while

Simple shell loop. More information: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_09.

• Read stdin and perform an action on every line:
  while read line; do echo "$line"; done
• Execute a command forever once every second:
  while :; do {{command}}; sleep 1; done