Bash (Unix shell)

Bash Scripting

Getting Started

Here is an example bash script

#!/bin/bash
# A simple variable example
myvariable=Hello
anothervar=Fred

echo $myvariable $anothervar

You can check your bash scripts using https://www.shellcheck.net/.

Usage

Basic If Statements

https://linuxize.com/post/bash-if-else-statement/

if [[ $VAR -gt 10 ]] then
  echo "The variable is greater than 10."
elif [[ $VAR -eq 10 ]] then
  echo "The variable is equal to 10."
else
  echo "The variable is less than 10."
fi

bash if statements

Comparisons

• -eq is ==
• -gt is >
• -ge is >=

Check if File/Folder Exists

See https://linuxize.com/post/bash-check-if-file-exists/#check-if-directory-exist.

Arrays

DRIVES=(
  /media/veracrypt1
  /media/veracrypt2
  /media/veracrypt3
  /media/veracrypt4
  /media/veracrypt5
  /media/veracrypt6
  /media/veracrypt7
  /media/veracrypt8
  /media/veracrypt9
)

for i in "${DRIVES[@]}"
do
        ls $i
done

Notes

• "${DRIVES[@]}" means every element will be a new word
• "${DRIVES[*]}" will expand the array into a single word

Functions

#!/bin/bash 
function say_hello {
    echo Hello $1
}
say_hello World

Redirects

Redirections

# `ls` is an example command

# Input to stdin
ls < word

# Output stdout to file
ls > file.txt

# Append stdout to file
ls >> file.txt

# Output stderr and stdout to file
ls > file.txt 2>&1
ls &> file.txt # Only works in bash, not sh

Trap

To make a bash script exit on ctrl c:

trap "exit" INT

Wait

Add wait at the end of a script to wait for all subprocesses.

dirname

Stack overflow

How to get the source directory of the bash script

#!/bin/bash

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"

.bashrc

Your .bashrc file will be loaded for each terminal.

Presentation of Shell Variable (PS1)

Add the following to show your current working directory.

PS1='\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$'

Brace Expansion

When you type the following:

echo {red,green,blue}_apple

bash will print out

red_apple green_apple blue_apple

If you want to save this into a variable, you can save it as an array:

APPLES=({red,green,blue}_apple)
echo "${APPLES[@]}"