Working with command line arguments (special variables, set and shift, getopt)

Command line arguments are required for the following reasons:

They inform the utility or command as to which file or group of files to process (reading/writing of files)
Command line arguments tell the command/utility which option to use

Check the following command line:

student@ubuntu:~$  my_program   arg1  arg2  arg3

If my_command is a bash Shell script, then we can access every command line positional parameters inside the script as follows:

$0 would contain "my_program"    # Command
$1 would contain "arg1"      # First parameter
$2 would contain "arg2"      # Second parameter
$3 would contain "arg3"      # Third parameter

The following is the summary of positional parameters:

$0	Shell script name or command
$1–$9	Positional parameters 1–9
${10}	Positional parameter 10
$#	Total number of parameters
$*	Evaluates to all the positional parameters
$@	Same as $*, except when double quoted
"$*"	Displays all parameters as "$1 $2 $3", and so on
"$@"	Displays all parameters as "$1" "$2" "$3", and so on

Let's create a script param.sh as follows:

#!/bin/bash
echo "Total number of parameters are = $#"
echo "Script name = $0"
echo "First Parameter is $1"
echo "Second Parameter is $2"
echo "Third Parameter is $3"
echo "Fourth Parameter is $4"
echo "Fifth Parameter is $5"
echo "All parameters are = $*"

Then as usual, give execute permission to script and then execute it:

./parameter.sh London Washington Delhi Dhaka Paris

Output:

Total number of parameters are = 5
Command is = ./parameter.sh
First Parameter is London
Second Parameter is Washington
Third Parameter is Delhi
Fourth Parameter is Dhaka
Fifth Parameter is Paris
All parameters are = London Washington Delhi Dhaka Paris

Understanding set

Many times we may not pass arguments on the command line, but we may need to set parameters internally inside the script.

We can declare parameters by the set command as follows:

$ set USA Canada UK France
$ echo $1
USA
$ echo $2
Canada
$ echo $3
UK
$ echo $4
France

We can use this inside the set_01.sh script as follows:

#!/bin/bash
set USA Canada UK France
echo $1
echo $2
echo $3
echo $4

Run the script as:

$ ./set.sh

Output:

USA
Canada
UK
France

Table declare Options
Option	Meaning
–a	An array will be created
–f	Displays function names and definitions
–F	Displays only the function names
–i	Makes variables integer types
–r	Makes variables read-only
–x	Exports variables

We give commands as follows:

set One Two Three Four Five
echo $0      # This will show command
echo $1       # This will show first parameter
echo $2
echo $*    # This will list all parameters
echo $#    # This will list total number of parameters
echo ${10} ${11}  # Use this syntax for parameters for 10th and
         # 11th parameters

Let us write script set_02.sh as follows:

#!/bin/bash
echo The date is $(date)
set $(date)
echo The month is $2
exit 0

Output:

In the script $(date), the command will execute and the output of that command will be used as $1, $2, $3 and so on. We have used $2 to extract the month from the output.

Let's write script set_03.sh as follows:

#!/bin/bash

echo "Executing script $0"
echo $1 $2 $3

set eins zwei drei
echo "One two three in German are:"
echo "$1"
echo  "$2"
echo  "$3"

textline="name phone address birthdate salary"
set $textline
echo  "$*"
echo 'At this time $1 = ' $1 'and $4 = ' $4

Output:

In this script, the following output shows:

Initially when the set is not called, then $1, $2, $3 do not contain any information.
Then, we set $1 to $3 as GERMAN numerals in words.
Then, set $1 to $5 as name, phone, address, birthdate, and salary, respectively.

Understanding shift

Using shift, we can change the parameter to which $1 and $2 are pointing to the next variable.

Create a script shift_01.sh as follows:

#!/bin/bash
echo "All Arguments Passed are as follow : "
echo $*
echo "Shift By one Position :"
shift
echo "Value of Positional Parameter $ 1 after shift :"
echo $1
echo "Shift by Two Positions :"
shift 2
echo "Value of Positional Parameter $ 1 After two Shifts :"
echo $1

Execute the command as follows:

$ chmod +x shift_01.sh
$ ./shift_01.sh One Two Three Four

Output:

student@ubuntu$ ./shift_01.sh One Two Three Four

All arguments passed are as follows:

One Two Three Four

Shift by one position.

Here, the value of the positional parameter $1 after shift is:

Two

Shift by two positions.

The value of the positional parameter $1 after two shifts:

Four

We observed that initially $1 was One. After shift, $1 will be pointing to Two. Once shift is done, the value in position 1 is always destroyed and is inaccessible.

Create a shift_02.sh script as follows:

#!/bin/bash

echo '$#: ' $#
echo '$@: ' $@
echo '$*: ' $*
echo
echo '$1 $2 $9 $10 are: ' $1 $2 $9 $10
echo

shift
echo '$#: ' $#
echo '$@: ' $@
echo '$*: ' $*
echo
echo '$1 $2 $9 are: ' $1 $2 $9

shift 2
echo '$#: ' $#
echo '$@: ' $@
echo '$*: ' $*
echo
echo '$1 $2 $9 are: ' $1 $2 $9

echo '${10}: ' ${10}

In this script execution, the following output shows:

Initially, $1 to $13 were numerical values 1 to 13, respectively.
When we called the command shift, then $1 shifted to number 2 and accordingly all $numbers are shifted.
When we called the command shift 2, then $1 shifted to number 4 and accordingly all $numbers are shifted.