while

Shell loop with conditional execution

TLDR

Basic while loop

$ while [condition]; do [command]; done

Infinite loop

$ while true; do [command]; sleep [1]; done

Read file line by line

$ while read -r line; do echo "$line"; done < [file.txt]

Loop until command fails

$ while [command]; do echo "still running"; done

Counter loop

$ i=0; while [ $i -lt 10 ]; do echo $i; i=$((i+1)); done

Process command output

$ [command] | while read -r line; do echo "$line"; done

Loop with break

$ while true; do if [condition]; then break; fi; done

SYNOPSIS

while CONDITION; do COMMANDS; done

while is a shell control structure that repeatedly executes a block of commands as long as the condition command returns a zero (success) exit status. The loop terminates when the condition returns non-zero.The condition is typically a test command (or its [ equivalent), but any command can be used. The loop executes as long as the command succeeds.Common patterns include reading files line by line with read, implementing retry logic, and creating daemon-like processes that run indefinitely.

CONTROL STATEMENTS

break

Exit the loop immediately.

break N

Exit N levels of nested loops.

continue

Skip remaining commands and start next iteration.

continue N

Continue at the Nth enclosing loop.

CAVEATS

Using a pipe to while creates a subshell, so variable changes inside the loop are not visible outside. Use process substitution or here-strings to avoid this: while read line; do ...; done < <(command). Always use read -r to prevent backslash interpretation.

HISTORY

The while loop has been a fundamental shell control structure since the original Bourne shell in Unix Version 7 (1979). The syntax is specified by POSIX and works identically across all POSIX-compliant shells including bash, dash, ksh, and zsh.