flock: add page (#6390)

pages/linux/flock.md

@@ -0,0 +1,17 @@
+# 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}} --command "{{command}}"`
+
+- Run a command with a file lock, and exit if the lock doesn't exist:
+
+`flock {{path/to/lock.lock}} --nonblock --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}} --nonblock --conflict-exit-code {{error_code}} -c "{{command}}"`