Page Comparison

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• grep -P '<pattern>' searches for <pattern> in its input, and only outputs lines containing it
  • always enclose '<pattern>' in single quotes to inhibit shell evaluation!
    • pattern-matching metacharacters are very different from those in the shell
  • -P says to use Perl patterns, which are much more powerful than standard grep patterns
  • -c says just return a count of line matchesv (inverse match) – only print lines with no match
  • -n says include the (line number) – prefix output with the line number of the matching linematch
  • -v (inverse match) says return only lines not matching the pattern-l i  (case insensitive) – ignore case when matching
  • -l says return only the names of files that do contain the pattern match
  • -L says return only the names of files that do not contain the pattern match
  • -Lc says return only the names of files containing no pattern matchesjust return a count of line matches
  • -A <n> (After) and -B <n> (Before) – output '<n>' number of lines after or before a match

A regular expression (regex) is a pattern of characters to search for and metacharacters that control and modify how matching is done.

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• here's a good general one: https://www.regular-expressions.info/
• and a perl regex tutorial: http://perldoc.perl.org/perlretut.html
  • perl regular expressions are the "gold standard" used in most other languages

Field delimiter summary

Be aware of the default field delimiter for the various bash utilities, and how to change them:

join -t $'\t' -j 2 file1 file12