Lesson 11.4: Loops: for, while and until

Repeating work with for over a list, a brace range and a wildcard; counting errors in every log with a for loop in a script; reading a file line by line with while read; counting down with while and retrying with until; skipping and stopping with continue and break; the C-style for (( )); renaming many files at once with ${f%.log}; and writing your own loop script in nano.

19 min

You will learn to

Loop over words, ranges and files with for
Read a file line by line with while read
Use while and until with a condition
Skip an item with continue and stop with break
Write a C-style for (( ))
Rename many files safely in a loop

Key terms

Loop: A block of commands that runs again and again: once per item (for) or as long as a condition holds (while, until).
Iteration: One pass through the loop body.
for … in … do … done: Runs the body once for each word in a list; the loop variable holds the current word.
while / until: while repeats while a command succeeds; until repeats until it succeeds.
break / continue: break leaves the loop at once; continue jumps to the next iteration.
Infinite loop: A loop whose condition never becomes false. Stop it with Ctrl+C.

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