`

# run some commands until the condition is no longer false

done

Listing 2-11

The until loop syntax

Listing 2-12 uses until to run some commands until a file’s size is greater

than zero (meaning it is not empty).

#!/bin/bash

FILE="output.txt"

touch "${FILE}"

until [[ -s "${FILE}" ]]; do

echo "$FILE is empty..."

echo "Checking again in 2 seconds..."

sleep 2

done

echo "${FILE} appears to have some content in it!"

Listing 2-12

An until loop to check a file’s size

We first create an empty file, then begin a loop that runs until the file is no

longer empty. Within the loop, we print some messages to the terminal. Save this

file as until_loop.sh and run it:

$ chmod u+x until_loop.sh

$ ./until_loop.sh

output.txt is empty...

Checking again in 2 seconds...

--snip--

At this point, the script has created the file output.txt, but it’s an empty file.

We can check this using the du command:

$ du -sb output.txt

output.txt

Open another terminal and navigate to the location at which your script is

saved, then append some content to the file so its size is no longer zero:

$ echo "until_loop_will_now_stop!" > output.txt

The script should exit the loop, and you should see it print the following:

output.txt appears to have some content in it!

This script is available at https://github.com/dolevf/Black-Hat-

Bash/blob/master/ch02/until_loop.sh.

for

The for loop iterates over a sequence, such as a list of filenames, variables,

or even group of values generated by running some command. Inside the for

Black Hat Bash (Early Access) © 2023 by Dolev Farhi and Nick Aleks