How to Find and Replace Text in a File in Linux

Replace text in one file or across a whole project, safely.

Short answer

$ sed -i 's/old/new/g' file.txt

Replaces every occurrence in place. Run it without -i first to preview the result before you commit to it.

sed edits streams, which makes it perfect for this: describe the change once and it applies to ten thousand lines instantly. The two things that separate a safe replacement from a bad afternoon are previewing before writing, and anchoring the pattern so it only matches what you intend.

Step by step

1 Preview first

$ sed 's/old/new/g' file.txt

Without -i, sed prints the result and leaves the file untouched. Read the output, confirm it is what you wanted, then add -i.

2 Replace in place, keeping a backup

$ sed -i.bak 's/old/new/g' file.txt

Writes the change and leaves file.txt.bak with the original. On a server config, this two-second insurance has saved countless afternoons.

3 Replace across many files

$ grep -rl "old" src/ | xargs sed -i "s/old/new/g"

grep -rl lists only the files that contain the text, xargs feeds them to sed. Efficient and precise: files without a match are never touched.

4 When the text contains slashes

$ sed -i "s|/var/www|/srv/web|g" config.conf

Any character can be the delimiter after s. Using | avoids escaping every slash in a path.

5 Anchor it to avoid collateral damage

$ sed -i "s/^PORT=.*/PORT=8080/" .env

The ^ anchors to the start of the line, so only the PORT setting changes — not a mention of "PORT" inside a comment.

Tips worth knowing

Always preview, or use -i.bak. sed is fast enough to ruin a file before you finish reading the command.
On macOS, BSD sed needs an argument for -i, even an empty one: sed -i '' 's/a/b/' file. Scripts meant for both should use -i.bak.
For interactive, per-occurrence confirmation, your editor is the better tool. sed is for bulk and for scripts.
In a git repository, run the replacement and then `git diff` — the safest preview there is.

Frequently asked questions

What does the g at the end do?

It replaces every occurrence on each line. Without it, only the first match per line changes.

How do I replace only in certain lines?

Prefix an address: sed -i "/^server/s/8080/9090/" conf replaces only inside lines starting with "server".

sed or awk for this?

sed for substitution — that is what it is for. awk when you also need columns, conditions or arithmetic.

Can I undo it?

Only if you used -i.bak or the file is in version control. That is precisely why both habits exist.

The commands behind it

sed

Stream-edit text: substitute, delete lines, and edit files in place with -i.

grep

Find lines matching a pattern in files or piped input — the workhorse of text search.

xargs

Turn a list of items into command arguments — the glue between pipelines and commands.

Concepts involved: Regular expression

Choosing between tools

sed vs awk

sed edits lines; awk understands columns. Substitution is sed, computation is awk.

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