bash_loops.md

Bash loops brief

for loops

for loops in Bash are used to iterate over a set of values. Here is the basic syntax of a for loop:

for var in values
do
    # commands
done

var is a variable that will be set to each value in values in turn. values can be a list of words separated by spaces, or a command that generates a list of values. Here is an example that prints out the numbers from 1 to 5:

for i in 1 2 3 4 5
do
    echo $i
done

You can also use the seq command to generate a sequence of numbers:

for i in $(seq 1 5)
do
    echo $i
done

🧐 Test yourself

Use command substitution $() and write a for loop that iterates over the files in your home directory.

while loops

while loops in Bash are used to repeat a block of commands as long as a certain condition is true (ends with exit code 0). Here is the basic syntax of a while loop:

while condition
do
    # commands
done

condition is a command or expression that returns either 0 (true) or non-zero (false). Here is an example that prints out the numbers from 1 to 5 using a while loop:

i=1
while [ $i -le 5 ]
do
    echo $i
    let i=$i+1
done

In this example, i is initialized to 1, and the loop continues as long as i is less than or equal to 5. i is incremented by 1 using the expression let i=$i+1.

Exercises

✏️ Blur an image

Create a script called blur.sh, which can be used to blur images. Use the convert command for the actual image processing. The script should expect as arguments multiple filenames of the images to be blurred. You need to test that the file content is indeed an image (file or stat). The script should generate a new file of the blurred image, and if the new image is successfully generated, replace the original image with the blurred one.

✏️ Bad elusive command

Say you have a command that fails rarely. In order to debug it you need to capture its output, but it can be time consuming to get a failure run. Write a bash script that runs the following script until it fails and captures its standard output and error streams to files and prints everything at the end. Report how many runs it took for the script to fail.

#!/bin/bash

n=$(( RANDOM % 100 ))
if [[ n -eq 42 ]]; then
   echo "Something went wrong"
   exit 1
fi

echo "Everything went according to plan"