Scripting with Parameters

Remarks#

• shift shifts the positional parameters to the left so that $2 becomes $1, $3 becomes $2 and so forth.
• "$@" is an array of all the positional parameters passed to the script/function.
• "$*" is an string composed of all the positional parameters passed to the script/function.

Multiple Parameter Parsing

To parse lots of parameters, the prefered way of doing this is using a while loop, a case statement, and shift.

shift is used to pop the first parameter in the series, making what used to be $2, now be $1. This is useful for processing arguments one at a time.

#!/bin/bash

# Load the user defined parameters
while [[ $# > 0 ]]
do
        case "$1" in

                -a|--valueA)
                        valA="$2"
                        shift
                        ;;

                -b|--valueB)
                        valB="$2"
                        shift
                        ;;

                --help|*)
                        echo "Usage:"
                        echo "    --valueA \"value\""
                        echo "    --valueB \"value\""
                        echo "    --help"
                        exit 1
                        ;;
        esac
        shift
done

echo "A: $valA"
echo "B: $valB"

Inputs and Outputs

$ ./multipleParams.sh --help
Usage:
    --valueA "value"
    --valueB "value"
    --help

$ ./multipleParams.sh
A: 
B:

$ ./multipleParams.sh --valueB 2
A: 
B: 2

$ ./multipleParams.sh --valueB 2 --valueA "hello world"
A: hello world
B: 2

Accessing Parameters

When executing a Bash script, parameters passed into the script are named in accordance to their position: $1 is the name of the first parameter, $2 is the name of the second parameter, and so on.

A missing parameter simply evaluates to an empty string. Checking for the existence of a parameter can be done as follows:

if [ -z "$1" ]; then
    echo "No argument supplied"
fi

Getting all the parameters

$@ and $* are ways of interacting with all the script parameters. Referencing the Bash man page, we see that:

• $*: Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable.
• $@: Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word.

Getting the number of parameters

$# gets the number of parameters passed into a script. A typical use case would be to check if the appropriate number of arguments are passed:

if [ $# -eq 0 ]; then
    echo "No arguments supplied"
fi

Example 1

Loop through all arguments and check if they are files:

for item in "$@"
do  
    if [[ -f $item ]]; then
        echo "$item is a file"
    fi  
done

Example 2

Loop through all arguments and check if they are files:

for (( i = 1; i <= $#; ++ i ))
do
    item=${@:$i:1}

    if [[ -f $item ]]; then
        echo "$item is a file"
    fi  
done

Argument parsing using a for loop

A simple example which provides the options:

#!/bin/bash
dr=''
install=false

skip=false
for op in "$@";do
    if $skip;then skip=false;continue;fi
    case "$op" in
        -v|--version)
            echo "$ver_info"
            shift
            exit 0
            ;;
        -h|--help)
            echo "$help"
            shift
            exit 0
            ;;
        -dr|--doc-root)
            shift
            if [[ "$1" != "" ]]; then
                dr="${1/%\//}"
                shift
                skip=true
            else
                echo "E: Arg missing for -dr option"
                exit 1
            fi
            ;;
        -i|--install)
            install=true
            shift
            ;;
        -*)
            echo "E: Invalid option: $1"
            shift
            exit 1
            ;;
    esac
done

Wrapper script

Wrapper script is a script that wraps another script or command to provide extra functionalities or just to make something less tedious.

For example, the actual egrep in new GNU/Linux system is being replaced by a wrapper script named egrep. This is how it looks:

#!/bin/sh
exec grep -E "$@"

So, when you run egrep in such systems, you are actually running grep -E with all the arguments forwarded.

In general case, if you want to run an example script/command exmp with another script mexmp then the wrapper mexmp script will look like:

#!/bin/sh
exmp "$@" # Add other options before "$@"
# or 
#full/path/to/exmp "$@"

Split string into an array in Bash

Let’s say we have a String parameter and we want to split it by comma

my_param="foo,bar,bash"

To split this string by comma we can use;

IFS=',' read -r -a array <<< "$my_param"

Here, IFS is a special variable called Internal field separator which defines the character or characters used to separate a pattern into tokens for some operations.

To access an individual element:

echo "${array[0]}"

To iterate over the elements:

for element in "${array[@]}"
do
    echo "$element"
done

To get both the index and the value:

for index in "${!array[@]}"
do
    echo "$index ${array[index]}"
done