pidfile 与 logfile的不同

原文答案：http://unix.stackexchange.com/questions/12815/what-are-pid-and-lock-files-for

大意翻译如下：

pidfile 是用户程序正在开始运行期间，将进程ID写进pidfile文件，主要目的有三个：

    1.为其他程序或操作系统标识本程序正在运行，或至少启动成功了。

    2.如果计划要用kill命令终止进行，它允许一个可写脚本非常容易地检测程序运行状态。

    3.有pidfile后，可以用非常少的代价去获取前一个运行实例因何没有退出成功

仅仅存在pidfile并不能保证进程id是否正在运行，这个方法不是100%简单，但对大多数程序来说是足够好的，检测进程id是否在进程表这个方式并不能跨平台使用，除非使用 ps工具。

lockfile 通常用来保证同一程序的两个实例不能同时运行，使用完后别忘记删除这个文件

pid files are written by some programs to record their process ID while they are starting. This has multiple purposes:

• It's a signal to other processes and users of the system that that particular program is running, or at least started successfully.

• It allows one to write a script really easy to check if it's running and issue a plain kill command if one wants to end it.

• It's a cheap way for a program to see if a previous running instance of it did not exit successfully.

Mere presence of a pid file doesn't guarantee that that particular process id is running, of course, so this method isn't 100% foolproof but "good enough" in a lot of instances. Checking if a particular PID exists in the process table isn't totally portable across UNIX-like operating systems unless you want to depend on the ps utility, which may not be desirable to call in all instances (and I believe some UNIX-like operating systems implement ps differently anyway).

Lock files are used by programs to ensure two (well-behaved) separate instances of a program, which may be running concurrently on one system, don't access something else at the same time. The idea is before the program accesses its resource, it checks for presence of a lock file, and if the lock file exists, either error out or wait for it to go away. When it doesn't exist, the program wanting to "acquire" the resource creates the file, and then other instances that might come across later will wait for this process to be done with it. Of course, this assumes the program "acquiring" the lock does in fact release it and doesn't forget to delete the lock file.

This works because the filesystem under all UNIX-like operating systems enforces serialization, which means only one change to the filesystem actually happens at any given time. Sort of like locks with databases and such.