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Bash: text processing and scripts / Stage 1 of 9

Regular expressions in grep

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Step 6 of 6 · summary · 2 min

Summary

  • grep looks for the pattern's characters in a row anywhere in a line, including inside another word.
  • Where a match sits is set by ^ (start of line), $ (end) and the -w option (whole word); square brackets are one character from a set.
  • grep -E understands | and +; the pattern 'ERROR|Error:|err=[1-9]' finds every error in the log and nothing else.
  • -o prints only the matches, and a sort | uniq -c pipeline counts them.
  • At the start of the lesson, grep error found the word terror and missed ERROR, Error: and err=5: the pattern was an exact substring and set neither the place of the match, nor case, nor variants.

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Reference for this lesson

Articles

  • sshd — the SSH server — The OpenSSH server; edit the config with a second session open — a mistake in PermitRootLogin locks you out immediately.
  • grep — searching text — The grep command finds lines matching a pattern in files and command output: regex, recursion, context.
  • sort — sort lines — The sort command orders lines of a file or command output: as text, as numbers (-n), by field (-k), reversed (-r).
  • uniq — remove or count repeated lines — The uniq command collapses adjacent identical lines and counts them with -c; sort almost always comes before it.
  • Regular expressions — patterns for grep, sed and awk — A pattern that many strings match: characters, classes like [0-9], repeats * and +, anchors ^ and $. grep searches lines by it, sed replaces text, awk selects lines.
Ubuntu 24.04
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