So far, we have seen the usage of the commands such as grep, head, ls, cat, and many more. These commands also support passing arguments to a command via a command line. Some of command line arguments are input files, output files, and options. Arguments are provided as per output needs. For example, ls -l filename is executed to get a long listing output, while ls -R filename is used to display recursively the contents of a directory.
Shell script also supports providing command line arguments that we can process further by a shell script.
The command line arguments can be given as follows:
<script_file> arg1 arg2 arg3 … argN
Here, script_file is a shell script file to be executed, and arg1, arg2, arg3, argN, and so on, are command line parameters.
Command line arguments are passed to a shell script as positional parameters. So, arg1 will be accessed in a script as $1, arg2 as $2, and so on.
The following shell demonstrates the usage of the command line arguments:
#!/bin/bash # Filename: command_line_arg.sh # Description: Accessing command line parameters in shell script # Printing first, second and third command line parameters" echo "First command line parameter = $1" echo "Second command line parameter = $2" echo "Third command line parameter = $3"
The following output is obtained after running the command_line_arg.sh script with arguments:
$ sh command_line_arg.sh Linux Shell Scripting First command line parameter = Linux Second command line parameter = Shell Third command line parameter = Scripting
The following table shows special variables that are useful to get more information about command line parameters:
|
Special variables |
Description |
|---|---|
|
|
Number of the command line arguments |
|
|
Complete set of command line arguments in a single string—that is, |
|
|
Complete set of command line arguments, but each argument is enclosed in separate quotes—that is, |
|
|
Name of the shell script itself |
|
|
Refers to argument1, argument2, …, argumentN, respectively |
Using $# in a script to check the number of command line arguments will be very helpful to process arguments further.
The following is another shell script example that takes command line arguments:
#!/bin/bash # Filename: command_line_arg2.sh # Description: Creating directories in /tmp # Check if at least 1 argument is passed in command line if [ $# -lt 1 ] then echo "Specify minimum one argument to create directory" exit 1 else pushd /tmp > /dev/null echo "Directory to be created are: $@" mkdir $@ # Accessing all command line arguments fi
The following output is obtained after executing the command_line_arg2.sh script:
$ sh command_line_arg2.sh a b Directory to be created are: a b $ sh command_line_arg2.sh Specify minimum one argument to create directory
To shift command line arguments towards the left, the shift built in can be used. The syntax is as follows:
shift N
Here, N is the number of arguments by which it can shift to the left.
For example, suppose the current command line arguments are arg1, arg2, arg3, arg4 and arg5. They can be accessed in a shell script as $1, $2, $3, $4, and $5, respectively; the $# value is 5. When we call shift 3, arguments get shifted by 3. Now, $1 contains arg4 and $2 contains arg5. Also, the $# value is now 2.
The following shell script demonstrates the usage of shift:
#!/bin/bash # Filename: shift_argument.sh # Description: Usage of shift shell builtin echo "Length of command line arguments = $#" echo "Arguments are:" echo "$1 = $1, $2 = $2, $3 = $3, $4 = $4, $5 = $5, $6 = $6" echo "Shifting arguments by 3" shift 3 echo "Length of command line arguments after 3 shift = $#" echo "Arguments after 3 shifts are" echo "$1 = $1, $2 = $2, $3 = $3, $4 = $4, $5 = $5, $6 = $6"
The following output is obtained after running the shift_argument.sh script with the arguments a b c d e f:
$ sh shift_argument.sh a b c d e f Length of command line arguments = 6 Arguments are: $1 = a, $2 = b, $3 = c, $4 = d, $5 = e, $6 = f Shifting arguments by 3 Length of command line arguments after 3 shift = 3 Arguments after 3 shifts are $1 = d, $2 = e, $3 = f, $4 = , $5 = , $6 =
Providing command line options make shell scripts more interactive. From the command line arguments, we can also parse options for further processing by a shell script.
The following shell script shows the command line usage with options:
#!/bin/bash
# Filename: myprint.sh
# Description: Showing how to create command line options in shell script
function display_help()
{
echo "Usage: myprint [OPTIONS] [arg ...]"
echo "--help Display help"
echo "--version Display version of script"
echo "--print Print arguments"
}
function display_version()
{
echo "Version of shell script application is 0.1"
}
function myprint()
{
echo "Arguments are: $*"
}
# Parsing command line arguments
if [ "$1" != "" ]
then
case $1 in
--help )
display_help
exit 1
;;
--version )
display_version
exit 1
;;
--print )
shift
myprint $@
exit 1
;;
*)
display_help
exit 1
esac
fiThe following output is obtained after executing the myprint.sh script:
$ sh myprint.sh --help Usage: myprint [OPTIONS] [arg ...] --help Display help --version Display version of script --print Print arguments $ sh myprint.sh --version Version of shell script application is 0.1 $ sh myprint.sh --print Linux Shell Scripting Arguments are: Linux Shell Scripting