Mastering sed: Essential Linux Text Processing Commands and Examples

About the Linux three musketeers

grep – filter keyword information in text.

sed – edit text data and modify original files.

awk – filter and extract file data, with formatted output.

What is sed

sed software itself.

Commands provided by sed.

Source data given to sed.

sed syntax format

sed [options] [sed built‑in command] [input file]

Explanation:
1. Each element (sed, options, command, input file) must be separated by at least one space.
2. The term "sed" refers to the software.
3. sed‑commands can be a single command or a combination.
4. input‑file is optional; sed can read from stdin or a pipe.

sed command execution process

Conceptual flow:
sed reads a line from a file or pipe, processes the line, outputs the line, then reads the next line, and so on.

sed parameters

sed modifies data in the pattern space (in‑memory). By default it does not write changes to the file; use -i to write changes back and suppress terminal output.
Example:
sed -i 's#old#new#' file.txt

sed options

-n Cancel default output, often used with the p command.

-e Execute multiple sed commands in one line.

-f Followed by a script file name.

-r Use extended regular expressions.

-i Edit the file in place; without -i changes exist only in memory.

sed commands

a   Append text after the specified line.
c   Replace the selected line with new text.
d   Delete the selected line.
D   Delete part of the pattern space up to a newline (multiline mode).
i   Insert text before the specified line.
h   Copy pattern space to hold space.
H   Append pattern space to hold space.
g   Copy hold space to pattern space.
G   Append hold space to pattern space.
x   Exchange pattern and hold spaces.
l   Print non‑printable characters.
n   Clear pattern space and read next line.
N   Read next line without clearing pattern space.
p   Print pattern space (usually with -n).
P   Print up to the first newline.
q   Quit sed.
r   Read data from a file.
s   Substitute (replace) text.
w   Write pattern space to a file.
y   Translate characters.
:label   Define a label.
t   If the previous command succeeded, jump to the label.

sed range matching

sed modify characters and cancel default output

sed insert, delete, and replace examples

Insert text after line 3: 3a Added line after line 3 Insert text before line 3: 3i Inserted line before line 3 Delete the second line: sed '2 d' file Delete lines 1 to 4: sed '1,4d' file Delete odd lines: sed '1~2d' file Delete even lines:

sed '2~2d' file

sed substitution examples

# Replace the first occurrence on each line
sed 's/old/new/' file
# Global replacement on each line
sed 's/old/new/g' file
# Use # as delimiter to avoid escaping slashes
sed 's#old#new#' file

Case‑insensitive substitution (use capital I):

sed 's#my#His#Ig' file

sed grouping and back‑references

# Example: extract the word after a comma
echo 'I am teacher yuchao,welcome my linux course' | sed -r 's/^.*,(.*)m.*/\1/g'
# Result: welcome

sed write command (w)

# Write lines containing "computer" to game2.log
sed '/computer/w game2.log' t1.log -n

sed -e option for multiple commands

sed -e '1p' -e '2p' -e '4p' t1.log -n

Using semicolon to chain commands

sed '1p;2p;4p' t1.log -n