What does `init [2]` mean in the COMMAND column of ps?

n611x007 just a moment. 1 answers, 0 views
executable command ps init sysvinit

Reading What do the brackets around processes mean? I understand that the executable name is printed.

Linux ps man page:

Sometimes the process args will be unavailable; when this happens, ps will instead print the executable name in brackets.

However with ps -Awwo pid,comm,args I get:

    1 init            init [2]

What does this mean? Does the "executable name" supposed to be init or [2]?

I suppose the executable is of course init - what is [2]? Why is it printed?

(Also, I don't really get why it can't show the full path if it knows the executable name.)

1 Answers

Gilles 08/06/2014.

Both the comm column and the first word of the args column in the ps output show the name of the executable program if everybody involved follows the default convention. However it is possible to have discrepancies for various reasons.

When a program starts, the command name as shown in the args column is chosen by the parent program that executes the program and passed as an argument (argv[0]). By convention, the parent chooses the base name of the executable (i.e. the path to the executable without the directory part), but this is not enforced. Once the program is running, it can overwrite that string.

Init (at least the traditional Linux SysVinit) overwrites its argv[0] to indicate the current runlevel.

On Linux, the comm column is initially filled in by the kernel to the first 16 characters of the base name of the executable. The process can change the content with the prctl system call.

If the executable is renamed or deleted, neither the comm column nor the args column will reflect this.

ps doesn't display the path to the executable, that's not in its job description. lsof can tell you with lsof -a -p 1 -d txt.

On Linux, you can see this information in files in /proc/PID/:

  • The process name (comm field) in in /proc/1/stat (second field in parentheses) and /proc/1/status (Name field).
  • The path to the executable via /proc/1/exe.
  • The arguments (starting with argv[0]) in /proc/1/cmdline (the arguments are separated by null bytes).

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