Shell 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, sh(1) implicitly loops over "$@".

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

While Loops

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

As an example, to infinitely loop and execute a command every 5 seconds, try:

while true; do
  date
  sleep 5
done

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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