Bash sed Command - Stream Editor

Using the sed Command

The sed command is a stream editor used to perform basic text transformations on an input stream (a file or input from a pipeline).

It's a powerful tool for making quick edits to files or streams of data.

All examples below use the example_text.txt file:

Hello World
Line 1
Line 2

Basic Usage

To replace the first occurrence of a pattern in a file, use sed 's/old/new/' filename:

sed 's/World/Bash/' example_text.txt
Hello Bash
Line 1
Line 2

Options

The sed command has options to change how it works:

• -i - Edit files directly without needing to save separately
• -e - Add the script to the commands to be executed
• -n - Don't automatically print lines
• -E - Use extended regular expressions
• -f - Add script from a file
• -l - Specify line length for l command

Edit Files In Place

The -i option allows you to edit files directly without needing to save separately.

Without this option, sed outputs the result to the standard output, and you must redirect it to a file to save changes.

cp example_text.txt copy.txt
sed -i 's/World/Bash/g' copy.txt
cat copy.txt
Hello Bash
Line 1
Line 2

This example edits a copy, so example_text.txt still says "Hello World" in the examples below.

On macOS, sed -i needs an empty string after it: sed -i '' 's/World/Bash/g' copy.txt.

Suppress Printing

The -n option suppresses automatic printing of pattern space.

By default, sed prints each line of input to the output. Using -n allows you to control which lines are printed, typically with the p command.

sed -n 's/World/Bash/p' example_text.txt
Hello Bash

Extended Regular Expressions

The -E option allows the use of extended regular expressions, which provide more powerful pattern matching capabilities than basic regular expressions.

Without this option, sed uses basic regular expressions. GNU sed also accepts -r, but -E works on both Linux and macOS.

sed -E 's/(World|Line)/Hello/g' example_text.txt
Hello Hello
Hello 1
Hello 2

Script from a File

The -f option allows you to add a script from a file, which is useful for executing complex or multiple sed commands.

Without this option, you must specify the script directly in the command line.

Content of script.sed file:

s/World/Bash/g

sed -f script.sed example_text.txt
Hello Bash
Line 1
Line 2

Specify Line Length

The -l option specifies the line length for the l command, which prints lines with non-printable characters.

This option is useful for formatting output when dealing with long lines.

sed -l 10 'l' example_text.txt
Hello Wor\
ld$
Hello World
Line 1$
Line 1
Line 2$
Line 2

The $ at the end of each line comes from the l command, which marks where each line ends. The \ shows where -l 10 wrapped a long line. Because -n is not used, each line is also printed normally.

Redirect Output to a File

To save the changes made by sed to a file, you can redirect the output to a new file. This is useful when you don't want to overwrite the original file.

sed 's/World/Bash/' example_text.txt > new_example_text.txt
cat new_example_text.txt
Hello Bash
Line 1
Line 2

Using sed for Advanced Text Processing

Sed can perform advanced text processing tasks. For example, sed 's/^/Prefix: /' example_text.txt adds a prefix to each line.

sed 's/^/Prefix: /' example_text.txt
Prefix: Hello World
Prefix: Line 1
Prefix: Line 2

Common Errors and Troubleshooting

When using sed, you might encounter errors such as:

• "sed: command garbled" - Check your command syntax.
• "sed: can't read file" - Ensure the file path is correct and accessible.

Debugging tips include using echo to print intermediate results and verify command logic.