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Shell script repeated execution of how to solve, I believe that many inexperienced people are helpless about this, this article summarizes the causes of the problem and solutions, through this article I hope you can solve this problem.

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flock is a file lock command that ensures secure access to critical resources between processes on Linux systems. In shell scripts, it can be used to control logical mutual exclusion.

Example 1

The existing script a.sh reads as follows

#!/ bin/bash echo "[`date +'%Y-%m-%d %H:%M:%S'`] begin pid:$$... " sleep 10 echo "[`date +'%Y-%m-%d %H:%M:%S'`] end pid:$$... "

Flock-xn./is executed in the terminal (denoted terminal 1) f.lock -c ./ a.sh command with the following results

[tt@ecs-centos-7 lock_test]$ flock -xn ./ f.lock -c ./ a.sh [2020-12-10 10:10:45] begin pid:5359... [2020-12-10 10:10:55] end pid:5359...

During the execution of the above command, another terminal (denoted terminal 2) is opened and the same command is executed, with the following results

[tt@ecs-centos-7 lock_test]$ flock -xn ./ f.lock -c ./ a.sh [tt@ecs-centos-7 lock_test]$

Command above flock -xn ./ f.lock -c ./ a.sh www.example.com

The-x option is an exclusive lock, sometimes called a write lock, which is the default option

The-n option is non-blocking, returning failure immediately if the lock cannot be acquired, rather than waiting for the lock to be released

The-c option is followed by the command to be executed

Perform flock -xn ./in Terminal 1 f.lock -c ./ a.sh command, lock the f.lock file, and execute./ a.sh command, the execution process will last about 10 seconds ( sleep 10 statement)

since flock -xn ./ f.lock -c ./ The a.sh command is executed during the execution of the terminal 1 command. At this time, terminal 1 has not released the f.lock file lock, and the-n option is non-blocking, so terminal 2 does not block waiting for the f.lock file lock, but immediately returns.

Terminal 2 if executed flock -x ./ f.lock -c ./ a.sh command will block and wait until terminal 1 releases the f.lock file lock, it will acquire the f.lock file lock and start executing./ a.sh Command

Example 2

In Example 1, you need to execute flock -xn file lock-c ./every time a.sh command, and each script that cannot be executed repeatedly must be assigned a file lock, and different scripts must use different file locks.

Is there any way to do it as long as it is executed./ The command a.sh does the same thing in Example 1.

Answer: Yes.

We slightly modified a.sh, and the modified content is as follows

1 #!/ bin/bash 2 3 4 echo "[`date +'%Y-%m-%d %H:%M:%S'`] 1111 pid:$$... MY_LOCK:${MY_LOCK}" 5 6 [ "${MY_LOCK}" != "$0" ] && exec env MY_LOCK="$0" flock -xn "$0" "$0" "$@" 7 8 echo "[`date +'%Y-%m-%d %H:%M:%S'`] begin pid:$$... MY_LOCK:${MY_LOCK}" 9 10 sleep 10 11 12 echo "[`date +'%Y-%m-%d %H:%M:%S'`] end pid:$$... "

Terminal 1 executes./ a.sh command, output as follows

[tt@ecs-centos-7 lock_test]$ ./ a.sh [2020-12-10 14:11:35] 1111 pid:5944... MY_LOCK: [2020-12-10 14:11:35] 1111 pid:5946... MY_LOCK:./ a.sh [2020-12-10 14:11:35] begin pid:5946... MY_LOCK:./ a.sh [2020-12-10 14:11:45] end pid:5946...

During the execution of the terminal 1 command, the terminal 2 executes./ a.sh command, output as follows

[tt@ecs-centos-7 lock_test]$ ./ a.sh [2020-12-10 14:11:44] 1111 pid:5976... MY_LOCK: [2020-12-10 14:11:44]

The new a.sh script adds lines 4 and 6 compared to the original

Line 4 is log printing

Line 6 Description

$0 is the script name, and the value here is./ a.sh

$@ is all parameters passed into the a.sh script

exec will execute the command immediately following it in the current process, and the command that the current script process has not yet executed will not be executed.

[ "${MY_LOCK}" != "$0" ] determines if the MY_LOCK environment variable is the same as the script name (a.sh)

If not, execute env MY_LOCK="$0" and flock -xn "$0" "$0" "$@"

env MY_LOCK="$0" Sets the environment variable MY_LOCK to the script name

flock -xn "$0" "$0" "$@" is flock -xn ./ a.sh ./ a.sh, which uses the current script name as the file lock

In Example 2, execute./ After the a.sh command, when running to line 6, the MY_LOCK variable is null, so [ "${MY_LOCK}" != "$0" ] results in true

The exec command ignores unexecuted commands, meaning that commands after line 6 in the current shell process are not executed.

Next, exec env MY_LOCK="$0" flock -xn "$0" "$0" "$@" command sets MY_LOCK variable to the current script name./ a.sh, while executing the flock -xn "$0" "$0" "$@" command, which is executed in a new subshell./ a.sh, so the process ID printed in the subsequent output of the script is different from the one at the beginning

Also, since env MY_LOCK="$0" was executed before flock -xn "$0" "$0" "$@," the MY_LOCK variable is set to./ a.sh, so flock -xn "$0" "$0" "$@" command re-executed./ a.sh command,[ "${MY_LOCK}" != on line 6 of the script The result of "$0" ] is false, the command after exec on line 6 will not be executed, and the script will continue to execute from line 7 to the end. The log output on lines 8 and 12 also indicates that the script has finished executing.

After reading the above, do you know how to solve the problem repeatedly in Shell scripts? If you still want to learn more skills or want to know more related content, welcome to pay attention to the industry information channel, thank you for reading!

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