Pass arguments into a function

• Shell functions have their own command line argument.
• Use variable $1, $2..$n to access argument passed to the function.
• The syntax is as follows:

name(){
  arg1=$1
  arg2=$2
  command on $arg1
}

• To invoke the the function use the following syntax:

 name foo bar

Where,

1. name = function name.
2. foo = Argument # 1 passed to the function (positional parameter # 1).
3. bar = Argument # 2 passed to the function.

Example

Create a function called fresh.sh:

#!/bin/bash
 
# write a function
fresh(){
   # t stores $1 argument passed to fresh()
   t=$1
   echo "fresh(): \$0 is $0"
   echo "fresh(): \$1 is $1"
   echo "fresh(): \$t is $t"
   echo "fresh(): total args passed to me $#"
   echo "fresh(): all args (\$@) passed to me -\"$@\""
   echo "fresh(): all args (\$*) passed to me -\"$*\""
}
 
# invoke the function with "Tomato" argument
echo "**** calling fresh() 1st time ****"
fresh Tomato
 
# invoke the function with total 3 arguments
echo "**** calling fresh() 2nd time ****"
fresh Tomato Onion Paneer

Save and close the file. Run it as follows:

chmod +x fresh.sh
./fresh.sh

Sample outputs:

**** calling fresh() 1st time ****
fresh(): $0 is ./fresh.sh
fresh(): $1 is Tomato
fresh(): $t is Tomato
fresh(): total args passed to me 1
fresh(): all args ($@) passed to me -"Tomato"
fresh(): all args ($*) passed to me -"Tomato"
**** calling fresh() 2nd time ****
fresh(): $0 is ./fresh.sh
fresh(): $1 is Tomato
fresh(): $t is Tomato
fresh(): total args passed to me 3
fresh(): all args ($@) passed to me -"Tomato Onion Paneer"
fresh(): all args ($*) passed to me -"Tomato Onion Paneer"

Function shell variables

• All function parameters or arguments can be accessed via $1, $2, $3,..., $N.
• $0 always point to the shell script name.
• $* or $@ holds all parameters or arguments passed to the function.
• $# holds the number of positional parameters passed to the function.

How Do I Display Function Name?

$0 always point to the shell script name. However, you can use an array variable called FUNCNAME which contains the names of all shell functions currently in the execution call stack. The element with index 0 is the name any currently-executing shell function.This variable exists only when a shell function is executing.

FUNCNAME in action

Create a shell script called funcback.sh:

#!/bin/bash
#  funcback.sh : Use $FUNCNAME
backup(){
	local d="$1"
	[[ -z $d ]] && { echo "${FUNCNAME}(): directory name not specified"; exit 1; }
	echo "Starting backup..."
}
 
backup $1

Save and close the file. Run it as follows:

chmod +x funcback.sh
funcback.sh /home
funcback.sh

Sample outputs:

backup(): directory name not specified

Example

Create a shell script to determine if given name is file or directory (cmdargs.sh):

#!/bin/bash
file="$1"
 
# User-defined function
is_file_dir(){
        # $f is local variable
	local f="$1"
        # file attributes comparisons using test i.e. [ ... ]
	[ -f "$f" ] && { echo "$f is a regular file."; exit 0; }
	[ -d "$f" ] && { echo "$f is a directory."; exit 0; }
	[ -L "$f" ] && { echo "$f is a symbolic link."; exit 0; }
	[ -x "$f" ] && { echo "$f is an executeble file."; exit 0; }
}
 
# make sure filename supplied as command line arg else die
[ $# -eq 0 ] && { echo "Usage: $0 filename"; exit 1; }
 
# invoke the is_file_dir and pass $file as arg
is_file_dir "$file"

Run it as follows:

./cmdargs.sh /etc/resolv.conf
./cmdargs.sh /bin/date
./cmdargs.sh $HOME
./cmdargs.sh /sbin

Sample outputs:

/etc/resolv.conf is a regular file.
/bin/date is a regular file.
/home/vivek is a directory.
/sbin is a directory.