sed

Understanding sed’s Basics

sed operates on a line-by-line basis. Its general syntax is:

sed [options] 'command' input_file

Where:

• [options]: Flags modifying sed’s behavior (e.g., -i for in-place editing).
• command: The action sed performs (e.g., substitution, deletion).
• input_file: The file sed processes. If omitted, sed reads from standard input.

Let’s start with some fundamental commands.

Substitution: The Workhorse of sed

The s command is arguably sed’s most used feature. It allows for text substitution. The syntax is:

s/pattern/replacement/flags

• pattern: The text to search for.
• replacement: The text to substitute.
• flags: Optional modifiers (explained below).

Example 1: Replacing a single occurrence:

Let’s say you have a file named my_file.txt containing:

This is a sample line.
This is another sample line.

To replace the first occurrence of “sample” with “example” on each line:

sed 's/sample/example/' my_file.txt

Output:

This is a example line.
This is another example line.

Example 2: Replacing all occurrences:

To replace all occurrences of “sample” with “example”, use the g flag:

sed 's/sample/example/g' my_file.txt

Output:

This is a example line.
This is another example line.

Example 3: Using delimiters:

If your pattern or replacement contains slashes, you can use a different delimiter. For example:

sed 's#This is a/#A different beginning:#' my_file.txt

Deletion: Removing Lines and Parts of Lines

The d command deletes lines matching a pattern.

Example 4: Deleting lines containing “another”:

sed '/another/d' my_file.txt

This will remove the second line because it contains “another”.

Example 5: Deleting lines with specific numbers:

You can delete lines based on their line number using address ranges. For example, to delete lines 1 and 2:

sed '1,2d' my_file.txt

Appending and Inserting Text

The a command appends text after a matched line, and i inserts text before a matched line.

Example 6: Appending text after a line containing “example”:

sed '/example/a\ This line was appended.' my_file.txt

Example 7: Inserting text before a line containing “example”:

sed '/example/i\ This line was inserted.' my_file.txt

In-place Editing with -i

By default, sed outputs the modified text to standard output. To modify the file directly, use the -i option:

sed -i 's/sample/example/g' my_file.txt

Caution: Always back up your files before using -i as it modifies the original file directly.

Advanced sed Techniques: Using Regular Expressions

sed’s true power lies in its ability to use regular expressions. This allows for much more complex pattern matching and manipulation. For instance, to replace all occurrences of one or more whitespace characters with a single space:

sed 's/[[:space:]]\+/ /g' my_file.txt

This is just a glimpse into the capabilities of sed. Experiment with different commands and options to realize its full potential for efficient text processing in your Linux workflows. Further exploration of regular expressions will improve your proficiency with this powerful tool.