SED Basics

sed (Stream Editor) is a powerful command-line tool used for text manipulation and transformation. It processes text files line by line and applies specified operations, such as searching, replacing, and deleting.

Basic Syntax

The basic syntax for sed is:

sed [options] 'command' file

• options: Additional flags to modify sed behavior.

• command: The editing operation(s) to perform.

• file: The input file to process.

If no file is specified, sed reads from standard input.

Common Commands

• s: Substitution command (search and replace).

• d: Delete lines.

• p: Print lines.

• a: Append text after a line.

• i: Insert text before a line.

Useful Options

• -i: Edit the file in place (without creating a backup).

• -n: Suppress automatic printing of lines (useful with p command).

• -e: Allows multiple commands.

• -f: Execute commands from a file.

Examples

Substitution (Search and Replace)

Replace the first occurrence of "foo" with "bar" on each line:

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

Replace all occurrences of "foo" with "bar":

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

Case-insensitive replacement:

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

Delete Lines

Delete lines containing "error":

sed '/error/d' file.txt

Delete the 5th line:

sed '5d' file.txt

Delete lines from 10 to 20:

sed '10,20d' file.txt

Print Lines

Print only lines containing "error":

sed -n '/error/p' file.txt

Print lines 5 to 10:

sed -n '5,10p' file.txt

Insert and Append Text

Insert "Header" before the first line:

sed '1i Header' file.txt

Append "Footer" after the last line:

sed '$a Footer' file.txt

Modify File In-Place

Replace "foo" with "bar" and save changes to the file:

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

Create a backup of the file before editing:

sed -i.bak 's/foo/bar/g' file.txt

Combine Multiple Commands

Replace "foo" with "bar" and delete lines containing "baz":

sed -e 's/foo/bar/g' -e '/baz/d' file.txt

Use a File for Commands

Store commands in a file (commands.sed):

s/foo/bar/g
/baz/d

Apply the commands:

sed -f commands.sed file.txt

Advanced Examples

Replace Only Specific Lines

Replace "foo" with "bar" on lines 2 to 5:

sed '2,5s/foo/bar/' file.txt

Remove Blank Lines

sed '/^$/d' file.txt

Add Line Numbers

sed = file.txt | sed 'N;s/\n/ /'

Summary

sed is a versatile tool for stream editing, offering powerful features for text processing. By mastering its commands and options, you can efficiently manipulate text files and automate complex editing tasks.