, I highly advise you to read the man test. If you did this then the following will make a lot of sense.

waldek@debian:~$ test -a does_exist 
waldek@debian:~$ echo $?
0
waldek@debian:~$ test -d does_exist 
waldek@debian:~$ echo $?
1
waldek@debian:~$ test -a does_not_exist
waldek@debian:~$ echo $?
1
waldek@debian:~$ 

The table below is taken from the bash reference manual you can find here.

command	status
-a file	True if file exists.
-b file	True if file exists and is a block special file.
-c file	True if file exists and is a character special file.
-d file	True if file exists and is a directory.
-e file	True if file exists.
-f file	True if file exists and is a regular file.
-g file	True if file exists and its set-group-id bit is set.
-h file	True if file exists and is a symbolic link.
-k file	True if file exists and its "sticky" bit is set.
-p file	True if file exists and is a named pipe (FIFO).
-r file	True if file exists and is readable.
-s file	True if file exists and has a size greater than zero.
-t fd	True if file descriptor fd is open and refers to a terminal.
-u file	True if file exists and its set-user-id bit is set.
-w file	True if file exists and is writable.
-x file	True if file exists and is executable.
-G file	True if file exists and is owned by the effective group id.
-L file	True if file exists and is a symbolic link.
-N file	True if file exists and has been modified since it was last read.
-O file	True if file exists and is owned by the effective user id.
-S file	True if file exists and is a socket.
file1 -ef file2	True if file1 and file2 refer to the same device and inode numbers.
file1 -nt file2	True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
file1 -ot file2	True if file1 is older than file2, or if file2 exists and file1 does not.
-o optname	True if the shell option optname is enabled. The list of options appears in the description of the -o option to the set builtin (see The Set Builtin).
-v varname	True if the shell variable varname is set (has been assigned a value).
-R varname	True if the shell variable varname is set and is a name reference.
-z string	True if the length of string is zero.
-n string string	True if the length of string is non-zero.
string1 == string2	True if the strings are equal. When used with the [[ command, this performs pattern matching as described above (see Conditional Constructs). ‘=’ should be used with the test command for POSIX conformance.
string1 = string2	True if the strings are equal. When used with the [[ command, this performs pattern matching as described above (see Conditional Constructs). ‘=’ should be used with the test command for POSIX conformance.
string1 != string2	True if the strings are not equal.
string1 < string2	True if string1 sorts before string2 lexicographically.
string1 > string2	True if string1 sorts after string2 lexicographically.

Nested if statements

It's worth pointing out we can nest if statements inside other if statements. There is no real theoretical limit to how deep we can go, but it's advised to keep the limit to two or three levels.

#!/bin/bash

num="40"

if [ "$num" -lt "300" ]; then
	echo "$num is a small number"
	if [ "$(( $num % 2))" -eq 0 ]; then
		echo "and it is even"
	else
		echo "$num is not even"
	fi
fi

Coding challenge - File information

Write a script that takes one argument which should be a valid file path. The program should print out what type of file this is, and if it is readable, print the first and last 5 lines. If the file does not exist, an error message should be shown. Something along these lines.

waldek@helloworld:~$ bash test.sh .bashrc 
.bashrc exists, I'll dig a little deeper
it is indeed a file
and I can read it!
here are the first 5 lines
# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

# If not running interactively, don't do anything
and here are the last 5 lines
  elif [ -f /etc/bash_completion ]; then
    . /etc/bash_completion
  fi
fi
. "$HOME/.cargo/env"
waldek@helloworld:~$ bash test.sh .not_a_file
that's not a file!
waldek@helloworld:~$ 

Spoiler warning!

#!/bin/bash

filepath=$1

if [ -e "$filepath" ]; then
	echo "$filepath exists, I'll dig a little deeper"
	if [ -f "$filepath" ]; then
		echo "it is indeed a file"
		if [ -r "$filepath" ]; then
			echo "and I can read it!"
			echo "here are the first 5 lines"
			head -n 5 $filepath
			echo "and here are the last 5 lines"
			tail -n 5 $filepath
		fi
	fi
else
	echo "that's not a file!"
fi

A modern version of test

I'll be the first to admit that the syntax of bash can be confusing and is rarely reader friendly. A nice, but brief, explication of the nuances of single and double brackets can be found in this stack overflow post. The double bracket command are called compound commands.

[[ ]]

waldek@debian:~$ [[ 3 = [[:digit:]] ]] ; echo $?
0
waldek@debian:~$ [ 3 = [[:digit:]] ] ; echo $?
1

TODO

(( ))

TODO

&& and ||

waldek@debian:~$ test true == true && echo "yes sir!" || echo "nope..."
yes sir!
waldek@debian:~$ test true == false && echo "yes sir!" || echo "nope..."
nope...
waldek@debian:~$ 

Ryan's tutorials

Loops - A variety of ways to perform repetitive tasks.

Ryan's tutorials

Write a script that sets all you cpu's to a desired governor.

Rename all files in a folder with an prefix or postfix.

Functions - Reuse code to make life easier.

defining a function

global vs local variable

return values

the command builtin

Ryan's tutorials

User Interface - Make your scripts user friendly.

• dialog tutorial
• better dialog tutorial
• Ryan's tutorials

Python

This repository has a twenty day course to learn python written by me. The main file you need to follow is this one. Some practical exercises can be found here together with the needed source files.

Vim as an IDE

I made a tutorial on the essentials of vim customization. My real world configuration can be found at this repository.