= Shell Looping =

<<TableOfContents>>

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== 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 [[Shell/Expansion|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
}}}

To immediately skip to the next iteration, use the `continue` [[Shell/BuiltinCommands#Continue|builtin]]. To immediately exit the loop, use the `break` [[Shell/BuiltinCommands#Break|builtin]].

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== 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
}}}

To immediately skip to the next iteration, use the `continue` [[Shell/BuiltinCommands#Continue|builtin]]. To immediately exit the loop, use the `break` [[Shell/BuiltinCommands#Break|builtin]].

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== 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
}}}

To immediately skip to the next iteration, use the `continue` [[Shell/BuiltinCommands#Continue|builtin]]. To immediately exit the loop, use the `break` [[Shell/BuiltinCommands#Break|builtin]].



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