Update cheatsheets

tldr/linux/flock

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# flock
+
+> Manage locks from shell scripts.
+> It can be used to ensure that only one process of a command is running.
+> More information: <https://manned.org/flock>.
+
+- Run a command with a file lock as soon as the lock is not required by others:
+
+`flock {{path/to/lock.lock}} {{[-c|--command]}} "{{command}}"`
+
+- Run a command with a file lock, and exit if the lock doesn't exist:
+
+`flock {{path/to/lock.lock}} {{[-n|--nonblock]}} {{[-c|--command]}} "{{command}}"`
+
+- Run a command with a file lock, and exit with a specific error code if the lock doesn't exist:
+
+`flock {{path/to/lock.lock}} {{[-n|--nonblock]}} {{[-E|--conflict-exit-code]}} {{error_code}} {{[-c|--command]}} "{{command}}"`