====== sed ======

Stream editor: applies edit commands to each line. GNU sed on Ubuntu.

===== The one command you'll use 95% of the time =====

<code bash>
sed 's/old/new/'          # substitute, first match per line
sed 's/old/new/g'         # all matches per line
sed 's/old/new/2'         # only the 2nd match per line
sed -i 's/old/new/g' f    # edit file in place
sed -i.bak 's/x/y/' f     # in place, keeps f.bak
sed -E 's/(a)(b)/\2\1/'   # extended regex + backrefs
</code>

Any delimiter works: useful for paths/URLs: ''sed 's|/a/b|/x/y|g'''.
''&'' in replacement = the whole match: ''sed 's/foo/[&]/g''' → ''[foo]''.

===== Addressing: //which// lines get the command =====

<code bash>
sed '5d' f                    # delete line 5
sed '5,10d' f                 # delete 5-10
sed '$d' f                    # last line
sed '2~3d' f                  # every 3rd line starting at 2
sed '/error/d' f              # lines matching /error/
sed '/START/,/END/d' f        # inclusive pattern range
sed '/START/,/END/!d' f       # keep ONLY the range (! inverts)
sed -n '5,10p' f              # print just lines 5-10 (-n + p)
sed -n '/error/p' f           # print only matches (grep-like)
</code>

===== The commands beyond `s` =====

<code bash>
d   delete line          p   print line (with -n)
a\  append after line    i\  insert before line
c\  change whole line    y/abc/ABC/  transliterate chars
</code>

<code bash>
sed '3i\inserted text' f        # insert before line 3
sed '/pattern/a\appended' f     # append after matches
sed 'y/abc/ABC/' f              # like tr, per char
sed -n 's/x/y/p' f              # print only changed lines
</code>

===== Multiple commands =====

<code bash>
sed -e 's/a/b/' -e '5d' f
sed 's/a/b/; s/c/d/' f          # ; separates commands
sed '/skip/!s/a/b/' f           # s on lines NOT matching
</code>

===== Useful regex bits (GNU sed BRE/ERE) =====

  * Default is **BRE**: ''+'', ''?'', ''('', '')'', ''<nowiki>|</nowiki>'' need backslashes.
  ''sed 's/\(ab\)\+/X/'': or just always use ''sed -E'' (ERE) and forget it.
  * ''\b'' word boundary, ''\w''/''\W'', ''\s''/''\S'' work in GNU sed.
  * ''\( \)'' groups become ''\1''-''\9'' backrefs: ''sed -E 's/([0-9]+)x([0-9]+)/\2 by \1/'''.
  * ''.*'' is greedy: ''sed -E 's/"(.*)"/\1/''' grabs the LAST quote pair.
  Non-greedy needs ''[^"]*'': ''sed -E 's/"([^"]*)"/\1/'''.

===== Classic one-liners =====

<code bash>
sed 's/^[[:space:]]*//;s/[[:space:]]*$//' f   # trim whitespace
sed 's/^/# /' f                              # comment out every line
sed -n 'x;$p' f || tail -1 f                 # last line (use tail)
sed -e :a -e 'N;$!ba' -e 's/\n/ /g' f        # join all lines
sed 's/./&\n/g' f                            # one char per line
seq 5 | sed 's/^/item/'                      # generate
</code>

===== Gotchas =====

  * ''-i'' without backup is irreversible: test without ''-i'' first.
  * ''sed -i '''' is macOS syntax; GNU is ''sed -i'' (no arg). These pages assume
  GNU sed on Linux.
  * ''sed -z'' treats input as NUL-separated: for filenames from ''find -print0''.
  * It processes line-by-line; multi-line matching needs ''N'', ''P'', ''D'': at that
  point reach for ''perl -0777 -pe'' or awk.


===== See Also =====

  * [[terminal:scripting:start|Scripting Index]]
  * [[terminal:tools:start|CLI Tools]]
  * [[terminal:start|Return to terminal]]
  * [[start|Return to wiki home]]