Bash shell functions are a way to group several UNIX / Linux commands for later execution using a single name for the group. Bash shell function can be executed just like a regular Unix command. Shell functions are executed in the current shell context without creating any new process to interpret them.
Both bash aliases and functions allow you to define shortcuts for longer or more complicated commands. However, aliases don’t allow control-flows, arguments, and other trickery things these functions will allow as explained in this tutorial.
This article is part of the on-going bash tutorial series. Refer to our earlier tutorial about bash introduction, bash exist status and bash alias examples.
Syntax to create a bash function:
function functionname() { commands . . }
- function is a keyword which is optional.
- functionname is the name of the function
- commands – List of commands to be executed in the functions.
Function accepts arguments. During execution, the arguments to the function becomes the positional parameters. Positional parameter 0 will have the scriptname which remains unchanged.
You can call the bash function from the command line as shown below:
$ functionname arg1 arg2
- When shell interprets a Linux command, it first looks into the special built-in functions like break, continue, eval, exec etc., then it looks for shell functions.
- The exit status of the bash function is the exit status of the last command executed in the function body.
Note: Place the shell function definitions in a shell start up file (for example, .bash_profile ). This way, the shell function is always available for you from the command line. Refer to our earlier bash execution sequence article to identify when .bash_profile will get executed.
Example 1: Function to display long list of files with the given extension
The function “lsext” is used to find the list of files in the current directory, which has the given extension as shown below. This function uses the combination of find command and ls command to get the job done.
$ function lsext() { find . -type f -iname '*.'${1}'' -exec ls -l {} \; ; } $ cd ~ $ lsext txt -rw-r--r-- 1 root root 24 Dec 15 14:00 InMorning.txt -rw-r--r-- 1 root root 184 Dec 16 11:45 Changes16.txt -rw-r--r-- 1 root root 458 Dec 18 11:04 Changes18.txt -rw-r--r-- 1 root root 1821 Feb 4 15:01 ChangesOfDB.txt
Example 2. Bash Function to execute a given Linux command on a group of files
In the following example, function “batchexec” finds the files with the given extension and executes the given command on those selected files.
$ function batchexec() { find . -type f -iname '*.'${1}'' -exec ${@:2} {} \; ; } $ cd ~ $ batchexec sh ls $ batchexec sh chmod 755 $ ls -l *.sh -rwxr-xr-x 1 root root 144 Mar 9 14:39 debug.sh -rwxr-xr-x 1 root root 5431 Jan 25 11:32 get_opc_vers.sh -rwxr-xr-x 1 root root 22 Mar 18 08:32 t.sh
In the above example, it finds all the shell script files with the .sh extension, and changes its permission to 755. (All permission for user, for group and others read and execute permission). In the function definition you could notice “${@:2}” which gives the second and following positional parameters (shell expansion feature).
Example 3. Bash Function to generate random password
The following function is used to generate the random strong passwords with special character for the given length. If length is not given by default it generates with 12 characters length.
$ function rpass() { cat /dev/urandom | tr -cd '[:graph:]' | head -c ${1:-12} } $ rpass 6 -Ju.T[[ $ rpass Gz1f!aKN^""k
In the above example, when rpass is executed with the argument 6, it generates random password with 6 characters and rpass without argument generates 12 character length password. ${1:-12} means if $1 is unset or null 12 will be returned else value of $1 will be substituted.
Example 4. Bash function to get IP address of a given interface
The following example defines a function called ‘getip’ which accepts interface name as an argument, and gives the IP address assigned on the given interface in the machine. ( by default it returns eth0 ip address ). This uses the ifconfig command to get the ip-address.
$ function getip() { /sbin/ifconfig ${1:-eth0} | awk '/inet addr/ {print $2}' | awk -F: '{print $2}'; } $ getip 15.110.106.86 $ getip eth0 15.110.106.86 $ getip lo 127.0.0.1
Example 5. Bash function to print the machines details
This example defines the function which gives all the required information about the machine. Users can define and call this function in the start up files, so that you will get these information during startup.
$ function mach() { echo -e "\nMachine information:" ; uname -a echo -e "\nUsers logged on:" ; w -h echo -e "\nCurrent date :" ; date echo -e "\nMachine status :" ; uptime echo -e "\nMemory status :" ; free echo -e "\nFilesystem status :"; df -h } $ mach Machine information: Linux dev-db 2.6.18-128.el5 #1 SMP Wed Dec 17 11:41:38 EST 2008 x86_64 GNU/Linux Users logged on: root pts/2 ptal.mot Wed10 0.00s 1.35s 0.01s w -h Current date : Thu Mar 18 11:59:36 CET 2010 Machine status : 11:59:36 up 7 days, 3 min, 1 user, load average: 0.01, 0.15, 0.15 Memory status : total used free shared buffers cached Mem: 2059768 2033212 26556 0 81912 797560 -/+ buffers/cache: 1153740 906028 Swap: 4192956 48164 4144792 Filesystem status : Filesystem Size Used Avail Use% Mounted on /dev/sda1 12G 12G 0 100% / tmpfs 1006M 377M 629M 38% /dev/shm /dev/sdc5 9.9G 409M 9.0G 5% /mydisk
Example 6: Bash function to format ls output better
The following function will clear the screen, place the cursor at the top of the screen, executes ls, and then places the cursor at the end of the screen.
$ function ll () { clear; tput cup 0 0; ls --color=auto -F --color=always -lhFrt; tput cup 40 0; } $ ll
Display the function code using type command
type is a shell built-in used to view the function code.
Syntax: type function-name
$ type ll ll is a function ll () { clear; tput cup 0 0; ls --color=auto -F --color=always -lhFrt; tput cup 40 0; alias ls="ls --color=auto -F" }
For your easy reference, get all the 6 functions mentioned this article from this sample .bash_profile functions file.
Add all these functions to your ~/.bash_profile file, to make sure you have access to these functions all the times without having to create it every time.