grep: Global Regular Expression print.

grep is a powerful program used to find text patterns within files. It uses regular expressions (shortly regex) to match the pattern.

When grep encounters a pattern in the file, it prints out the lines containing it. If no file is given, it will search recursively search the given pattern in the files in current directory.

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grep pattern [file...]

grep <search-pattern> file.txt
  • To show the line number in the output, use -n option:
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grep -n <search-pattern> file.txt

grep is case-sensitive.

  • To ignore case-sensitive in grep, use -i option:
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grep -i <search-pattern> file.txt

Use some special characters or regular expressions in grep:

  • ^ : search at the beginning of the line.
  • $ : search at the end of the line.
  • . : search any character.

Some handy options for grep:

  • -v : tells grep to print only those lines that do not match the pattern.

Variants

grep has two variants:

  1. egrep : Extended grep.
  2. fgrep : fast grep.

Egrep

Egrep allows to use complicated regex.

For example, search any words that start with either I or o:

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egrep '^(i/o)' file.txt

search any words starts in the range between i to u:

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egrep '^[i-u]' file.txt