Master the Bash Shell/

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Loops

Loops

In this lesson, you will cover for loops, case statements, and while loops in bash. This lesson will quickly take you through the various forms of looping that you might come across, including 'while' loops, 'case' statements, 'for' loops, and 'C-style' loops.

We'll cover the following...

How Important is this Lesson?

For and while loops are a basic construct in most programming languages, and bash is no exception.

for Loops

First you’re going to run a for loop in a ‘traditional’ way:

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    for (( i=0; i < 20; i++ ))
    do
      echo $i
      echo $i > file${i}.txt
    done
    ls
Terminal 1
Terminal
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  • You just created twenty files, each with a number in them using a for loop in the ‘C’ language style (line 1)

  • Note there’s no $ sign involved in the variable when it’s in the double parentheses!

  • Line 3 echoes the value of i in each iteration

  • line 4 redirects that value to a file called file{$i}.txt, where ${i} is replaced with its value in that iteration

  • Line 6 shows the listing of files created

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for f in $(ls *txt)
do
    echo "File $f contains: $(cat $f)"
done

It’s our old friend the command substitution! The command substitution lists all the files we have.

This for loop uses the in keyword to separate the variable each iteration will assign to f and the list to take items from. Here bash ...