Mastering sed: Essential Commands for Fast Text Processing

sed is a widely used, simple yet powerful command‑line tool for fast text processing. It is considered a hard skill that every programmer should master because of its versatility and similarity to vim and regular‑expression syntax.

A Simple Introduction

A basic sed command consists of three parts: options, address range, and the editing command, followed by the target file. The -f option can load a script file (advanced usage, not covered here).

Parameters

-n

(or --quiet / --silent) suppresses the default output, printing only the results you request. -i edits files in place, which is very dangerous because it overwrites the original file.

Using -i will modify the original file directly; be extremely careful.

Address Ranges

Line numbers can specify ranges, e.g., 1,4 selects lines 1‑4. Other examples:

sed -n '5p' file</code><code>sed -n '2,5p' file</code><code>sed -n '1~2p' file</code><code>sed -n '2~2p' file</code><code>sed -n '2,+3p' file</code><code>sed -n '2,$p' file

Regular‑expression ranges are also supported:

sed -n '/sys/,+3p' file</code><code>sed -n '/^sys/,/mem/p' file

Editing Commands

The most common command is p (print). Others include:

p prints matching lines. d deletes matching lines (requires dropping -n ). w writes matching lines to another file.

Less frequently used commands such as a, i, and c are omitted for brevity.

sed -n '2,5p' file</code><code>sed '2,5d' file</code><code>sed -n '2,5w output.txt' file

Substitution Mode

The s command replaces matched text. Its basic form is s/pattern/replacement/flags. Flags include g (global replace), p (print when used with -n), w (write changed lines), i (ignore case), and e (execute the result as a command).

sed -n 's/a/b/gipw output.txt' file</code><code>sed 's/^/ls -la/e' file

The special placeholder & represents the entire matched text, allowing constructions such as:

sed 's/.*/"&"/' file

Alternative Delimiters

Because slashes often appear in patterns, you can use other delimiters:

sed '/aaa/s@/etc@/usr@g' file</code><code>sed '/aaa/s|/etc|/usr|g' file

In‑Place Editing with Backup

Use -i.bak to edit a file while keeping a backup:

sed -i.bak 's/a/b/' file

Practical One‑Liners

Print lines longer than 50 characters: sed -n '/^.{50}/p' Count word occurrences: sed 's/ /\n/g' file | sort | uniq -c Remove comments from Python files:

find ./ -name "*.py" | xargs sed -i.bak '/^[ ]*#/d'

Show specific line ranges: sed -n -e '5,7p' -e '10,13p' file Extract only IP addresses:

ip route show | sed -n '/src/p' | sed -e 's/  */ /g' | cut -d' ' -f9

sed also offers an x (exchange) command and can even be used to implement games like Sokoban ( GitHub link ).