Update cheatsheets

tldr/until

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# 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!"}}`