until: add page (#15233)

* Create until.md

* Update until.md

* Update until.md

* Update pages/common/until.md

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/until.md

@@ -0,0 +1,12 @@
+# until
+
+> Simple shell loop that repeats until it receives zero as return value.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#index-until>.
+
+- Execute a command until it succeeds:
+
+`until {{command}}; do :; done`
+
+- Wait for a systemd service to be active:
+
+`until systemctl is-active --quiet {{nginx}}; do {{echo "Waiting..."}}; sleep 1; done; {{echo "Launched!"}}`