How to acquire a lock file in Linux Bash

Question

I have an existing lock file sometimes used by other processes. I want to temporarily acquire this lock file so other programs that potentially use it have to wait for me to unlock. Then I want to run a few commands, and then unlock it. How do I do this? I thought this would be easy but for some reason I cannot figure it out at all. I understand that I would most likely need to use flock for this, but what arguments should I be using in this scenario? flock seems to always need a command or second file to work, however in this situation there doesn't seem to be one.

Context: A bash script I am using is running into race conditions around a particular lock file (/var/lib/apt/lists/lock to be specific), and to test my solution I want to reliably be able to lock this file so I can check if my changes to the script work.

[This](https://en.wikipedia.org/wiki/File_locking) is an overview on the topic. Look especially at the sectons labelled _Problems_ and _Lock files_ — user1934428, Feb 26 '21 at 07:59

score 3 · Answer 1 · answered Feb 26 '21 at 08:19

3

You have an example in the flock (1) man page. You should execute the commands in a subshell:

(
    flock -n 9 || exit 1
    ...
) 9>/var/lib/apt/lists/lock

Using this form the lock is released when the subshell exits.

answered Feb 26 '21 at 08:19

Piotr P. Karwasz

12,857
3
20
43

Command doesn't work. `/var/lib/apt/lists/lock: Permission denied` – Dan Feb 26 '21 at 15:17
You need write permission on the file, which for `/var/lib/apt/lists/lock` means that you must be `root`. – Piotr P. Karwasz Feb 26 '21 at 15:20

How to acquire a lock file in Linux Bash

1 Answers1