Bash Scripting: Arguments

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Bash Script Arguments

In Bash scripting, arguments are the inputs passed to a script at the time of its execution. They enable scripts to be more flexible and reusable, as they can perform different operations based on the provided arguments.

How to Pass Arguments to a Bash Script

Arguments are passed to a Bash script after the script name, separated by spaces.

For example, let's say we have a script called myscript.sh, and we want to pass three arguments to it (arg1, arg2, and arg3). We would do so as follows:

./myscript.sh arg1 arg2 arg3

In the script, these arguments are accessed as $1, $2, $3, etc., where the number corresponds to the position of the argument. Here's an example script that prints the first three arguments:

#!/bin/bash
echo "First argument: $1"
echo "Second argument: $2"
echo "Third argument: $3"

Special Variables for Arguments

There are several special variables you can use when dealing with arguments in a Bash script:

• $0: The name of the script itself.
• $1 to $9: The first 9 arguments to the script. $1 is the first argument and so on.
• ${10} to ${N}: The tenth argument and beyond. Note the use of curly braces for accessing arguments with a number greater than 9.
• $#: The number of arguments passed to the script.
• $@: All arguments passed to the script as a list of separate words.
• $*: All arguments passed to the script as a single word.
• $$: The process ID (PID) of the current script.
• $?: The exit status of the last command executed in the script.

For instance, if we update our script as follows:

#!/bin/bash
echo "Script name: $0"
echo "Number of arguments: $#"
echo "All arguments with \$@: $@"
echo "All arguments with \$*: $*"

And run it with ./myscript.sh arg1 arg2 arg3, it would output:

Script name: ./myscript.sh
Number of arguments: 3
All arguments with $@: arg1 arg2 arg3
All arguments with $*: arg1 arg2 arg3

Shift Command

The shift command in Bash allows you to "shift" the positional parameters of your script. This is useful when you want to iterate over each argument in a loop.

When you call shift, all positional parameters move one position to the left. $2 becomes $1, $3 becomes $2, and so on. $0 (the script name) is unaffected.

Here's an example script that uses shift to print all arguments:

#!/bin/bash
while (( $# )); do
echo "Argument: $1"
shift
done

Getopts Command

For more complex scripts where arguments might have options associated with them, the getopts command is useful. It allows you to handle arguments with options in a more organized way.

Here's an example of how to use getopts:

#!/bin/bash

while getopts ":a:b:c:" opt; do
case ${opt} in
    a ) echo "Option -a has value $OPTARG"
    ;;
    b ) echo "Option -b has value $OPTARG"
    ;;
    c ) echo "Option -c has value $OPTARG"
    ;;
    \? ) echo "Invalid Option: -$OPTARG"
    ;;
    : ) echo "Option -$OPTARG requires an argument"
    ;;
esac
done

In this script, the getopts command is used in a while loop to iterate over the options. The options are defined in the getopts command itself: :a:b:c:. This means the script expects options -a, -b, and -c, and each of these options requires a value.

$OPTARG holds the value of the current option, and $opt is the current option itself. The case command is used to handle each option separately.

Conclusion

Bash script arguments are a powerful tool for making your scripts flexible and reusable. They allow your scripts to accept inputs directly from the command line, enabling them to perform different tasks depending on those inputs.

By understanding and using these features, you can write more adaptable and powerful Bash scripts. Keep practicing and exploring different use cases to strengthen your command over Bash script arguments.

• Bash Scripting
• Bash Scripting: Introduction
• Variables
• Basic Math
• Control Statements
• For Loops
• While Loops
• Functions
• Case Statements

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