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
- Some common expressions
# File exists and is a regular file.
[ -f $FILE ]
# File is non-zero size
[ -s $FILE ]
# String has length zero
[ -z $STR ]
# String has length non-zero
[ -n $STR ]
Comparisons
-eq
is==
-gt
is>
-ge
is>=
Check if File/Folder Exists
See https://linuxize.com/post/bash-check-if-file-exists/#check-if-directory-exist.
Arrays
DRIVES=(
/media/veracrypt1
/media/veracrypt2
/media/veracrypt3
/media/veracrypt4
/media/veracrypt5
/media/veracrypt6
/media/veracrypt7
/media/veracrypt8
/media/veracrypt9
)
for i in "${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
# `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
trap 'kill $(jobs -p)' EXIT
Wait
Add wait
at the end of a script to wait for all subprocesses.
Brace Expansion
When you type the following:
echo {red,green,blue}_apple
bash will print out
red_apple green_apple blue_apple
If you want to save this into a variable, you can save it as an array:
APPLES=({red,green,blue}_apple)
echo "${APPLES[@]}"
--
--
explicitly ends non-positional arguments for many commands
dirname
How to get the source directory of the bash script
#!/bin/bash
DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
Check if running as sudo
if [ $EUID -ne 0 ]
then echo "Please run as root"
exit
fi
.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\[\033[00m\]\n\$ '