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while / until / do / done

Repeats the body while (or until) the condition command exits 0. 'while' loops while the condition succeeds; 'until' is the inverse and is idiomatic for polling until a service becomes ready.

while COMMAND; do ...; done   |   until COMMAND; do ...; done

Common flags / operators

Flag / Operator Purpose
do / done Delimits the loop body
while read Idiomatic line-by-line input consumption
while :; Infinite loop (: is the true builtin)
until Same shape as while but inverts the condition
break / continue Exit or skip to the next iteration

Examples

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

Reads file.txt line by line; -r prevents backslash mangling.

while (( i < 10 )); do echo "$i"; ((i++)); done

Arithmetic-tested loop with in-place increment.

until curl -sf http://localhost:8080/health; do sleep 2; done

Poll a health endpoint every 2s until it returns 2xx.

while IFS=, read -r a b c; do echo "$a $b"; done < data.csv

CSV parsing: IFS=, splits on commas per iteration.

Gotcha

Piping into 'while' (cmd | while read ...) runs the loop in a subshell — variable changes inside are lost. Use process substitution 'while ... done < <(cmd)' to preserve them.

Related

read
Reads a single line from stdin (or a file descriptor) and splits it into the given variables using IFS. The workhorse for line-by-line input processing and interactive prompts.
for ... in / for ((;;))
Iterates a variable over a word list, glob, or command substitution; the C-style form iterates using arithmetic. Executes the body once per iteration and exits with the status of the last command run.
break / continue
Loop-control builtins for for/while/until/select loops. 'break' exits the enclosing loop; 'continue' skips to the next iteration; both accept an integer N to affect N nested loops at once.
if / then / elif / else / fi
Conditional branching based on the exit status of a command (0 = true, non-zero = false). Runs the then-branch when the tested command succeeds; supports optional elif and else clauses and must be terminated with fi.
case ... esac
Pattern-matches a word against glob-style patterns (not regex) and runs the first matching branch. Terminators control fall-through: ;; ends, ;& falls through, ;;& tries next pattern.

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