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This guide explores sed, the powerful stream editor used for text processing and pattern-based transformations on the command line. You’ll learn how sed combines pattern matching with built-in editing capabilities—making it a versatile tool for everything from simple substitutions to complex in-place file edits.

Comparing grep, awk, and sed

Before diving into sed, it helps to see how it relates to other common utilities:

# List disk usage
$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/root       7.7G  2.9G  4.9G  38% /
devtmpfs        486M     0  486M   0% /dev
...

# grep for "root"
$ df -h | grep "root"
/dev/root       7.7G  2.9G  4.9G  38% /

# Print second line with awk
$ df -h | awk 'NR == 2 { print }'
/dev/root       7.7G  2.9G  4.9G  38% /

Tool	Strengths	Limitations	Example
grep	Fast plain-text pattern matching	No built-in substitution	`grep "root" disk-usage.txt`
awk	Column/record data processing	Complex syntax for edits	`awk 'NR==2' disk-usage.txt`
sed	Stream editing, inline substitutions	Regex-based commands	`sed 's/foo/bar/g' file.txt`

sed Basics: Substitution Command

The core sed substitution syntax is:

sed 's/<pattern>/<replacement>/<flags>' [file]

s: substitute command
pattern: a regular expression to match
replacement: text to replace each match
flags: modifiers (e.g., g for global replacement on each line)

Example: Replace every sample with are in poem.txt:

$ sed 's/sample/are/g' poem.txt
Roses are red,
Violets are blue,
Sugar are sweet,
And so are you.

Breakdown:

s: substitution
/sample/: text to find
/are/: replacement text
g: apply to all matches on each line

In this tutorial, we’ll start with basic operations such as printing specific lines, then move on to advanced techniques like in-place editing and generating custom output.

The image is an introduction to "sed," a stream editor, highlighting topics like constructing sed commands, printing text from a file, and replacing string patterns in input data.

GNU vs. BSD sed

sed comes in two main flavors:

GNU sed: Default on most Linux distributions, supports extended features
BSD sed: Bundled with macOS, lacks some GNU-specific options

Most basic sed commands work across both implementations, but when you need advanced flags (e.g., -r for extended regex), GNU sed is the recommended choice.

Note

On macOS, install GNU sed via Homebrew:

brew install gnu-sed

Then use gsed instead of sed for full compatibility.

The image is a diagram titled "sed Introduction" showing two paths: one from MacOS to BSD Unix, and another from Linux to the GNU Project.

What's Next

Printing and deleting lines
Addressing ranges and patterns
In-place (-i) file edits
Advanced scripting with semicolons and labels

For more details, see the GNU sed Manual.

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