How to truncate long matching lines returned by grep or ack

user_232417 0
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I want to run ack or grep on HTML files that often have very long lines. I don't want to see very long lines that wrap repeatedly. But I do want to see just that portion of a long line that surrounds a string that matches the regular expression. How can I get this using any combination of Unix tools?

ack grep unix
10 Answers

You could use the grep option -o, possibly in combination with changing your pattern to ".{0,10}<original pattern>.{0,10}" in order to see some context around it:

       -o, --only-matching
              Show only the part of a matching line that matches PATTERN.

..or -c:

       -c, --count
              Suppress normal output; instead print a count of matching  lines
              for  each  input  file.  With the -v, --invert-match option (see
              below), count non-matching lines.
user_404680

Pipe your results thru cut. I'm also considering adding a --cut switch so you could say --cut=80 and only get 80 columns.

user_84540

grep -oE ".\{0,10\}error.\{0,10\}" mylogfile.txt

In the unusual situation where you cannot use -E, use lowercase -e instead.

Explanation: enter image description here

user_85345880

The Silver Searcher (ag) supports its natively via the --width NUM option. It will replace the rest of longer lines by [...].

Example (truncate after 120 characters):

 $ ag --width 120 '@patternfly'
 ...
 1:{"version":3,"file":"react-icons.js","sources":["../../node_modules/@patternfly/ [...]

In ack3, a similar feature is planned but currently not implemented.

user_7835100

I put the following into my .bashrc:

grepl() {
    $(which grep) --color=always $@ | less -RS
}

You can then use grepl on the command line with any arguments that are available for grep. Use the arrow keys to see the tail of longer lines. Use q to quit.

Explanation:

  • grepl() {: Define a new function that will be available in every (new) bash console.
  • $(which grep): Get the full path of grep. (Ubuntu defines an alias for grep that is equivalent to grep --color=auto. We don't want that alias but the original grep.)
  • --color=always: Colorize the output. (--color=auto from the alias won't work since grep detects that the output is put into a pipe and won't color it then.)
  • $@: Put all arguments given to the grepl function here.
  • less: Display the lines using less
  • -R: Show colors
  • S: Don't break long lines
user_24227020

Here's what I do:

function grep () {
  tput rmam;
  command grep "$@";
  tput smam;
}

In my .bash_profile, I override grep so that it automatically runs tput rmam before and tput smam after, which disabled wrapping and then re-enables it.

user_14838610

ag can also take the regex trick, if you prefer it:

ag --column -o ".{0,20}error.{0,20}"
user_49415300
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