Scripting with BASH

Most Linux distributions come with the bash shell. It is a very simple shell, but it is also a very powerful one. It is an updated version of the Bourne shell, which is the most common shell on UNIX systems. As a result, it is the most commonly used shell on UNIX systems and knowning how to script in the bash shell is a prerequisite to use Linux as a power user on any system.

Variables

Variables in shells are strings that are assigned to a name. The name is followed by an equals sign, and then the value of the variable. The value can be a string, a number, or a special value.

Assigning a value to a variable

#!/bin/bash

i=0
echo $i

Command substitution

Assinging a value to a variable can also be done by command substitution where the output of a command is assigned to a variable.

Command substitution

#!/bin/bash

i=`date +%s`
echo $i

Exit-codes

`$?`

Conditional structures

If-then-else

Conditional structures are used to evaluate a condition and execute a command if the condition is true. Based on this a command is executed if the condition is true or another if the condition is false.

If-then-else

#!/bin/bash

i=1
if [ $i -eq 1 ]
then
    echo "i equals 1"
else
    echo "i does not equal 1"
fi

The if-then-else construct can also be extended to include an else-if statement.

If-then-else

#!/bin/bash

i=1
j=2
if [ $i -eq 1 ]
then
    echo "i equals 1"
elif [ $j -eq 2 ]
then
    echo "j equals 2"
else
    echo "i and j do not equal 1 or 2"
fi

Short-circuit evaluation

The shell also allows to write short form conditional statements. This is can be used by using /usr/bin/[ or [ as the first character of the condition. It is an alternative to the test command and with the && and || operators the action can be defined.

If-then-else

#!/bin/bash

i=1
[ $i -eq 1 ] && echo "i equals 1"

Case statement

Beside the if-then-else construct, the shell also allows to define a case statement. This is used to evaluate a value and execute a command based on the value. This makes the script more flexible and can be used to evaluate multiple values.

Case statement

#!/bin/bash

i=1

case $i in
    1) echo "One";;
    2) echo "Two";;
    3) echo "Three";;
    *) echo "Unknown";;
esac

Loops

For loops

Iterating over a list of values is done by using a for loop. The for loop is used to iterate over a list of values. The for loop is defined by the keyword for, followed by the name of the variable, followed by the keyword in, followed by the list of values. The list of values is separated by a space.

For loop

#!/bin/bash

for i in {1..10}
do
    echo $i
done

While loops

Counter based loops are used to iterate over a list of values. The while loop is defined by the keyword while, followed by the condition, followed by the keyword do, followed by the commands, followed by the keyword done.

While loop

#!/bin/bash

while :
do
    echo "Hello World!"
    sleep 1
done

While loop

#!/bin/bash

while :; do
    echo "Hello World!"
    sleep 1
done