[linux] How to get the start time of a long-running Linux process?

Is it possible to get the start time of an old running process? It seems that ps will report the date (not the time) if it wasn't started today, and only the year if it wasn't started this year. Is the precision lost forever for old processes?

As a follow-up to Adam Matan's answer, the /proc/<pid> directory's time stamp as such is not necessarily directly useful, but you can use

awk -v RS=')' 'END{print $20}' /proc/12345/stat

to get the start time in clock ticks since system boot.¹

This is a slightly tricky unit to use; see also convert jiffies to seconds for details.

awk -v ticks="$(getconf CLK_TCK)" 'NR==1 { now=$1; next }
    END { printf "%9.0f\n", now - ($20/ticks) }' /proc/uptime RS=')' /proc/12345/stat

This should give you seconds, which you can pass to strftime() to get a (human-readable, or otherwise) timestamp.

awk -v ticks="$(getconf CLK_TCK)" 'NR==1 { now=$1; next }
    END { print strftime("%c", systime() - (now-($20/ticks))) }' /proc/uptime RS=')' /proc/12345/stat

Updated with some fixes from Stephane Chazelas in the comments; thanks as always!

If you only have Mawk, maybe try

awk -v ticks="$(getconf CLK_TCK)" -v epoch="$(date +%s)" '
  NR==1 { now=$1; next }
  END { printf "%9.0f\n", epoch - (now-($20/ticks)) }' /proc/uptime RS=')' /proc/12345/stat |
xargs -i date -d @{}

¹ man proc; search for starttime.

 ps -eo pid,etime,cmd|sort -n -k2

ls -ltrh /proc | grep YOUR-PID-HERE

For example, my Google Chrome's PID is 11583:

ls -l /proc | grep 11583
dr-xr-xr-x  7 adam       adam                     0 2011-04-20 16:34 11583

    ps -eo pid,cmd,lstart | grep YOUR-PID-HERE

The ps command (at least the procps version used by many Linux distributions) has a number of format fields that relate to the process start time, including lstart which always gives the full date and time the process started:

# ps -p 1 -wo pid,lstart,cmd
  PID                  STARTED CMD
    1 Mon Dec 23 00:31:43 2013 /sbin/init

# ps -p 1 -p $$ -wo user,pid,%cpu,%mem,vsz,rss,tty,stat,lstart,cmd
USER       PID %CPU %MEM    VSZ   RSS TT       STAT                  STARTED CMD
root         1  0.0  0.1   2800  1152 ?        Ss   Mon Dec 23 00:31:44 2013 /sbin/init
root      5151  0.3  0.1   4732  1980 pts/2    S    Sat Mar  8 16:50:47 2014 bash

For a discussion of how the information is published in the /proc filesystem, see https://unix.stackexchange.com/questions/7870/how-to-check-how-long-a-process-has-been-running

(In my experience under Linux, the time stamp on the /proc/ directories seem to be related to a moment when the virtual directory was recently accessed rather than the start time of the processes:

# date; ls -ld /proc/1 /proc/$$ 
Sat Mar  8 17:14:21 EST 2014
dr-xr-xr-x 7 root root 0 2014-03-08 16:50 /proc/1
dr-xr-xr-x 7 root root 0 2014-03-08 16:51 /proc/5151

Note that in this case I ran a "ps -p 1" command at about 16:50, then spawned a new bash shell, then ran the "ps -p 1 -p $$" command within that shell shortly afterward....)