Bash Scripting

Common Functions

Here are some common functions I use in my scripts.

Prompt

You can prompt the user for an yes or no answer using this function. An optional default values are accepted as the second argument.

Example Usage:

 if prompt "Is this okay?" Y ; then
    echo "It's Okay!"
 fi
 if ! prompt "Is this okay?" N ; then
    echo "It's not okay!"
 fi

Function:

function prompt() {
	# credit: http://djm.me/ask
	while true; do
		if [ "${2:-}" = "Y" ]; then
			prompt="Y/n"
			default=Y
		elif [ "${2:-}" = "N" ]; then
			prompt="y/N"
			default=N
		else
			prompt="y/n"
			default=
		fi

		# Ask the question
		read -p "$1 [$prompt] " REPLY

		# Default?
		if [ -z "$REPLY" ]; then
			REPLY=$default
		fi

		# Check if the reply is valid
		case "$REPLY" in
			Y*|y*) return 0 ;;
			N*|n*) return 1 ;;
		esac
	done
}

die

Similar to the die command in PHP which prints out a message before stopping execution.

Function:

function die {
	echo $@
	exit
}

The caveat is that exit will kill the current shell - if running in a subshell, this will terminate the subshell and not the parent shell. To get around this, you may need to kill the parent PID instead.

Indent

Indents text piped to the function.

Function:

function Indent {                                                                                                    
	Prepend="\t"

	if [ $# -eq 1 ] ; then
		Indent=$1

		while [ $Indent -ge 0 ] ; do 
			Prepend="\t$Prepend"
			Indent=$(($Indent - 1))
		done
	fi

	while read i ; do echo -e "${Prepend}$i" ; done
}

Usage:

echo "test" | Indent
	test
echo "test" | Indent 2
		test

Extracting / Parsing Filenames

Use the basename to extract filenames from a path and and dirname to extract the path of a filename.

Eg:

dirname /etc/hosts
/etc

basename /etc/hosts
hosts

To extract only the extension or the filename without the extension, use bash's string matching:

Filename=$(basename /etc/resolv.conf)
Ext="${Filename##*.}"
Name="${Filename%.*}"

Arithmetic Operations

Do integer arithmetic operations on variables within double brackets. For example:

Value=1
ValuePlusOne=$((Value + 1))
ValueTimes12=$((Value * 12))

The operators that are supported are:

+ - / * % 

Bitwise operators also work:

^ (XOR)   | (OR)   & (AND)

Control Blocks

Switch

read -p "What do you want to do?: " Option

case "$Option" in
	"K")
		install_new_kernel
		die "Complete!"
		;;
	"H")
		usage
		;;
	*)
		die "Stopping installation."
		;;
esac

Snippets

Get First IP Address

ip a | /bin/grep inet | grep -v inet6 | tail -n+2 | head -n 1 | awk '{print $2}' | cut -d'/' -f1