sed: Stream Editor

The s/from/to/ engine

sed applies a small program to each line of input and prints the result. The most common command is s/pattern/replacement/ — substitute. Once you've used this, you'll never copy-paste-find-replace in a GUI editor again.

Substitution

sed 's/foo/bar/' — first occurrence per line.
sed 's/foo/bar/g' — every occurrence (global).
sed 's/foo/bar/I' — case-insensitive (GNU).
sed 's|/usr/local|/opt|g' — change delimiter when substituting paths (any single char works).
sed -E 's/(foo|bar)/[\1]/g' — extended regex with capture groups; \1 is the first captured group.

Other commands

sed '/pattern/d' — delete matching lines.
sed -n '5,10p' file — print only lines 5-10 (-n suppresses default output).
sed '$d' file — delete the last line.
sed '1i\\ header' file — insert text at line 1.

In-place editing

sed -i rewrites the file. BSD sed (macOS) requires an empty argument: sed -i '' 's/foo/bar/g' file. GNU sed: sed -i 's/foo/bar/g' file. The mismatch trips up everyone the first time. brew install gnu-sed + alias if you want consistency.

Sed vs awk vs perl

sed for line-by-line substitutions. awk for field-aware processing. perl -pe for serious regex (full PCRE with lookaheads). Each has its place; sed is the smallest and the most portable.

Code

Substitution patterns·bash

# Replace foo with bar globally, output to stdout
sed 's/foo/bar/g' file.txt
# In place — macOS BSD
sed -i '' 's/foo/bar/g' file.txt
# In place — GNU (linux or brew install gnu-sed)
sed -i 's/foo/bar/g' file.txt
# Slash in pattern? change delimiter
sed 's|/usr/local|/opt/homebrew|g' install.sh

Delete + extract·bash

# Drop blank lines
sed '/^$/d' file
# Print only lines 100-110
sed -n '100,110p' big.log
# Strip trailing whitespace
sed -E 's/[[:space:]]+$//' file

The s/from/to/ engine

Substitution

Other commands

In-place editing

Sed vs awk vs perl

Code

External links

Exercise

Progress

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