Re: why does ps |grep sometimes not return itself?
On Thursday 10 June 2010 03:30:14 Pieter de Goeje wrote: On Wednesday 09 June 2010 09:34:40 Matthew Seaman wrote: On 09/06/2010 08:15:23, Eitan Adler wrote: Why do I sometimes see the grep in ps's output and sometimes not see it? When you run that pipeline the OS doesn't start both programs exactly simultaneously. [...] It's a race condition. I would like to add that you can avoid the issue entirely by using this command: % ps aux -p `pgrep sh` [output snipped due to bad wrapping] Or the old trick: ps | grep '[s]h' Jonathan ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
why does ps |grep sometimes not return itself?
Why do I sometimes see the grep in ps's output and sometimes not see it? [ei...@alphabeta ~ ]% ps aux|grep Me eitan 96325 0.0 0.0 1856 724 5 RL+ 10:14AM 0:00.00 grep Me [ei...@alphabeta ~ ]% ps aux|grep Me [ei...@alphabeta ~ !1! ]% -- Eitan Adler ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: why does ps |grep sometimes not return itself?
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 09/06/2010 08:15:23, Eitan Adler wrote: Why do I sometimes see the grep in ps's output and sometimes not see it? [ei...@alphabeta ~ ]% ps aux|grep Me eitan 96325 0.0 0.0 1856 724 5 RL+ 10:14AM 0:00.00 grep Me [ei...@alphabeta ~ ]% ps aux|grep Me [ei...@alphabeta ~ !1! ]% When you run that pipeline the OS doesn't start both programs exactly simultaneously. It starts ps(1) first, then grep(1) together with creating the pipeline by connecting ps's stdout to grep's stdin. Depending on system load and various other factors, this may allow ps(1) to grab the snapshot of the process table that it works on before grep(1) has started. It's a race condition. Whether you see this effect or not will depend very much on system conformation and load. I can't reproduce the effect on a lightly loaded dual processor machine, which always shows the grep process, whereas on a single processor virtual machine running under VirtualBox, I never see grep in the ps output unless I renice the ps(1) process. Cheers Matthew - -- Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard Flat 3 PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate JID: matt...@infracaninophile.co.uk Kent, CT11 9PW -BEGIN PGP SIGNATURE- Version: GnuPG/MacGPG2 v2.0.14 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkwPRBAACgkQ8Mjk52CukIxkWgCeNSQX37XoGvaZn2A/0vYrmUka 1yMAnjtZJoYNqOdo9UDkWks/4cYpVFPy =Rkg3 -END PGP SIGNATURE- ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: why does ps |grep sometimes not return itself?
On Wednesday 09 June 2010 09:34:40 Matthew Seaman wrote: On 09/06/2010 08:15:23, Eitan Adler wrote: Why do I sometimes see the grep in ps's output and sometimes not see it? [ei...@alphabeta ~ ]% ps aux|grep Me eitan 96325 0.0 0.0 1856 724 5 RL+ 10:14AM 0:00.00 grep Me [ei...@alphabeta ~ ]% ps aux|grep Me [ei...@alphabeta ~ !1! ]% When you run that pipeline the OS doesn't start both programs exactly simultaneously. It starts ps(1) first, then grep(1) together with creating the pipeline by connecting ps's stdout to grep's stdin. Depending on system load and various other factors, this may allow ps(1) to grab the snapshot of the process table that it works on before grep(1) has started. It's a race condition. Whether you see this effect or not will depend very much on system conformation and load. I can't reproduce the effect on a lightly loaded dual processor machine, which always shows the grep process, whereas on a single processor virtual machine running under VirtualBox, I never see grep in the ps output unless I renice the ps(1) process. I would like to add that you can avoid the issue entirely by using this command: % ps aux -p `pgrep sh` USERPID %CPU %MEM VSZ RSS TT STAT STARTED TIME COMMAND root 1326 0.0 0.1 6680 3664 ?? Is1:09AM 0:00.00 /usr/sbin/sshd pyotr 1460 0.0 0.1 3972 2696 v0 I 1:09AM 0:00.02 -zsh (zsh) -- Pieter de Goeje ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
