Linux Bash Scripting Part3 – Parameters and Options

So far, you’ve seen how to write Linux bash scripts that do the job without user inputs. Today we will continue our series about Linux bash scripting.

I recommend you review the previous posts if you want to know what we are talking about.

Bash scripting basics

Bash scripting – For loops

 

 

Today we will know how to retrieve input from the user and deal with that input, so our script becomes more interactive.

To pass data to your shell script, you should use command line parameters.

$ ./myscript 3 5

Here we send two parameters (3 and 5) to the script. So how to read these parameters in our bash script?

Read parameters

The shell gives you some easy to use variables to process input parameters:

• $0 is the script’s name.
• $1 is the 1st parameter.
• $2 is the 2nd parameter.

The shell can read the 9th parameter, which is $9.

Let’s see those variables in action:

#!/bin/bash
echo $0 # Script name
echo $1 # 1st parameter
echo $2 # 2nd parameter
echo $3 # 3rd parameter

Check the results of the following command:

./myscript 5 10 15

Here is another example of how we can use two parameters and calculate the sum of them.

#!/bin/bash
total=$(( $1 + $2 ))
echo First passed parameter is $1.
echo Second passed parameter is $2.
echo Total is $total.

$ ./myscript 5 10

The parameters are not restricted to numbers; they could be strings like this:

#!/bin/bash
echo Hello $1, how do you do

./myscript Adam

And the result is as expected.

What if our parameter contains a space, and we want to pass it as one value? I guess you know the answer from the previous posts. The answer is to use quotations.

If your script requires over nine parameters, you should use braces like this ${10}.

Check parameters

If you don’t pass parameters and your code is expecting it, your script will exit with an error.

That’s why we should use a Linux if statement to make sure that they exist.

#!/bin/bash
if [ -n "$1" ]; then # If first parameter passed then print Hi
	echo Hi $1.
else
	echo "No parameters found. "
fi

Counting parameters

To get how many parameters passed, you can use this variable ($#).

#!/bin/bash
echo There were $# parameters passed.

./myscript 1 2 3 4 5

How excellent is Linux bash scripting? This variable also provides a geeky way to get the last parameter. Look at this trick:

#!/bin/bash
echo Last passed parameter is ${!#}

Iterate over parameters

The $* variable holds all the parameters as one value.

The $@ variable holds all the parameters as separate values so that you can iterate over them.

This code shows how to use them:

#!/bin/bash
echo "The first variable \$* method: $*"
echo "-----------"
echo "The second variable \$@ method: $@"

The same result, but if you want to know the difference look at the following example:

#!/bin/bash
total=1
for var in "$*"; do
	echo "\$* =======> #$total = $var"
	total=$(($total + 1))
done
total=1
for var in "$@"; do
	echo "\$@ =======> #$total = $var"
	total=$(($total + 1))
done

Check the result to see the difference:

The result is pretty clear. You can use any method according to your needs.

Shifting parameter variable

The shift command moves every parameter variable to the left:

variable $3 ==> variable $2.

variable $2 ==> variable $1.

variable $1 ==> dropped.

variable $0 ==> (the script name) as it is.

You can use the shift command to iterate over parameters like this:

#!/bin/bash
total=1
while [ -n "$1" ]; do # while loop starts
	echo "#$total = $1"
	total=$(($total + 1))
	shift
done

Here, we have a while loop checking $1 length. If $1 becomes zero, the loop stops. And the shift command is shifting all passed parameters to the left.

Careful when using the shift command, since you can’t recover the shifted parameter.

Bash scripting options

Options are single letters with a dash before it.

#!/bin/bash
while [ -n "$1" ]; do # while loop starts
	case "$1" in
	-a) echo "-a option passed" ;; # Message for -a option
	-b) echo "-b option passed" ;; # Message for -b option
	-c) echo "-c option passed" ;; # Message for -c option
	*) echo "Option $1 not recognized" ;; # In case you typed a different option other than a,b,c
	esac
	shift
done

$ ./myscript -op1 -op2 -op3 -op4

The above code checks for one of the correct options. If you typed one of them, the suitable commands would run.

Separate options from parameters

Sometimes you need to use options and parameters in the same script. You have to separate them. By doing this, you are telling the bash where the parameters are and where the options are.

We use the double dash (–) at the end of the options list. After the shell sees the double dash, it treats the remaining inputs as parameters and not as options.

#!/bin/bash
while [ -n "$1" ]; do # while loop starts
	case "$1" in
	-a) echo "-a option passed" ;; # Message for -a option
	-b) echo "-b option passed" ;; # Message for -b option
	-c) echo "-c option passed" ;; # Message for -c option
	--)
		shift # The double dash which separates options from parameters
		break
		;; # Exit the loop using break command
	*) echo "Option $1 not recognized" ;;
	esac
	shift
done
total=1
for param in $@; do
	echo "#$total: $param"
	total=$(($total + 1))
done

$ ./myscript -a -b -c -- 5 10 15

As you can see from the result, all the parameters after the double dash treated as parameters and not options.

Process options with values

When you dig deep into Linux bash scripting, sometimes you need options with additional parameter values like this:

./myscript -a value1 -b -c value2

There should be a way to identify any additional parameter for the options and be able to process it.

#!/bin/bash
while [ -n "$1" ]; do # while loop starts
	case "$1" in
	-a) echo "-a option passed" ;;
	-b)
		param="$2"
		echo "-b option passed, with value $param"
		shift
		;;
	-c) echo "-c option passed" ;;
	--)
		shift # The double dash makes them parameters
		break
		;;
	*) echo "Option $1 not recognized" ;;
	esac
	shift
done
total=1
for param in "$@"; do
	echo "#$total: $param"
	total=$(($total + 1))
done

And if we run it with these options:

$ ./myscript -a -b test1 -d

From the results, you can see that we get the parameter for the -b option using the $2 variable.

Standard options

When you start your Linux bash scripting, you are free to choose which letter is suitable for your option.

However, some letters are commonly used in Linux programs.

And here is the list of the common options:

-a	List all items.
-c	Get the count of items.
-d	Output directory.
-e	Expand items.
-f	To specify a file.
-h	To show the help page.
-i	To ignore character case.
-l	To list a text.
-n	To say no for a question.
-o	To send output to a file or so.
-q	Keep silent; don’t ask the user.
-r	To process something recursively.
-s	Stealth mode.
-v	Verbose mode.
-x	Specify executable.
-y	To say yes without prompting the user.

It’s good to follow standards. If you work with Linux, many of these options may look familiar to you.

 

Getting user input using the read command

Sometimes you need data from the user while the bash scripting is running.

The bash shell uses the read command for this purpose.

The read command reads input from standard input (the keyboard) or a file descriptor and stores it in a variable:

#!/bin/bash
echo -n "What's your name: "
read name
echo "Hi $name,"

We use the -n option to disable the newline so that you can type your text in the same line.

You can specify multiple inputs like this:

#!/bin/bash
read -p "What's your name: " first last
echo "Your data for $last, $first…"

If you don’t specify variables for the read command, it will save all incoming inputs in the REPLY variable.

#!/bin/bash
read -p "What's your name: "
echo Hello $REPLY,.

You can use the -t option to specify a timeout for input in seconds.

#!/bin/bash
if read -t 5 -p "What's your name: " name; then
	echo "Hi $name, how are you?"
else
	echo "You took much time!"
fi

If you do not enter data for five seconds, the script will execute the else clause and print a sorry message.

 

Reading password

In Linux bash scripting, sometimes, you don’t want the user input to be displayed on the screen, like entering a password.

The -s option suppresses the output from appearing on the screen.

#!/bin/bash
read -s -p "Enter password: " mypass
echo "Your password is $mypass? "

 

Read files

The read command can read files one line on each call.

Now, if you want to get all file data, you can get the content using the cat command, then send it to the read command using while loop like this:

#!/bin/bash
count=1
# Get file content then pass to read command by iterating over lines using while command
cat myfile | while read line; do
	echo "#$count: $line"
	count=$(($count + 1))
done
echo "Finished"

Or we can make it simpler by redirecting the file content to the while loop like this:

#!/bin/bash
while read line; do
	echo $line
done <myfile

We just pass the file content to the while loop and iterate over every line and print the line number and the content, and each time you increase the count by one.

I hope you find this post interesting. Keep coming back.

Thank you.