sed - Find and Replace from the Command Line

sed: Find and Replace Without Opening a File

You need to replace "foo" with "bar" in a file. You could open it in an editor, Ctrl+H, type both strings, click replace all, save, close. Or:

sed 's/foo/bar/g' file.txt

Done. No editor, no clicking, no saving. That's sed.

The One Pattern You Need

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

That's the whole thing. s means substitute. old is what you're replacing. new is what you're replacing it with. g means "all occurrences" (without it, sed only replaces the first match per line).

The slashes are just delimiters - they separate the parts.

How sed Thinks

sed reads each line, applies your rules, and prints the result. It doesn't modify the original file by default - it just outputs the transformed text.

Want to actually change the file? Add -i:

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

Now the file itself is modified. Use with care.

Common Patterns

Replace text everywhere:

sed 's/error/warning/g' log.txt

Delete lines containing a pattern:

sed '/DEBUG/d' log.txt

The d command deletes matching lines entirely.

Replace only on matching lines:

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

Only runs the substitution on lines containing "header".

Delete blank lines:

sed '/^$/d' file.txt

^$ is regex for "start followed immediately by end" - an empty line. You'd know that if you were keeping up with your Regexgolf.

Case-insensitive replace:

sed 's/error/warning/gi' file.txt

The i flag ignores case.

The Delimiter Trick

Slashes are traditional, but they're annoying when your text contains slashes:

sed 's/\/path\/to\/file/\/new\/path/g'  # ugly escape hell
sed 's|/path/to/file|/new/path|g'       # clean

You can use any character as the delimiter. Pipes |, colons :, and # are popular alternatives.

Real Example: Repurposing a Cover Letter (don't do this)

Switching the name:

sed -i 's/companyA/companyB/g' cover_letter.txt

Capitalizing a name that is also a word:

sed -i 's/Mrs\. will/Mrs. Will/' cover_letter.txt

The Mrs\. makes sure you only replace instances referring to the person, not the verb.

sed vs Other Tools

• Interactive editing? Use your editor.

• Just searching? Use grep.

• Need columns? Use awk.

• Find and replace? That's sed.

sed shines in scripts and pipelines where you can't open an editor. It's also faster than any GUI for bulk changes.

Quick Reference

Practice

sed is essential for CTF challenges where you need to transform data formats or clean up output for the next command in a pipeline.

Three slashes and a letter. That's sed. s/this/that/g and you're done.