Linux Command sed and 3 Options

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The Linux command sed stands for “Stream Editor” and is used to read text streams, search for specific patterns, and modify or replace them. It’s a highly efficient tool for managing large text files, making it essential for developers and system administrators. In this post, we’ll explore the basic usage of the sed command and its key options.

Basic Structure of the Linux Command sed

The sed command is generally used in the following format:

sed [options] 'command' file
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Here, options are optional, while the command defines what action sed will perform. The most commonly used command is s (substitute), which finds a specific text pattern and replaces it with another.

Text Substitution

sed 's/old-text/new-text/' filename
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This command finds ‘old-text’ in the file and replaces it with ‘new-text’. If ‘old-text’ appears multiple times in a line, by default only the first occurrence in each line is replaced.

Hello, Jacob! Hello, Mary!
Hello, Jemma! Hello, Davidson!
Hello, John! Hello, Jack!
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If you use the command s/Hello/Hi/, only the first occurrence of “Hello” in each line will be replaced with “Hi.”

Figure 1. Replacing the first occurrence of "Hello" with "Hi" in each line using the Linux command sed
Figure 1. Replacing the first occurrence of “Hello” with “Hi” in each line using the Linux command sed

The g (global) Command

By default, the sed command only changes the first matching pattern in each line. However, if you want to replace all matching patterns in a line, you need to use the g command.

sed 's/old-text/new-text/g' filename
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By adding the g command at the end, all occurrences of ‘old-text’ in a line are replaced with ‘new-text’. Below is an example where every “Hello” in the example.txt file is replaced with “Hi.”

Figure 2. Replacing all occurrences of "Hello" with "Hi" using the Linux command sed
Figure 2. Replacing all occurrences of “Hello” with “Hi” using the Linux command sed

Key Options You Can Use with sed

The sed command is incredibly powerful, offering a variety of options. Here, we’ll introduce some frequently used ones.

The -i (in-place) Option

By default, sed doesn’t modify the original file but outputs the changes. However, by using the -i option, you can modify the original file directly.

sed -i 's/old-text/new-text/' filename
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This command directly replaces ‘old-text’ with ‘new-text’ in the original file. Note that when using the -i option, the file is immediately modified, so it’s a good idea to create a backup beforehand.

Figure 3. Directly modifying the original file using the Linux command sed with the -i option
Figure 3. Directly modifying the original file using the Linux command sed with the -i option

The -n (silent) Option

By default, the sed command outputs all lines of the file. However, if you use the -n option, it only outputs the lines where you’ve used the p (print) command.

sed -n 's/old-text/new-text/p' filename
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If you only use the -n option, the modified content won’t be printed. But when used together with the p command, only the lines where ‘old-text’ is replaced by ‘new-text’ will be printed.

Figure 4. Printing only the modified lines using the Linux command sed with the -n option and p command
Figure 4. Printing only the modified lines using the Linux command sed with the -n option and p command

The -e (multi-command) Option

When you need to execute multiple commands in sed, you can use the -e option.

sed -e 's/text1/text2/' -e 's/text3/text4/' filename
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This command first replaces ‘text1’ with ‘text2’, and then replaces ‘text3’ with ‘text4’. The -e option is useful when executing multiple commands in sequence.

Practical Examples

Modifying Specific Lines

If you want to modify only specific lines in a text file, you can specify the line number.

sed '2s/old-text/new-text/' filename
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This command changes ‘old-text’ to ‘new-text’ only on the second line of the file. By specifying the line number, you can easily modify a specific part of a large file.

Using Regular Expressions

sed supports regular expressions, allowing you to define more flexible patterns. For example, if you want to find and replace only numbers, you can use the following command:

sed 's/[0-9]/#/g' filename
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This command replaces all numbers in the file with #. Regular expressions are one of sed‘s most powerful features, enabling easy handling of complex pattern matching.

Figure 5. Replacing all numbers with "#" using regular expressions and the Linux command sed
Figure 5. Replacing all numbers with “#” using regular expressions and the Linux command sed

Backing Up Files While Modifying Them

When using the -i option, you can create a backup file by specifying an extension.

sed -i.bak 's/old-text/new-text/' filename
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This command creates a backup of the original file with the .bak extension before modifying it. It’s recommended to back up important files to avoid losing data due to accidental modifications. In this case, the original file will be saved as filename.bak.

Figure 6. Creating a backup file using the Linux command sed with the -i.bak option
Figure 6. Creating a backup file using the Linux command sed with the -i.bak option

Precautions When Using sed

  • Backup Recommended for File Modifications: When using the -i option to modify a file directly, always create a backup. Once sed is executed, it’s difficult to reverse the changes, so take extra care when dealing with important data.
  • Be Careful with Regular Expressions: When using regular expressions in sed, always ensure that the pattern behaves as expected. Special characters like . or * have specific meanings in regular expressions, so use \ to escape them if you want them treated as plain text.
  • Combine with Other Commands: While sed is powerful on its own, you can combine it with other commands like grep or awk to create even more robust scripts.

Summary

The Linux command sed is a powerful tool for modifying files and processing text data in Linux. It can handle a wide range of tasks, from simple text substitutions to complex transformations using regular expressions. By understanding the basic usage and key options discussed in this post, you can process text more efficiently.

Additionally, always practice safe habits by backing up your data, and consider using sed with other utilities to create more powerful automation scripts. sed is an indispensable tool in almost every Linux task involving text files.

References

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