Scripting with Parameters

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

Argument parsing using a for loop

A simple example which provides the options:

Opt	Alt. Opt	Details
-h	–help	Show help
-v	–version	Show version info
-dr path	–doc-root	An option which takes a secondary parameter (a path)
-i	–install	A boolean option (true/false)
-*	—	Invalid option

#!/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 "$@"

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

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