#36 Fixing grep

Some versions of grep offer a remarkable variety of capabilities, including the particularly useful ability to show the context (a line or two above and below) of a matching line in the file. Additionally, some rare versions of grep can highlight the region in the line (for simple patterns, at least) that matches the specified pattern.

Both of these useful features can be emulated in a shell script, so that even users on older commercial Unixes with relatively primitive grep commands can enjoy them.

The Code
#!/bin/sh

# cgrep - grep with context display and highlighted pattern matches.

context=0
esc="^["
bOn="${esc}[1m" bOff="${esc}[22m"
sedscript="/tmp/cgrep.sed.$$"
tempout="/tmp/cgrep.$$"

function showMatches
{
  matches=0

  echo "s/$pattern/${bOn}$pattern${bOff}/g" > $sedscript

  for lineno in $(grep -n "$pattern" $1 | cut -d: -f1)
  do
    if [ $context -gt 0 ] ; then
      prev="$(($lineno - $context))"
      if [ "$(echo $prev | cut -c1)" = "-" ] ; then
        prev="0"
      fi
      next="$(($lineno + $context))"

      if [ $matches -gt 0 ] ; then
        echo "${prev}i\\" >> $sedscript
        echo "----" >> $sedscript
      fi
      echo "${prev},${next}p" >> $sedscript
    else
      echo "${lineno}p" >> $sedscript
    fi
    matches="$(($matches + 1))"
  done
  if [ $matches -gt 0 ] ; then
    sed -n -f $sedscript $1 | uniq | more
  fi
}

trap "/bin/rm -f $tempout $sedscript" EXIT

if [ -z "$1" ] ; then
  echo "Usage: $0 [-c X] pattern {filename}" >&2; exit 0
fi

if [ "$1" = "-c" ] ; then
  context="$2"
  shift; shift
elif [ "$(echo $1|cut -c1-2)" = "-c" ] ; then
  context="$(echo $1 | cut -c3-)"
  shift
fi

pattern="$1"; shift

if [ $# -gt 0 ] ; then
  for filename ; do
    echo "----- $filename -----"
    showMatches $filename
  done
else
  cat - > $tempout      # save stream to a temp file
  showMatches $tempout
fi

exit 0

How It Works
This script uses grep -n to get the line numbers of all matching lines in the file and then, using the specified number of lines of context to include, identifies a starting and ending line for displaying each match. These are written out to the temporary sed script, along with a word substitution command (the very first echo statement in the showMatches function) that wraps the specified pattern in bold-on and bold-off ANSI sequences. That's 90 percent of the script, in a nutshell.

Running the Script
This script works either with an input stream (in which case it saves the input to a temp file and then processes the temp file as if its name had been specified on the command line) or with a list of one or more files on the command line. To specify the number of lines of context both above and below the line matching the pattern that you specified, use -c value, followed by the pattern to match.

The Results
$ cgrep -c 1 teacup ragged.txt
----- ragged.txt -----
in the wind, and the pool rippling to the waving of the reeds--the
rattling teacups would change to tinkling sheep-bells, and the
Queen's shrill cries to the voice of the shepherd boy--and the

Hacking the Script
A useful refinement to this script would return line numbers along with the matched lines.

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