Bash Looping

For Loops

The for loop iterates over a list. The list can be anything from a literal list of items (1 2 3) to a process expansion ($(seq 1 3)) to a filename expansion (*). See here for more details.

for filename in *; do
  chmod 755 "$filename"
done

If a list is not specified, bash(1) implicitly loops over "$@".

for arg; do
  if [[ "$arg" = "-h" ]]; then
    echo "$help_message"
  fi
done

C-Style

For more complex looping structures, consider using a C-style for loop.

As an example, to check if a word is a palindrome, try:

word="$1"
len=${#word}
for (( i=0,j=len-1; i<(len/2); i++,j-- )); do
  [[ "${word:i:1}" != "${word:j:1}" ]] && exit 1    
done                                            
exit 0

This example makes use of substring expansion; see here for details.

The key to this grammar is the semicolons (;). There must be three semicolon delimited expressions, all of which are parsed as arithmetic. The first is evaluated once. The second is evaluated repeatedly until it evaluates to 0. For each evaluation of the second, all of the commands of the for loop are executed and the third expression is evaluated.

While Loops

The while loop iterates as long as the condition evaluates to true.

As an example, to replace spaces with underscores (_), try:

word="$1"
i=0
while [[ "$i" < "${#word}" ]]; do
  [[ "${word:i:1}" = " " ]] && word="${word::i}_${word:i+1}"
  (( i++ ))
done
echo $word

This example makes use of substring expansion and arithmetic; see those articles for more details.

Until Loops

until loops are the inverse of a while loops.

As an example, to repeatedly prompt a user until a valid response is detected, try:

echo "Create file 'config.toml'? [y/N]: "
until [[ -e "config.toml" ]]; do
  read RESPONSE
  case "$RESPONSE" in
    [Yy]) touch "config.toml";;
    [Nn]) exit 1;;
    *) echo "Please enter Y or N";;
  esac
done

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