home user wrote: > After coming to think that login and interactive modes are not the answer, I > started digging into ps, and piping its output into grep. Unfortunately, > that generates another process "grep ksysguard ...". Piping into "grep -v > ..." also doesn't help. After groping all over grep and bash today and > trying lots of things, I got lucky. In the GNU Grep 3.3 web page > "http://www.gnu.org/software/grep/manual/grep.html";, > I found this: > ----- > 7. Why do people use strange regular expressions on |ps| output? > > ps -ef | grep '[c]ron' You could also use 'pgrep ksysguard' instead of piping ps to grep. That avoids both a pipe and the need to hide the grep command from the grep. > The main problem is solved. But this leaves me with two follow-up > questions: > 1. How does the [first_letter] prevent matching the ps output line for > grep? (I'm almost certain I'm not the only member of this list not knowing > the answer to this!) It works, and I'm using it, but it would be good to > really understand how it works (beyond what the GNU grep page says). I saw > no hint of an answer in the regular expression write-up. When you run ps -ef | grep ksysguard, the ps list includes entries for both ps -ef and grep ksysguard. Adding the bracets around a character in the grep command means that the grep pattern is looking for that one character followed by the others, in the case of 'grep [k]sysguard' the regular expression is k followed by sysguard. In the ps -ef output, the string [k]sysguard doesn't match because there are literal brackets there. In the grep regex, those brackets have special meaning. So this is a good method for filtering such matches in the output of ps. But pgrep is generally easier and purpose-built for that particular problem. > 2. Is there a way to have a *single* grep match one string and not another. > Example: find all lines containing "ksysguard" but not "grep". Note that > grep ksysguard | grep -v grep > is *not* what I'm looking for. Yes, by ensuring the regex is precise and doesn't match both lines, which is just what using [k]sysguard does. You might be able to do this with something fancy like a negative lookbehind, using 'grep -P' but I'm not sure. Or you could use a regex which took advantage of the ps -ef output format, like ps -ef | grep '[0-9] ksysguard'. That won't match the grep for ksysguard because it's only looking for a number followed by a space, followed by ksysguard. Both are far less elegant/readable than grep [k]sysguard. :) Which is itself less elegant than using pgrep. And in this case, I think, pgrep is far less elegant than using the desktop's autostart functions, as Ed showed for both Gnome and KDE. -- Todd
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