• Posts
  • RSS
  • ◂◂RSS
  • Contact

  • How do you get the PID in bash?

    January 27th, 2014
    bash, tech  [html]
    If you ask how to get the PID in bash someone will tell you:
    The variable $$ contains the PID.
    This is usually correct, but not always. Sometimes it can be incorrect, but in a way that's actually closer to what you wanted.

    The basic problem is that many bash constructs run in a subshell. This means the shell will fork itself, running your function in a separate process. This allows simple concurrency, but it also means a new process id. For example:

        $ function a { echo $$; }
        $ function b { echo $BASHPID; }
    
    Function a uses $$ while b uses $BASHPID. When run as normal functions they both give the PID of bash running them:
        $ a
        6042
        $ b
        6042
    
    When run in the background, however, they diverge:
        $ a &
        6042
        $ b &
        14774
    
    To run a function in the background bash uses a multiprocess model, forking a subshell for each one, so we should expect both a & and b & to give new PIDs. In order to make this process faster, though, and because POSIX says so, bash doesn't reinitialize itself when creating a subshell. This means $$, which was computed once when bash started, continues to have its original value. $BASHPID always looks up the current PID of the executing process, however, so it does change when you make a subshell.

    While this definition of $$ is frustrating if you actually want to get the current process id, for example to let each background function have free reign of /tmp/PID, in many cases this definition is actually helpful. There are many simple constructs in bash that require subshells, and it's convenient that $$ stays consistent. For example:

        $ ls $SOME_PLACE | while read fname ; do
            some_command $fname >> /tmp/$$.output
          done
        $ other_command /tmp/$$.output
    
    Because pipes run in a subshell if $$ were defined like $BASHPID all of these some_command outputs would build up in some random file in /tmp and other_command would look in a different file. Instead $$ happens to mean the right thing, and everything works.

    Comment via: google plus, facebook

    Recent posts on blogs I like:

    Streaming the Biden Infrastructure Plan

    I streamed my thoughts about the Biden infrastructure plan, and unlike previous streams, I uploaded this to YouTube. I go into more details (and more tangents) on video, but, some key points: Out of the nearly $600 billion in the current proposal that is …

    via Pedestrian Observations April 11, 2021

    Collections: Clothing, How Did They Make it? Part IVb: Cloth Money

    This is the second half of the fourth part of our four part (I, II, III, IVa) look at the production of textiles, particularly wool and linen, in the pre-modern world. Last time, we looked at commercial textile workers and the finishing processes for text…

    via A Collection of Unmitigated Pedantry April 9, 2021

    Notes from “Don’t Shoot the Dog”

    I just finished Karen Pryor’s “Don’t Shoot the Dog: the New Art of Teaching and Training.” Partly because a friend points out that it’s not on Audible and therefore she can’t possibly read it, here are the notes I took and some thoughts. It’s a quick, eas…

    via The whole sky April 2, 2021

    more     (via openring)


  • Posts
  • RSS
  • ◂◂RSS
  • Contact