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lazarus
Asked: 2014-09-27 10:42:07 +0800 CST

how to see the PID for currently running command?

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while reading about linux I got :

By default, variables created within a script are only available to the current shell; child processes (sub-shells) will not have access to values that have been set or modified. Allowing child processes to see the values, requires use of the export command.

I tried to find about subshells then I came across Are there any command to see how processes are working?, and I found ltrace could be used for that, when I searched for how to use ltrace or strace, I found that PID is required attribute for that.

now, If I want to know the PID for $ cp file1 file2 ,how would I do that?? so that I can use ltrace to it??

command-line

2 Answers

  1. Best Answer

    PID of current command

    The PID of a command in your own shell is shown when you start the command in background. If the current command runs in the foreground, the shell mostly waits for it to finish, we have to wait too - except we want to use a tool outside this shell; See below for that option.

    As an example command, I'll use this ping that sends a request every 5 seconds for 10 times:

    ping -i 5 -c 10 example.com

    Now, I'll use & to run it in the background:

    $ ping -i 5 -c 10 example.com &
[1] 12238
$ PING example.com (93.184.216.119) 56(84) bytes of data.
64 bytes from 93.184.216.119: icmp_seq=1 ttl=55 time=118 ms
64 bytes from 93.184.216.119: icmp_seq=2 ttl=55 time=117 ms
64 bytes from 93.184.216.119: icmp_seq=3 ttl=55 time=117 ms

    The [1] 12238 tells us that the process has the process id, short PID 12238; And that it's the first background job currently running in this shell.

    Example: tracing library calls

    In the same terminal we get the output of ping, so we better run ltrace in another one (ping has special root permissions, so we need sudo to mess with its inside):

    $ sudo ltrace -p 12238      
[sudo] password for siegel: 
gettimeofday(0x7fff7f0b6900, 0)                                           = 0
gettimeofday(0x7fff7f0b68e0, 0)                                           = 0
sendmsg(3, 0x60a300, 2048, 24)                                            = 64
recvmsg(3, 0x7fff7f0b6970, 0, 0)                                          = 84
__printf_chk(1, 0x40813b, 64, 0x60a560)                                   = 29
__printf_chk(1, 0x4077ea, 4, 28)                                          = 11
__printf_chk(1, 0x40814d, 55, 11)                                         = 7
__printf_chk(1, 0x408162, 118, 7)                                         = 12
_IO_putc('\n', 0x7f7836d89400)                                            = 10
fflush(0x7f7836d89400)                                                    = 0
gettimeofday(0x7fff7f0b6900, 0)                                           = 0
poll(0x7fff7f0b6950, 1, 4880, 0)                                          = 0
gettimeofday(0x7fff7f0b6900, 0)                                           = 0
gettimeofday(0x7fff7f0b68e0, 0)                                           = 0
sendmsg(3, 0x60a300, 2048, 24)                                            = 64
recvmsg(3, 0x7fff7f0b6970, 0, 0)                                          = 84
__printf_chk(1, 0x40813b, 64, 0x60a560)                                   = 29
__printf_chk(1, 0x4077ea, 5, 28)                                          = 11
__printf_chk(1, 0x40814d, 55, 11)                                         = 7
__printf_chk(1, 0x408162, 118, 7)                                         = 12
_IO_putc('\n', 0x7f7836d89400)                                            = 10
fflush(0x7f7836d89400)                                                    = 0
gettimeofday(0x7fff7f0b6900, 0)                                           = 0
poll(0x7fff7f0b6950, 1, 4881, 0)

    That's a trace of the library calls during two 'pings'.


    Finding PIDs of running processes in general

    As the title question is not very specific, here are some other ways to find PIDs of currently running commands:

    • Search for processes by command name
      pgrep -x cmd Note that the variant pgrep cmd is wrong: it mathes as substring

    • Search by command line with arguments
      pgrep -f cmdarg

    • List processes that belong to the current shell (session), like background processes, or the shell itself
      ps -s $$

    • List all processes: "process status"
      ps aux

    • Listing processes in a more interactive way: "table of processes"
      top

    • And all you need at once: a table of processes, integrated with ltrace and strace:
      htop
      It's a variant of top with enhanced UI and configurability, plus some extras:
      Choose a process with the cursor line, and press L for ltrace, or s for strace!

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  2. bash, dash (and many other shells) have built-in commands for job control. If you run a command in the background

    sleep 9999 &

    you can discover all the background commands of the current shell:

    $ jobs
[1]+  Running                 sleep 9999 &

    If you need the process ID of one of them, you can get it with jobs -p %N, where N is the job ID in the job list (first column) above. There's also a special variable $! that the shell will substitute with the PID of the command executed in the background most recently.

    For details and more options, have a look at the relevant section of the manuals of bash(1) (direct link to job control section in HTML version), dash(1) or your shell of choice. TLDP has a great section on job control as well.

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