Mastering sed: Powerful Text Editing and Stream Processing in Linux

1. Stream editor sed and the sed command

In Linux, the stream editor sed is commonly used for text replacement. Unlike interactive editors such as vim, sed processes files in batch mode, making it much faster for large‑scale data editing.

Typical sed processing steps:

Read one line from input.

Match the line against the provided editing commands.

Modify the line according to the commands.

Output the new line to STDOUT.

sed repeats this cycle until all input is processed.

<code>sed [options] edit_commands [file]    # items in brackets are optional</code>

2. Using sed for text replacement

The

s

command performs substitution. Basic syntax:

<code>sed 's/srcStr/dstStr/' file</code>

Use different delimiters if needed, e.g.

!

:

<code>sed 's!/bin/bash!/BIN/BASH!' /etc/passwd</code>

By default only the first occurrence per line is replaced; add the

g

flag to replace all matches.

<code>sed 's/root/ROOT/g' /etc/passwd</code>

The

p

flag prints matching lines, often combined with

-n

to suppress automatic output:

<code>sed -n 's/root/ROOT/gp' /etc/passwd</code>

3. Line addressing

sed can target specific lines using numeric addresses or ranges:

<code>sed '3 s/bin/BIN/g' /etc/passwd          # replace "bin" with "BIN" on line 3
sed '2,5 s/bin/BIN/g' /etc/passwd        # replace on lines 2 through 5
sed '10,$ s/bin/BIN/g' /etc/passwd        # replace from line 10 to the end</code>

Pattern addressing filters lines that match a regular expression:

<code>sed -n '/root/s/bin/BIN/p' /etc/passwd</code>

Range patterns apply commands from the first matching line to the second:

<code>sed '/pattern1/,/pattern2/ edit_command' file</code>

4. Deleting lines

The

d

command removes lines. Combine with addressing to delete specific lines:

<code>sed '/root/d' /etc/passwd                # delete lines containing "root"
sed '2,$d' /etc/passwd                    # delete from line 2 to the end</code>

5. Inserting and appending text

Use

i

to insert a line before the addressed line and

a

to append after it:

<code>sed 'iInsert a line before every line' /etc/passwd
sed '1iInsert a line before the first line' /etc/passwd
sed '3aAppend a line after the third line' /etc/passwd
sed '$aAppend a line at the last line' /etc/passwd</code>

6. Modifying lines

The

c

command replaces the entire addressed line(s) with new text:

<code>sed '3 cNew text' /etc/passwd                     # replace line 3
sed '/root/ cNew text' /etc/passwd                # replace lines matching "root"
sed '2,4 cNew text' /etc/passwd                   # replace lines 2‑4 with a single new line</code>

7. Transliteration

The

y

command performs character‑by‑character conversion:

<code>echo abcdefggfedcba | sed 'y/acg/ACG/'</code>

Result:

AbCdefGGfedCbA

.

8. Working with files

Write selected lines to a file using

w

:

<code>sed '1,2w test.txt' /etc/passwd</code>

Read and insert a file’s content using

r

:

<code>sed '3 r test.txt' /etc/passwd</code>

These commands enable powerful, scriptable text manipulation directly from the command line.