Learning objectives

• Utilize bash to run for loops and if conditions
• Edit text with sed

Introduction

Bash can automate simple tasks in the command line; some of the most commonly used tools here are: for loops, if statements, and text editing with sed commands.

Try and Learn

Iterating a task in bash can be done with a bash for loop. The variable i loops through the specified range, here from 1 to 10. Bash commands between the do and done use a counter variable (in this case, i). For illustration we print here the value of the variable i with the command echo:

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

Simple logic can be implemented in bash with the if statement. If the condition is true, the commands between then and else are executed. If the condition is wrong, the commands between else and fi are executed. In this illustration case the condition checked in the if statement is the existence of a test.txt file.

if [[ -f test.txt ]]; then
echo "file exists"
else
echo "file does not exist"
fi

A commonly encountered text editing task is the replacement of specific works in a text file. This can be one with the bash command sed. The following sed command replaces any occurrence of oldvalue in the file test.txt with the newvalue.

sed -i -e "s/oldvalue/newvalue/g" test.txt