sed

Additional Notes

sed is a stream editor. It reads text, applies editing commands, and prints the result. It is often used for search-and-replace, deleting lines, printing selected lines, and simple text transformations.

By default, sed writes changed text to standard output. It does not modify the original file unless you use in-place editing.

Syntax

sed [options] 'script' [file...]

Parameters

• options: Flags that change how sed behaves.
• file: Text file to read or process.
• script: Sed editing command or script, such as s/old/new/.

Common Options

• -n: Suppress automatic printing. Useful with explicit p.
• -i: Edit files in place.
• -i.bak: Edit in place and create backup files.
• -E: Use extended regular expressions.
• -e SCRIPT: Add a script command.
• -f FILE: Read sed commands from a file.

Common Commands

• s/old/new/: Replace first match on each line.
• s/old/new/g: Replace all matches on each line.
• p: Print matching lines.
• d: Delete matching lines.
• 1,5p: Print lines 1 through 5.
• /pattern/p: Print lines matching a pattern.
• /pattern/d: Delete lines matching a pattern.

Examples

sed 's/http/https/' urls.txt

Replace the first http on each line.

sed 's/http/https/g' urls.txt

Replace every http on each line.

sed -n '1,20p' app.log

Print lines 1 through 20.

sed '/^#/d' config.conf

Remove comment lines from output.

sed '/^$/d' file.txt

Remove empty lines from output.

sed -i.bak 's/old-name/new-name/g' config.conf

Edit a file in place while creating config.conf.bak.

cat access.log | sed -n '/404/p'

Print only lines containing 404.

Practical Notes

• Quote sed scripts so the shell does not interpret special characters.
• Use -i.bak when learning in-place editing.
• The delimiter does not have to be /; for paths, s#/old/path#/new/path#g can be easier.
• For field-based processing, use awk.
• For simple fixed-string replacement in one file, a text editor may be safer.