Mastering sed: Powerful Text Editing Commands for Linux

Overview of the stream editor sed

sed

is a non‑interactive stream editor used on Unix‑like systems for batch text processing. It reads input line‑by‑line, applies a sequence of editing commands, and writes the transformed stream to STDOUT. Because it works in a pipeline, sed is much faster than interactive editors such as vim for large‑scale modifications.

Basic command syntax

sed [options] 'edit_commands' [file]

Square brackets denote optional parts. If file is omitted, sed reads from standard input.

Important options

-e – Append an editing command to the script. Useful when multiple commands are supplied on the command line. Example: sed -e 's/foo/bar/' -e 's/baz/qux/' file.txt -f – Read editing commands from a file. This is convenient when the script contains many commands.

# script.sed
s/root/ROOT/
s/bin/BIN/
s/home/HOME/

sed -f script.sed /etc/passwd

-n – Suppress automatic printing of each pattern space. Typically combined with the p command to output only lines that match a substitution.

sed -n 's/foo/bar/p' file.txt

Core sed commands

s

– Substitute (text replacement). d – Delete the current line. i – Insert a line before the addressed line. a – Append a line after the addressed line. c – Change the addressed line(s) to new text. y – Transliterate characters (one‑to‑one mapping). p – Print the current line. = – Print the line number. w – Write selected lines to a file. r – Read a file and insert its contents.

Text substitution with s

Basic form: sed 's/srcStr/dstStr/' file The delimiter / can be replaced by any character to avoid escaping slashes, e.g.: sed 's!/bin/bash!/BIN/BASH!' /etc/passwd Flags that can follow the closing delimiter:

g – Replace all occurrences on the line.

n – Replace only the n ‑th occurrence on the line (e.g., sed 's/root/ROOT/2' file replaces the second root on each line).

Printing matched lines

Combine -n with the p flag to output only lines that were actually substituted:

sed -n 's/root/ROOT/p' /etc/passwd

Writing results to a file

Use the w command to write only the modified lines to a separate file:

sed -n 's/root/ROOT/g w change.txt' /etc/passwd

Line addressing

Addressing places a selector before a command, limiting the command to specific lines.

Numeric line: sed '3 s/bin/BIN/g' file – apply only to line 3.

Range: sed '2,5 s/bin/BIN/g' file – apply to lines 2 through 5.

From a line to the end: sed '10,$ s/bin/BIN/g' file.

Pattern address: sed -n '/root/ s/bin/BIN/p' file – apply only to lines containing root.

Pattern range: sed -n '/root/,/nologin/ s/bin/BIN/p' file.

Deleting lines

Use d with an address. Without an address, d would delete every line.

# Delete lines that contain "root"
sed '/root/d' /etc/passwd
# Delete from line 2 to the end of the file
sed '2,$d' /etc/passwd

Inserting and appending text

The i command inserts before the addressed line; a appends after it. These commands cannot be combined with other text on the same line.

# Insert a line before every line (for demonstration only)
sed 'i\Inserted line' file
# Insert before the first line
sed '1i\Header line' file
# Append after the third line
sed '3a\Footer line' file
# Append after the last line
sed '$a\End of file' file

Changing entire lines with c

The c command replaces the whole addressed line or range with a single new line.

# Replace line 3 entirely
sed '3c\New text for line 3' file
# Replace a range (lines 2‑4) with one line
sed '2,4c\Combined replacement' file

Character transliteration with y

Syntax: [address]y/inchars/outchars/. It performs a one‑to‑one mapping of characters.

echo abcdefggfedcba | sed 'y/acg/ACG/'
# Output: AbCdefGGfedCbA

File I/O commands

Write ( w ) : [address]w filename writes the addressed lines to filename.

# Write the first two lines to test.txt
sed '1,2w test.txt' /etc/passwd

Read ( r ) : [address]r filename inserts the contents of filename after the addressed line.

# Insert test.txt after line 3
sed '3r test.txt' /etc/passwd

Combining addressing with other commands

Addressing works with any command, not only s. For example, to replace bin with BIN only on lines that contain root: sed -n '/root/ s/bin/BIN/p' /etc/passwd Or to apply a substitution over a pattern range:

sed -n '/root/,/nologin/ s/bin/BIN/p' /etc/passwd

Be aware that a pattern range starts processing at the first match of the start pattern and continues until the end pattern is found; overlapping ranges can cause a command to be applied multiple times.

Summary of typical usage patterns

Simple substitution on the whole file: sed 's/old/new/g' file Substitution with selective output: sed -n 's/old/new/gp' file Delete lines matching a pattern: sed '/pattern/d' file Insert a header line: sed '1i\Header text' file Append a footer line: sed '$a\Footer text' file Replace a block of lines with a single line: sed '5,10c\New block content' file Transliterate characters: sed 'y/abc/ABC/' file Write only changed lines to another file:

sed -n 's/pattern/repl/g w changes.txt' file