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Sed

sed -<PARAMETER> '<RESTRICTION> <FLAG1>/<PATTERN1>/<PATTERN2>/<FLAG2>' <FILE1> <FILE2>…

Examples

Recursive find and replace:

shopt -s globstar && sed -E -i 's/foo/bar/g' ./**/*.yml

# Use this to test matching.
shopt -s globstar && sed -E -n 's/foo/p' ./**/*.yml

Sed example using comments:

sed \
  --regexp-extended \
  "
  # Remove boilerplate
  s|<p>Necessary cookies.*</p>||g
  s|<p>Any cookies that may.*</p>||g

  # Remove inline links
  s/<(a|span)[^>]*>//g

  # Remove other HTML formatting elements.
  s!<(\/a|\/span|\/b|\/br|\/sup|p|b|br)\s*\/*>!!g

  # Convert HTML escape sequences.
  s|&#821[6|7];|'|g
  s|&#822[0|1];|\"|g
  s|&#8211;| - |g
  s|&#8212;| -- |g
  s|&amp;|\&|g
  s|<sup>|^|g
  s|</p>|\n|g

  # Remove leading spaces.
  s|^\s*||g
  "
  • sed -n '!spam!,!eggs!p' = Print all lines between spam and eggs, inclusive.
  • sed -En '/([a-z]+) \1/p' = Print instances of duplicated words.
  • sed '1 s/^/spam/ file.txt = Insert spam at the first line of file.txt.
    • 1 = Restrict operations to the first line of the input.
    • s = Substitute mode.
    • ^ = (First string) Replace the start of the first line with the second string.
    • spam = (Second string) This string will replace the start of the first line in file.txt.
  • sed 's|spam|eggs|3' file.txt = Replace the third occurrence of spam with eggs in file.txt.

  • sed -n 2p = Print second line of input.
  • sed 10q = Print first ten lines of input, emulates head.
  • sed '5,10d' = Delete lines 5 through 10.
  • sed 's|.||2g' = Remove all periods, skipping the first match on each line.

  • sed '$d' = Delete the last line.
  • sed '/^$/d' = Delete all blank lines.
  • sed '/./,$!d' = Delete all leadig blank lines at top of file.

Restrictions and Ranges

  • sed '/^#/ s/[0-9]+//' = Delete the first number on all lines starting with #.
    • '/^#/ = Restriction: Operate only on lines matching expression.
  • sed '2,3 s/[0-9]+//' = Same thing, but only on lines 2 and 3.
  • sed '2,$ s/[0-9]+//' = Same thing, but between lines 2 and the end of the file.
  • sed '/start/,/stop/ s/#.*//' = Remove comments between the lines start and stop, inclusive.

Flags

  • Before patterns (sed '<FLAG>/pattern1/pattern2/')

    • s (substitute) = Perform a string substitution.
    • i (insert) = Insert input above match.
    • a (append) = Insert input below match.

  • After patterns (sed '/pattern1/pattern2/<FLAG>')

    • <NUMBER> = Only operate on <NUMBER>th match.
    • <NUMBER>g = Operate on all matches from the <NUMBER>th match onwards (GNU sed only).
    • g (global) = Match pattern multiple times on the same line, if possible.
    • p (print) = Force-print match to stdout. Usually used with -n to only print matching lines.
    • q (quit) = Stop processing input.
    • w (write) = Write to the provided file.
    • I (insensitive) = Make regex case-insensitive.

Parameters

  • -e (expression) = Combine multiple invocations into a single command.
  • -r (regex) = Use extended regular expressions, allowing the use of characters like +.
  • -n (nullify) = Suppress printing modified input to stdout.
  • -i (in-place) = Don't print result to stdout, just go ahead and immediately edit file.

Patterns

  • & = Current regex match (ex: echo "123 abc ABC" | sed 's/abc/& def/' = 123 abc def ABC).
  • $ = The end of the file.

Other commands and examples

  • tr 'a-z' 'A-Z' (translate) = Find first parameter ('a-z') and replace matches with second parameter ('A-Z').

  • sort -rk 2 = Reverse (r) sort results by the second column (k) of output.

    Filter out only the host's primary IPv4 address:
# Good:
ip -4 -br a | awk '!/127\.0\.0/ {gsub(/\/[0-9]{1,2}/,""); print $3}'
# Bad:
ifconfig ens32 | grep "inet" | grep -v "inet6" | tr -s " " ":" | cut -f 3 -d ":"