sed Command in Linux: Edit Text Streams in Place

sed — stream editor — rewrites text as it flows past. Its name is the whole job description: unlike an editor you open, sed applies edits to a stream, which makes it the tool of choice for scripted, repeatable changes across files and pipelines.

The substitution everyone uses #

Replace text in a stream

$ echo "deploy to prod" | sed 's/prod/staging/'
deploy to staging

The pattern reads s/old/new/. Applied to a file, the change streams to the terminal; the file itself is untouched until you ask for it:

Edit a file in place, keeping a backup

$ sed -i.bak 's/localhost/aospix.com/' app.conf

-i writes the file back, and .bak keeps the original beside it — cheap insurance on config edits.

Global, selective, and deleted lines #

By default sed replaces the first match per line. A trailing g handles every match, and line addresses narrow the scope:

Replace every match, and only on lines 10-20

$ sed 's/error/fault/g' app.log
$ sed '10,20s/#/;/g' notes.txt

Deleting lines rounds out the daily trio:

Command	Effect
`sed 's/a/b/'`	Replace first `a` per line
`sed 's/a/b/g'`	Replace all matches per line
`sed '3d' file`	Delete line 3
`sed '/^#/d' file`	Delete comment lines
`sed -n '5,8p' file`	Print only lines 5–8
`sed '/error/!d' file`	Keep only matching lines

That last row is territory grep usually owns — reach for grep when you are only filtering, and for sed when you are changing.

Real examples #

Strip comments and blank lines from a config for a clean view:

Show only active settings

$ sed -e 's/#.*//' -e '/^\s*$/d' /etc/ssh/sshd_config

Swap a setting across many files at once:

Update one setting across a directory

$ for f in conf.d/*.conf; do sed -i 's/^Port .*/Port 2222/' "$f"; done

For column-aware edits rather than pattern edits, awk is the right neighbour, and for combined pipelines see unix pipes in practice. More from the terminal notes series lands weekly.