So pulling open a file with cat and then using grep to get matching lines only gets me so far when I am working with the particular log set that I am dealing with. It need a way to match lines to a pattern, but only to return the portion of the line after the match. The portion before and after the match will consistently vary. I have played with using sed or awk, but have not been able to figure out how to filter the line to either delete the part before the match, or just return the part after the match, either will work. This is an example of a line that I need to filter:

2011-11-07T05:37:43-08:00 <0.4> isi-udb5-ash4-1(id1) /boot/kernel.amd64/kernel: [gmp_info.c:1758](pid 40370="kt: gmp-drive-updat")(tid=100872) new group: <15,1773>: { 1:0-25,27-34,37-38, 2:0-33,35-36, 3:0-35, 4:0-9,11-14,16-32,34-38, 5:0-35, 6:0-15,17-36, 7:0-16,18-36, 8:0-14,16-32,34-36, 9:0-10,12-36, 10-11:0-35, 12:0-5,7-30,32-35, 13-19:0-35, 20:0,2-35, down: 8:15, soft_failed: 1:27, 8:15, stalled: 12:6,31, 20:1 }

The portion I need is everything after "stalled".

The background behind this is that I can find out how often something stalls:

cat messages | grep stalled | wc -l

What I need to do is find out how many times a certain node has stalled (indicated by the portion before each colon after "stalled". If I just grep for that (ie 20:) it may return lines that have soft fails, but no stalls, which doesn't help me. I need to filter only the stalled portion so I can then grep for a specific node out of those that have stalled.

For all intents and purposes, this is a freebsd system with standard GNU core utils, but I cannot install anything extra to assist.

  • @Gilles, Odd how that didn't pop up when I searched, though I didn't use the title I eventually went with...but it didn't show up in the screen below my title. Anyway, that aside, that might get me where I want, though I need the entire line after the match, not the first word - but might not take much of a change. – MaQleod Nov 7 '11 at 23:52
  • Its title sucked. I stole yours which is very nice. Take the sed solution and don't treat whitespace specially. – Gilles Nov 7 '11 at 23:55
  • @Gilles, that is something I'm not entirely sure how to do. I am still learning sed. – MaQleod Nov 8 '11 at 0:06
  • similar to unix.stackexchange.com/questions/24089/… as well. – Tim Kennedy Nov 8 '11 at 0:43
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    @shaa0601 I don't understand your question, it's especially difficult to follow in a comment with no formatting. Ask a new, self-contained question. – Gilles Aug 28 '14 at 15:37
up vote 118 down vote accepted

The canonical tool for that would be sed.

sed -n -e 's/^.*stalled: //p'

Detailed explanation:

  • -n means not to print anything by default.
  • -e is followed by a sed command.
  • s is the pattern replacement command.
  • The regular expression ^.*stalled: matches the pattern you're looking for, plus any preceding text (.* meaning any text, with an initial ^ to say that the match begins at the beginning of the line). Note that if stalled: occurs several times on the line, this will match the last occurrence.
  • The match, i.e. everything on the line up to stalled:, is replaced by the empty string (i.e. deleted).
  • The final p means to print the transformed line.

If you want to retain the matching portion, use a backreference: \1 in the replacement part designates what is inside a group \(…\) in the pattern. Here, you could write stalled: again in the replacement part; this feature is useful when the pattern you're looking for is more general than a simple string.

sed -n -e 's/^.*\(stalled: \)/\1/p'

Sometimes you'll want to remove the portion of the line after the match. You can include it in the match by including .*$ at the end of the pattern (any text .* followed by the end of the line $). Unless you put that part in a group that you reference in the replacement text, the end of the line will not be in the output.

As a further illustration of groups and backreferences, this command swaps the part before the match and the part after the match.

sed -n -e 's/^\(.*\)\(stalled: \)\(.*\)$/\3\2\1/p'
  • I've tried the first two examples and it just seems to hang. I don't get an error message, nor do I get a new prompt, just nothing. – MaQleod Nov 8 '11 at 1:00
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    @MaQleod Oh, it's waiting for input on standard input, which here is the terminal because you haven't redirected it. Here you'd do an input redirection sed … <messages, since you want to process data from a file. To act on data produced by another command, you'd use a pipe: somecommand | sed …. – Gilles Nov 8 '11 at 1:02
  • right, end of day blackout there. command works perfectly, thanks. – MaQleod Nov 8 '11 at 16:37
  • Best sed explanation I've seen so far -- thanks! – Jon Wadsworth Sep 16 '16 at 17:47
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    @ungalcrys Shorter version of what? This isn't equivalent to any of the commands in my answer. I'd recommend writing it as sed 's/^.*stalled//' since -r is specific to Linux and doesn't work on other systems such as macOS and here you aren't getting any benefit from it. – Gilles Aug 9 '17 at 10:19

The other canonical tool you already use: grep:

For example:

grep -o 'stalled.*'

Has the same result as the second option of Gilles:

sed -n -e 's/^.*\(stalled: \)/\1/p'

The -o flag returns the --only-matching part of the expression, so not the entire line which is - of course - normally done by grep.

To remove the "stalled :" from the output, we can use a third canonical tool, cut:

grep -o 'stalled.*' | cut -f2- -d:

The cut command uses delimiter : and prints field 2 till the end. It's a matter of preference of course, but the cut syntax I find very easy to remember.

  • 1
    Thanks for mentioning the -o option! I wanted to point out that grep doesn't recognize the \n as a newline, so your first example only matches to the first n character. For example, echo "Hello Anne" | grep -o 'A[^\n]*' returns the string A. However, echo "Hello Anne" | grep -o 'A.*' returns the expected Anne, since . matches any character except the newline. – adamlamar Mar 16 '15 at 21:52
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    Note that the quotes around the cut delimiter -d':' are removed by @poige. I find it easier to remember with quotes, e.g. with -d' ' or -d';'. – Anne van Rossum Jul 10 '17 at 20:44
  • According to your finding it should be easier to remember to use quotes with -f 2 too. Seriously, why not? – poige Aug 26 '17 at 10:26
  • Because a delimiter like a semi-colon ; rather than a colon : will be interpreted differently if not quoted. Of course that's logical behavior, but still I like to rely on muscle memory. I don't like to quote the delimiter one time but not the other time. Just personal preference, like I said before: easier to remember. – Anne van Rossum Oct 7 '17 at 18:09
  • the period that is part of the .* is needed, worked well for me: cat filename | grep 'Return only this line xyz text' | grep -o 'xyz.*' returns xyz text – ron Dec 12 '17 at 19:01

I used ifconfig | grep eth0 | cut -f3- -d: to take this

    [root@MyPC ~]# ifconfig
    eth0  Link encap:Ethernet  HWaddr AC:B4:CA:DD:E6:F8
          inet addr:192.168.0.2  Bcast:192.168.0.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:78998810244 errors:1 dropped:0 overruns:0 frame:1
          TX packets:20113430261 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:110947036025418 (100.9 TiB)  TX bytes:15010653222322 (13.6 TiB)

and make it look like this

    [root@MyPC ~]# ifconfig | grep eth0 | cut -f3- -d:
    C4:7A:4D:F6:B8

Hope this helps.

  • 1
    Does this answer the question? – Stephen Rauch Mar 31 '17 at 4:56
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    You can use cat /sys/class/net/*/address, no parsing required. – Anne van Rossum Dec 13 '17 at 16:58

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