5. Bash scripting¶

A bash script is a file containing commands that can run on the bash shell. They usually have the .sh extension.

A minimal example¶

Create a file named myscript.sh and insert the code from the example below.

#!/usr/bin/env bash
# A simple bash script

echo "Hello World"

Note

#! is called the “shebang”. It indicates the path to the program/interpreter that should be used to execute the script.

Execution¶

The permissions on the script you created should be set so you can execute it with either ./myscript.sh or bash myscript.sh. Sometimes the file might not have appropriate permissions set. For example:

$ ./myscript.sh
bash: ./myscript.sh: Permission denied
$ ls -l myscript.sh
-rw-r--r-- 18 dkishore users 4096 Jun 10 09:12 myscript.sh

To run this file, the permissions on the script need to be changed to allow for execution: chmod +x myscript.sh

Variables¶

#!/bin/bash
cp $1 $2
# Verification
echo Details for $2
ls -lh $2

$1 and $2 are the first and second arguments to the script.

Let’s experiment here. First, create a file named variables_test.sh and copy-paste the code above there. Next, try calling this file with myscript.sh as the first argument, and myscript2.sh as the second argument. What happened?

Note

$0 refers to the name of the bash script

$# refers to the number of arguments passed to the script

$@ refers to all the arguments supplied

$? refers to the exit status of the most recent process

You can also set your own variables inside the script:

#!/bin/bash
myvariable=Hello
anothervar=Fred
echo $myvariable $anothervar
sampledir=/etc
echo $sampledir

Note

Use quotes if your value has a space. For example: myvar="Hello World!"
When referring to or reading a variable we place a $ sign before the variable name
When setting a variable we leave out the $ sign
Do not use white-space around the =

Input¶

Run the command read and save the users response into the variable varname as shown below. Then create a new shell script with the code below and run it. This is how you can have an interactive program.

#!/bin/bash
# Ask the user for their name
echo Hello, who am I talking to?
read varname
echo It\'s nice to meet you $varname

Conditional statements¶

Play with the scripts below. Do you understand what they are doing? If you don’t understand, make sure to test them out and ask us questions!

if statements

#!/bin/bash
# Basic if statement
if [ $1 -gt 100 ]
then
    echo "Hey that\'s a large number."
    pwd
fi

The square brackets in the if statement is a reference to the test command.
-gt is equivalent to >=. Similarly there are =, != and many more.
0 means TRUE and 1 means FALSE (fail).

if-else-if

#!/bin/bash
# elif statements
if [ $1 -ge 18 ]
then
    echo "You may go to the party."
elif [ $2 == 'yes' ]
then
    echo "You may go to the party but be back before midnight."
else
    echo "You may not go to the party."
fi

Loops¶

while loop

#!/bin/bash
# Basic while loop
counter=1
while [ $counter -le 10 ]
do
    echo $counter
    ((counter++))
done
echo "All done"

until loop (opposite of the while loop)

#!/bin/bash
# Basic until loop
counter=1
until [ $counter -gt 10 ]
do
    echo $counter
    ((counter++))
done
echo "All done"

for loops

#!/bin/bash
# Basic for loop
names='Stan Kyle Cartman Kenny'  # is one way to define lists
for name in $names
do
    echo $name
done
echo All done

Ranges and iterators

#!/bin/bash
# Basic range in for loop
for value in {1..5}
do
    echo $value
done
echo All done

Note

You can have custom range by providing a step. For example: {10..0..2}

Functions¶

Minimal example

#!/bin/bash
# Basic function
print_something () {
    echo Hello I am a function
}
print_something

Passing arguments

#!/bin/bash
# Passing arguments to a function
print_something () {
    echo Hello $1
}
print_something Mars
print_something Jupiter

Returning values

Bash functions don’t allow for return values, but they do allow for a return status.

#!/bin/bash
# Setting a return status for a function
print_something () {
    echo Hello $1
    return 5
}
print_something Mars
print_something Jupiter
echo The previous function has a return value of $?

Variable scope

#!/bin/bash
# Experimenting with variable scope
var_change () {
    local var1='local 1'
    echo Inside function: var1 is $var1 : var2 is $var2
    var1='changed again'
    var2='2 changed again'
}
var1='global 1'
var2='global 2'
echo Before function call: var1 is $var1 : var2 is $var2
var_change
echo After function call: var1 is $var1 : var2 is $var2

Results¶

Before function call: var1 is global 1 : var2 is global 2
Inside function: var1 is local 1 : var2 is global 2
After function call: var1 is global 1 : var2 is 2 changed again