Bash Scripting

Test

test is a bash built-in function to add conditionals in your script. [ is an alias of the test command and are analogous.

Use the test operators listed below with test or a single bracket test condition like the following example.

$ test -e /etc/hosts && echo "File exists"
File exists

## Note that the white space around [ and ] and also the operator are required.
$ [ -e /etc/hosts ] && echo "File exists"
File exists

$ if [ -e /etc/hosts ] ; then echo "File exists" ; fi
File exists

Operator	Description
`-e`	file exists
`-a`	file exists (deprecated) This is identical in effect to `-e`
`-f`	file is a regular file (not a directory or device file)
`-s`	file is not zero size
`-d`	file is a directory
`-b`	file is a block device. Eg. `/dev/sda`.
`-c`	file is a character device. Eg. `/dev/tty0`.
`-p`	file is a pipe. Eg. `/dev/fd/0`.
`-h`	file is a symbolic link
`-L`	file is a symbolic link
`-S`	file is a socket
`-t`	file (descriptor) is associated with a terminal device This test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal.
`-r`	file has read permission (for the user running the test)
`-w`	file has write permission (for the user running the test)
`-x`	file has execute permission (for the user running the test)
`-g`	file or directory has set-group-id (sgid) flag set If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory.
`-u`	file has set-user-id (suid) flag set
`-k`	file or directory has sticky bit set If set on a file, that file will be kept in cache memory, for quicker access. If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. This restricts altering or deleting specific files in that directory to the owner of those files.
`-O`	you are owner of file
`-G`	group-id of file same as yours
`-N`	file modified since it was last read
`f1 -nt f2`	file f1 is newer than f2
`f1 -ot f2`	file f1 is older than f2
`f1 -ef f2`	files f1 and f2 are hard links to the same file
`!`	"not" -- reverses the sense of the tests above (returns true if condition absent).

Integer

Integer tests, with test or inside single brackets.

Eg.

$ test 2 -eq 2 && echo equal

Operator	Description
`-eq`	is equal to
`-ne`	is not equal to
`-gt`	is greater than
`-ge`	is greater than or equal to
`-lt`	is less than
`-le`	is less than or equal to

Integer tests can aso be done inside double parentheses. Eg.

$ ((2==2)) && echo equal

Operator	Description
`==`	is equal
`<`	is less than
`<=`	is less than or equal to
`>`	is greater than
`>=`	is greater than or equal to

Strings

Operator	Description
`-z`	string is null, that is, has zero length
`-n`	string is not null.
`=`	is equal to ## Does not work in zsh. Works only in sh and bash. $ if [ "$a" = "$b" ] ; then echo "Equal" fi
`==`	is equal to ## Does not work in zsh. Works only in sh and bash. $ if [ "$a" == "$b" ] ; then echo "Equal" fi This operator inside a double bracket `[[ ]]` construct does pattern matching.
`!=`	is not equal to $ if [ "$a" != "$b" ] ; then echo "Not equal" fi This operator inside a double bracket `[[ ]]` construct does pattern matching.
`=~`	Match using regex $ if [ zone36-ta =~ .ta$ ] ; then echo "matches" fi $ if [ zone36-ta =~ ta ] ; then echo "matches" fi

Useful Functions / Utilities

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

Source: <phorkie>https://phorkie.leo.home.steamr.com/13/embed</phorkie>

die

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

Function:

function die {
	echo $@
	exit
}

Caveat when using in a subshell

This will not work inside a subshell as it will just quit the subshell rather than the script. To get around this, you may need to kill the parent PID instead.

Indent

Indents text piped to the function.

Source: <phorkie>https://phorkie.leo.home.steamr.com/14/embed</phorkie>

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