Shell Scripting In Linux

Why Shell Scripting?

It’s just another way to improve your productivity. Just like how you’d write a program to do the repetitive tasks for you, you can achieve the same with shell scripts. Maybe you need to compile and deploy multiple applications. Instead of manually running these commands one by one, you can simply run a single shell script that will run all of the commands for you.

There are various shell languages (bash, csh, zsh, etc) that you can choose from. You can check which languages are installed on your server by using the command cat etc/shells. But since bash is the most commonly used one, I’ll be using bash syntax.

Shell Script Files

All executable shell script files have a file extension of .sh and will start with a shebangs on the first line.

#!/bin/bash or #!/bin/csh (depending on the language you’re using)

This tells the shell interpreter what shell language you’ll be using. If not, it will use the system default, which you can check with echo $SHELL.

Running Shell Scripts

Make sure you have execution permission on the sript file. Check out this post for more info on linux file permissions.

chmod +x [filename]

There are a couple ways you can execute the script file:

• ./scriptname.sh¹
• sh scriptname.sh
• bash scriptname.sh

If the script is in another directory, use the absolute path along with the filename.

You can also run shell scripts within a shell script with the same commands mentioned above. Say we have a file, print_hello.sh, which simply contains echo hello. We’ll have another file called print_helloworld.sh.

#!/bin/bash
./print_hello.sh
echo world

This will execute the print_hello.sh file, which will print hello, then it will move on and print world.

Functions

There are two ways we can define functions in bash.

#!/bin/bash
# define functions
func1 () {
    # body
    echo func1
}

function func2 {
    # body
    echo func2
}

# call the functions
func1
func2

In shell scripts, you are limited to returning integer values, usually to signify the status of the executed function. If you want to return some value, you will have to pass it through a global variable.

myfunc () {
    return 5
}
myfunc
echo $? # exit status of the last command executed -> 5

You can also pass in parameters to the function. The numbers signify the position of the arguments passed. $@ will print out all the arguments.

myfunc () {
    echo $1 $2
    echo $@
}
myfunc hello world  # outputs 'hello world'

Functions can be defined on a single line as well. Just separate the commands with semicolons. They also have to be defined before they are called on the script.

Variables

Global variables are variables that can be accessed anywhere in the script. Local variables are defined inside functions and their scope is only within the function. You can declare them using the word local.

var=glo
func1 () {
    local var=loc
}

func2 () {
    var=loc
}

echo $var   # outputs glo
func1
echo $var   # outputs glo (global var not affected)
func2
echo $var   # outputs loc (gloval var changed)

Saving User Input To a Variable

You can also get the user input and pass that to the function.

func1 () {
    echo $1
}
echo 'Input the word:'
read userInput  # save user input to the variable, userInput
func1 $userInput

If Statements

Here is the basic syntax:

if [ condition ]
then 
    # action
elif [ condition ]; then    # separata by semicolon if on the same line
    # action
else 
    # action
fi

Here are some of the conditions you can use in the if statement.

For examaple:

a=10
b=20
if [ $a -lt $b ] 
then 
   echo '$a is less than $b'    # this is executed
else
    echo '$a is not less than $b'
fi

Note: make sure to include spaces between the brackets

Double square brackets are an “upgraded” version of the single square brackets, which has additional functionality, such as pattern matching.

# checks if string ends with '/' using pattern matching
if [[ $path != */ ]]; then
        path="$path/"
fi

You can also have nested if statements.

Loops

For Loops

The basic syntax is:

for [ loop condition ]
do
    # action
done

Here are a few examples.

for i in {1..5}
do
    echo "output: $i"
done

max=10
for (( i=2; i <= $max; ++i ))
do
    echo $i
done

for f in $(ls):     # prints out result from the ls command
do
    echo "-> $f"
done

Note: The double parenthesis² is for evaluating arithmetic expressions.

If we want to loop through an array of strings:

arr=( a b c )
for i in "${arr[@]}"
do
	echo "$i"
done

Note: Make sure to use double quotes.

While/Until Loops

Here’s the basic syntax:

while [ condition ]
do
    # action
done

Here’s an example:

i=0
while [ $i -lt 10 ]
do
    echo $i
    (( i++ ))
done

The syntax is the same for until loops. They’re similar to the while loops but instead will run until the stated condition becomes true (or keeps executing while the condition is false).

Case Statements

Here is an example.

echo Select from the following options:
echo 1. a
echo 2. b
echo 3. c
read selection

case $selection in
    a) echo 'You selected a';;
    b) echo 'You selected b';;
    c) echo 'You selected c';;
    *) echo 'Wrong input';;     
esac

We define the possible case values using the syntax caseValue). So if the value of selection is a, the statement in a) will be executed. *) is similar to the default case and will execute when no values are matched.

Other Things

If your line gets too long, you can continue on the next line by using a backslash.

arr=( a b c \
d e f g h i )