Cheatsheet & Examples: sed

The sed command is a stream editor used to perform basic text transformations on an input stream (like a file or input from a pipeline). It can search, replace, delete, insert, and manipulate text based on patterns or line numbers.

Replace a specific pattern in a file

Example Usage: sed 's/pattern/replacement/' file.txt

What it does: Replaces the first occurrence of pattern with replacement on each line of the input file.

Command-line Arguments Explained:

• s: The substitute command, which replaces text.
• pattern: The text or regular expression to search for.
• replacement: The text to replace the matched pattern.

Delete lines matching a pattern

Example Usage: sed '/pattern/d' file.txt

What it does: Deletes all lines that match the specified pattern.

Command-line Arguments Explained:

• /pattern/: The text or regular expression to search for.
• d: The delete command.

Insert text before a matching line

Example Usage: sed '/pattern/i\New line' file.txt

What it does: Inserts a new line New line immediately before every line that matches pattern.

Command-line Arguments Explained:

• /pattern/: The text or regular expression to search for.
• i\: The insert command, followed by the text to add.

Append text after a matching line

Example Usage: sed '/pattern/a\New line' file.txt

What it does: Appends a new line New line immediately after every line that matches pattern.

Command-line Arguments Explained:

• /pattern/: The text or regular expression to search for.
• a\: The append command, followed by the text to add.

Substitute all occurrences in a line

Example Usage: sed 's/pattern/replacement/g' file.txt

What it does: Replaces all occurrences of pattern with replacement on each line of the input file.

Command-line Arguments Explained:

• s: The substitute command.
• g: The global flag, which ensures all matches in the line are replaced.

Print specific lines

Example Usage: sed -n '3p' file.txt

What it does: Prints only the third line of the input file (suppresses default output).

Command-line Arguments Explained:

• -n: Disables default output (quiet mode).
• 3p: The address 3 and the print command p.

In-place editing of a file

Example Usage: sed -i 's/foo/bar/' file.txt

What it does: Modifies the file file.txt in-place, replacing foo with bar on each line.

Command-line Arguments Explained:

• -i: Edits the file directly (in-place).
• s/foo/bar/: The substitute command.

Delete blank lines

Example Usage: sed '/^$/d' file.txt

What it does: Removes empty lines from the input file.

Command-line Arguments Explained:

• /^$/: Regular expression matching empty lines.
• d: Delete command.

Apply multiple commands

Example Usage: sed -e 's/foo/bar/' -e 's/one/two/' file.txt

What it does: Applies multiple substitution commands to the input file.

Command-line Arguments Explained:

• -e: Specifies an additional script to apply.
• s/foo/bar/: First substitution command.
• s/one/two/: Second substitution command.

Print lines containing a specific pattern

Example Usage: sed -n '/error/p' file.txt

What it does: Prints only the lines that contain the word error.

Command-line Arguments Explained:

• -n: Suppresses default output.
• /error/: The pattern to match.
• p: Print command.

Delete all lines except those matching a pattern

Example Usage: sed '/pattern/!d' file.txt

What it does: Deletes all lines that do not match the specified pattern.

Command-line Arguments Explained:

• /pattern/: The address for lines to keep.
• !: Inverts the address match.
• d: Delete command.

Quit after a specific line

Example Usage: sed '/pattern/q' file.txt

What it does: Exits immediately after the first line matching pattern is found.

Command-line Arguments Explained:

• /pattern/: The text or regular expression to search for.
• q: Quit command.

Substitute using backreferences

Example Usage: sed 's/\(hello\) world/\1/' file.txt

What it does: Replaces hello world with hello, using a backreference to capture groups.

Command-line Arguments Explained:

• s: Substitute command.
• \(hello\) and \1: Capture group and backreference in the pattern.
• world: The text to replace.

Escape special characters in replacement

Example Usage: sed 's/\/pattern/\/replacement/' file.txt

What it does: Replaces /pattern with /replacement in the input file, properly escaping the slashes.

Command-line Arguments Explained:

• s: Substitute command.
• \/pattern and \/replacement: Escaped slashes in the pattern and replacement.

Substitute with case-insensitive matching

Example Usage: sed 's/pattern/replacement/i' file.txt

What it does: Replaces pattern with replacement, ignoring case in the match.

Command-line Arguments Explained:

• s: Substitute command.
• i: Case-insensitive flag for the substitution.

Print line numbers with content

Example Usage: sed '=' file.txt

What it does: Prints line numbers before each line of the input file.

Command-line Arguments Explained:

• =: Print the line number of each line.

Replace text in a range of lines

Example Usage: sed '3,5s/foo/bar/' file.txt

What it does: Replaces foo with bar in lines 3 to 5 of the input file.

Command-line Arguments Explained:

• 3,5: The range of lines to apply the command to.
• s/foo/bar/: The substitute command.

Use a script file

Example Usage: sed -f script.sed file.txt

What it does: Reads and executes commands from the script file script.sed.

Command-line Arguments Explained:

• -f: Specifies the script file to use.
• script.sed: The file containing sed commands.
• file.txt: The input file to process.

Substitute using a new line character

Example Usage: sed 's/pattern/replacement\n/' file.txt

What it does: Replaces pattern with replacement followed by a newline.

Command-line Arguments Explained:

• s: Substitute command.
• replacement\n: Includes a newline in the replacement text.