shell-scripting.md

Shell Scripting

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Comments

Comments are defined by a leading #. They start at the position of the #-symbol until the end of the line. The Shebang (see below) is also a comment.

# single line comment

:' Multiline comments
work like
this

Shebang

The Shebang defines which shell the script uses.

#!/bin/bash
#!/usr/bin/env bash
#!/usr/bin/env sh

For example, #!/bin/bash tells your shellscript to use bash. Even better would be #!/user/bin/env bash as its uses the environemnt variable of the user instead of the binary. bash can be exchanged with zsh, csh or any other shell. If your script is POSIX compliant, you can use #!/usr/bin/env sh. This uses the deault shell of the user.

Variables

Defining Variables

Variables can be defined by assigning a value to a variable name. It is important to note, that there must be no spaces between the variable name, the = and the value, otherwise the shell interprets these as commands.

Variable=1
Variable="Hello World"

You can also define empty variables.

Variable=       # null
Variable=""     # empty string

Working with Variables

When using the variable itself (assigning values or exporting), you use the variable name. When the value of a variable is needed, a $-symbol is used.

echo $Variable
echo "$Varibale"
echo '$Variable'    # returns the string "$VARIABLE"

The first two echo-commands return the value of the variable, while the last echo returns the String $Variable. It is also advices to always use double quotes " around variables to avoid parameter expansion and other unexpected behavior.

Parameter Expansion

Variables can be expanded, allowing their value to be modified during the time of the expansion.

Variable="Hello World"
echo ${$Variable}            # Hello World
echo ${Variable/World/You}  # Hello You

The example above replaces the first occurance of the string World with the string You. The get the length of a string, use:

echo ${#Variable}   # 11

Arrays

Arrays are not supportet by POSIX, but can be used in more modern shells like bash or zsh. An array is defined with () and the elements are seperated by spaces.

array=(Hello World 1 2 3)

The elements of an array can be addressed in various ways:

echo "$array"           # Hello (first element)
echo "${array[0]}"      # Hello (first element)
echo "${array[@]}"      # Hello World 1 2 3 (whole array)
echo "${#array[@]}"     # 5 (number of elements)

Brace extension { }

This can be used to generate a sequence of characters or numbers.

echo {1..10}
echo {a..z}
echo {A, B}.sh  # A.sh B.sh

Control Flow

If-Statements

If statements can be used to check for a specific condition and change the behavior of the script. It is important to leave a space between the square brackets and the condition itself.

if [ "$Variable" = 10 ]; then
    echo "$Variable is 10"
else
    echo "$Variable is not 10"
fi

The logic parameters and and or are represented by && and || respectively. To use these in an if statement, you need to have multiple pairs of square brackets.

if [ "Contidion 1" ] || [ "Condition 2 "]; then
    # etc.
fi

Conditionals can also be used with shell commands.

git commit && git push
cd ~/.config/alacritty || exit 0

Bash also supports double square brackets [[ ]] that allow additional features found in more common programming languages, but these are at the same time not POSIX compliant.

For-Loops

For-loops are use to iterate over a sequence of values.

for i in {1..5}; do
    echi "$i"
done

This can also be used with arrays.

for i in "${array[@]}"; do
    echo "$i"
done

# This will return the indices of the array
for i in "${!array[@]}"; do
    echo "$i"
done

Funcitons

Functions take input parameters and return output parameters. Note that the function keyword is not necessarily needed.

function myfunc() {
    # code here
}