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# Scripting # bash
## Bash
A `bash` script is a sequence of command that are executed one by one. 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. Most of the time we just execute one command and wait for the result to then make a decision and execute an other command.
@ -274,6 +272,7 @@ waldek@helloworld:~$
If you open up a new shell this `$name` variable will not be defined because variables are local to each instance of `bash` that is running. If you open up a new shell this `$name` variable will not be defined because variables are local to each instance of `bash` that is running.
This can be observed as follows. This can be observed as follows.
We can *export* variables to children with the `export` keyword.
``` ```
waldek@helloworld:~$ name="wouter gordts" waldek@helloworld:~$ name="wouter gordts"
@ -293,7 +292,7 @@ exit
waldek@helloworld:~$ waldek@helloworld:~$
``` ```
### Using variables to store the output of command # Using variables to store the output of command
Bash only really knows *characters*, both for sending and receiving. Bash only really knows *characters*, both for sending and receiving.
We can temporarily store the output of a command using variables. We can temporarily store the output of a command using variables.
@ -310,7 +309,7 @@ waldek:x:1000:1000:waldek,,,:/home/local/waldek:/bin/zsh
waldek@metal:~$ waldek@metal:~$
``` ```
A little but more complicated. A little bit more complicated.
``` ```
waldek@metal:~$ count=$(grep "/home" /etc/passwd | wc -l) waldek@metal:~$ count=$(grep "/home" /etc/passwd | wc -l)
@ -350,9 +349,29 @@ it is 02:17:02 PM and we are a Tuesday in 2022
waldek@metal:~$ waldek@metal:~$
``` ```
### Getting input into the script # Coding challenge - Output system stats
#### With `read`
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*. 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! It's not really complicated but it will demonstrate we can do arithmetic in bash scripts as well!
@ -379,7 +398,40 @@ echo "your are probably around $(( $this_year - $year ))..."
waldek@metal:~$ waldek@metal:~$
``` ```
#### With command line arguments # 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:~$
```
<details>
<summary>Spoiler warning!</summary>
```bash
#!/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"
```
</details>
### `read` multiple variables
TODO
## With command line arguments
We can create a similar behaviour but 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. By doing so we don't have to answer any questions the script poses at runtime.
@ -405,6 +457,14 @@ This variable represents the *first* argument on the command line.
Knowing this, what would `$4` mean? Knowing this, what would `$4` mean?
Indeed, the *fourth* argument... Indeed, the *fourth* argument...
## From a file
TODO
## From a pipe
TODO
# Coding Challenge - output the exact output below # Coding Challenge - output the exact output below
``` ```
@ -428,59 +488,334 @@ echo "here are all of them on one line: $@"
</details> </details>
# More math!
## The `let` keyword
### What is a Bash Script - What are they, how do they work and how to run them. TODO
[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-script.php) ## The `expr` keyword
### Variables - Store data temporarily for later use. TODO
[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-variables.php) ## Double parenthesis
Write a program that prints information about your computer such as: We've already seen the basic syntax before but here are some more examples.
* the hostname ```
* the FQDN waldek@metal:~$ a=11
* number of cpus waldek@metal:~$ b=202
* type of cpu waldek@metal:~$ echo $(( $a + $b ))
* amount of RAM 213
waldek@metal:~$ echo $(( $a - $b ))
-191
waldek@metal:~$ echo $(( $a * $b ))
2222
waldek@metal:~$ echo $(( $a / $b ))
0
waldek@metal:~$ echo $(( $a % $b ))
11
```
Write a program that prints information about a given user such as: Incrementing variables can also be done with the double parenthesis syntax.
We can't use the `$` to reference the variable though.
This is a *classic* example of bash's finicky behaviour.
* name ```
* UID waldek@metal:~$ echo $(( b++ ))
* their default shell 202
* groups they are a member of waldek@metal:~$ echo $(( b++ ))
* number of files in their home directory 203
* amount of disk space they use waldek@metal:~$ echo $(( b++ ))
204
waldek@metal:~$ echo $(( b++ ))
205
waldek@metal:~$ echo $(( b++ ))
206
waldek@metal:~$
```
### Input - Different ways to supply data and directions to your Bash script. ## Variable length
[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-input.php) 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.
### Arithmetic - Perform various arithmetic operations in your Bash script. ```
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](https://ryanstutorials.net/bash-scripting-tutorial/bash-arithmitic.php) [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-arithmitic.php)
### If Statements - How to make decisions within your Bash script. # If Statements - How to make decisions within your Bash script.
The small *password* checker we made before could use some [conditional logic](https://en.wikipedia.org/wiki/Conditional_(computer_programming)).
We can easily implement this in `bash` with the following syntax.
```bash
#!/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.
```bash
#!/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.
```bash
#!/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.
```bash
#!/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?
TODO - explain `test` and exit status
The table below is taken from the bash reference manual you can find [here](https://www.gnu.org/software/bash/manual/bash.html#Bash-Conditional-Expressions).
| <div style="width:300px">command</div> | 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.
```bash
#!/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:~$
```
<details>
<summary>Spoiler warning!</summary>
```bash
#!/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
```
</details>
## A *modern* version of `test`
### `[[ ]]`
TODO
### `(( ))`
TODO
### `&&` and `||`
TODO
[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php) [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php)
Write a script that takes one argument which is a filepath. # Loops - A variety of ways to perform repetitive tasks.
The program should print out what type of file this is, and if it is readable, print the first and last 5 lines.
### Loops - A variety of ways to perform repetitive tasks.
[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-loops.php) [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-loops.php)
Write a script that sets all you cpu's to a desired governor. Write a script that sets all you cpu's to a desired governor.
### Functions - Reuse code to make life easier. Rename all files in a folder with an prefix or postfix.
# Functions - Reuse code to make life easier.
[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-functions.php) [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-functions.php)
### User Interface - Make your scripts user friendly. # User Interface - Make your scripts user friendly.
[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-user-interfaces.php) [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-user-interfaces.php)