Loops

Loops repeatedly execute a sequence of commands, either by iterating over a list or while a condition holds. They are useful for automating repetitive tasks or processing multiple items.

For loops

A for loop iterates over a list of strings, executing a block of commands for each string. In each iteration, the string is assigned to a variable for use in the commands.

$ for user in alice bob charlie; do
>     echo "Hello, $user!"
> done
Hello, alice!
Hello, bob!
Hello, charlie!

The word after for is the loop variable, assigned to each string in the list after in. The do reserved word starts the command block, and done ends the loop.

The semicolon after the list is optional if do is on a new line:

$ for user in alice bob charlie
> do
>     echo "Hello, $user!"
> done
Hello, alice!
Hello, bob!
Hello, charlie!

The in reserved word can also be on a separate line:

$ for user
> in alice bob charlie; do
>     echo "Hello, $user!"
> done
Hello, alice!
Hello, bob!
Hello, charlie!

Word expansion is performed on the list:

$ for file in *.txt; do
>     echo "$file contains $(wc -l -- "$file") lines"
>     echo "First line: $(head -n 1 -- "$file")"
> done
file1.txt contains 10 lines
First line: This is the first line of file1.
file2.txt contains 5 lines
First line: This is the first line of file2.

If the list is empty, the loop does not run:

$ for user in; do
>     echo "Hello, $user!"
> done

If in and the list are omitted, the loop iterates over the positional parameters as if in "$@" were specified:

$ set alice bob charlie
$ for user do
>     echo "Hello, $user!"
> done
Hello, alice!
Hello, bob!
Hello, charlie!

The exit status of a for loop is the exit status of the last command run in the loop, or 0 if the loop does not run.

While and until loops

A while loop executes commands as long as a condition is true. An until loop is similar, but continues until the condition becomes true. The do reserved word separates the condition from the loop body, and done ends the loop.

$ count=1
$ while [ $count -le 5 ]; do
>     echo "Count: $count"
>     count=$((count + 1))
> done
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

$ count=1
$ until [ $count -gt 3 ]; do
>     echo "Count: $count"
>     count=$((count + 1))
> done
Count: 1
Count: 2
Count: 3

See Exit status and conditionals for details on how exit status affects loop conditions.

The exit status of a while or until loop is that of the last command run in the loop body, or 0 if the loop body does not run. Note that the exit status of the condition does not affect the exit status of the loop.

Break and continue

The break utility exits the current loop. The continue utility skips to the next iteration.

$ for i in 1 2 3 4 5; do
>     if [ $i -eq 3 ]; then
>         echo "Breaking at $i"
>         break
>     fi
>     echo "Iteration $i"
> done
Iteration 1
Iteration 2
Breaking at 3

$ for i in 1 2 3 4 5; do
>     if [ $i -eq 3 ]; then
>         echo "Skipping $i"
>         continue
>     fi
>     echo "Iteration $i"
> done
Iteration 1
Iteration 2
Skipping 3
Iteration 4
Iteration 5

By default, break and continue affect the innermost loop. You can specify a numeric operand to affect the n’th outer loop:

$ for i in 1 2; do
>     for j in a b c; do
>         if [ "$j" = "b" ]; then
>             echo "Breaking outer loop at $i, $j"
>             break 2
>         fi
>         echo "Inner loop: $i, $j"
>     done
> done
Inner loop: 1, a
Breaking outer loop at 1, b