Replacing Part of a String with sed

Oct 13, 2025 by wbrum.org 2 views

Introduction

Sed (Stream Editor) is a powerful command-line tool used for text processing and manipulation. It is widely used in Unix-like operating systems for tasks such as searching, replacing, and filtering text. In this article, we will explore how to use sed to replace only part of a string.

Understanding sed

Sed is a non-interactive command-line tool that reads input from a file or standard input, performs operations on the input, and writes the output to standard output. The basic syntax of sed is as follows:

sed 'command' input_file

Where command is the sed command to be executed, and input_file is the file from which sed reads the input.

Replacing Part of a String with sed

To replace part of a string with sed, we use the s command, which is used for substitution. The basic syntax of the s command is as follows:

sed 's/old_string/new_string/' input_file

Where old_string is the string to be replaced, and new_string is the string that replaces the old string.

Example: Replacing "bobearl" with "jim"

Let's consider the following string:

"billy" "bobearl" and "johnny"

We want to replace "bobearl" with "jim". We can use the following sed command:

sed 's/bobearl/jim/' /tmp/text.txt

However, this will replace all occurrences of "bobearl" in the string, including the "bob" in "bobearl". If we want to replace only the "bob" part of "bobearl", we can use the following sed command:

sed 's/bob/jim/' /tmp/text.txt

This will replace only the "bob" part of "bobearl" with "jim", resulting in the following output:

"billy" "jimarl" and "johnny"

Using sed with Regular Expressions

Sed supports regular expressions, which are powerful patterns used to match and manipulate text. Regular expressions can be used to match specific parts of a string, making it easier to replace only part of a string.

For example, to replace only the "bob" part of "bobearl" with "jim", we can use the following sed command:

sed 's/bob\?/jim/' /tmp/text.txt

The \? in the regular expression matches a single character (in this case, the "b" in "bob"), making it possible to replace only the "bob" part of "bobearl".

Using sed with Capture Groups

Sed also supports capture groups, which are used to capture and reference specific parts of a string. Capture groups can be used to replace only part of a string by referencing the captured group in the replacement string.

For example, to replace only the "bob" part of "bobearl" with "jim", we can use the following sed command:

sed 's/${bob}$/jim/' /tmp/text.txt

The ${ and }$ in the regular expression create a capture group, which captures the "bob" part of the string. The ${bob}$ in the replacement string references the captured group, making it possible to replace only the "bob" part of "bobearl".

Conclusion

In this article, we explored how to use sed to replace only part of a string. We discussed the basic syntax of sed, the s command, and how to use regular expressions and capture groups to replace only part of a string. We also provided examples of how to use sed to replace "bobearl" with "jim" in a string.

Best Practices

When using sed to replace only part of a string, it is essential to use regular expressions and capture groups to ensure that only the desired part of the string is replaced. It is also essential to test the sed command thoroughly to ensure that it produces the desired output.

Common sed Commands

Here are some common sed commands that can be used to replace only part of a string:

s/old_string/new_string/: Replaces the old string with the new string.
s/old_string/new_string/g: Replaces all occurrences of the old string with the new string.
s/${old_string}$/new_string/: Replaces the old string with the new string, using a capture group to reference the old string.
s/${old_string}$/new_string/g: Replaces all occurrences of the old string with the new string, using a capture group to reference the old string.

sed Command Options

Here are some common options that can be used with sed commands:

-n: Suppresses the output of the sed command.
-r: Enables extended regular expressions.
-E: Enables extended regular expressions.
-i: Edits the input file in place.
-e: Specifies a sed command to be executed.

sed Command Examples

Here are some examples of sed commands that can be used to replace only part of a string:

sed 's/bob/jim/' /tmp/text.txt: Replaces all occurrences of "bob" with "jim" in the file /tmp/text.txt.
sed 's/bob/jim/g' /tmp/text.txt: Replaces all occurrences of "bob" with "jim" in the file /tmp/text.txt.
sed 's/${bob}$/jim/' /tmp/text.txt: Replaces all occurrences of "bob" with "jim" in the file /tmp/text.txt, using a capture group to reference the old string.
sed 's/${bob}$/jim/g' /tmp/text.txt: Replaces all occurrences of "bob" with "jim" in the file /tmp/text.txt, using a capture group to reference the old string.

sed Command Best Practices

Here are some best practices to keep in mind when using sed commands:

Use regular expressions and capture groups to ensure that only the desired part of the string is replaced.
Test the sed command thoroughly to ensure that it produces the desired output.
Use the -n option to suppress the output of the sed command.
Use the -r or -E option to enable extended regular expressions.
Use the -i option to edit the input file in place.
Use the -e option to specify a sed command to be executed.
sed Q&A: Frequently Asked Questions =====================================

Q: What is sed?

A: Sed (Stream Editor) is a powerful command-line tool used for text processing and manipulation. It is widely used in Unix-like operating systems for tasks such as searching, replacing, and filtering text.

Q: What is the basic syntax of sed?

A: The basic syntax of sed is as follows:

sed 'command' input_file

Where command is the sed command to be executed, and input_file is the file from which sed reads the input.

Q: What is the `s` command in sed?

A: The s command in sed is used for substitution. It is used to replace a specified pattern with a replacement string.

Q: How do I use the `s` command in sed?

A: The basic syntax of the s command is as follows:

sed 's/old_string/new_string/' input_file

Where old_string is the string to be replaced, and new_string is the string that replaces the old string.

Q: How do I use regular expressions in sed?

A: Regular expressions can be used in sed to match and manipulate text. They are specified in the s command using the following syntax:

sed 's/old_string/new_string/' input_file

Where old_string is a regular expression that matches the string to be replaced.

Q: How do I use capture groups in sed?

A: Capture groups can be used in sed to capture and reference specific parts of a string. They are specified in the s command using the following syntax:

sed 's/${old_string}$/new_string/' input_file

Where old_string is the string to be replaced, and new_string is the string that replaces the old string.

Q: How do I use the `-n` option in sed?

A: The -n option in sed is used to suppress the output of the sed command.

Q: How do I use the `-r` or `-E` option in sed?

A: The -r or -E option in sed is used to enable extended regular expressions.

Q: How do I use the `-i` option in sed?

A: The -i option in sed is used to edit the input file in place.

Q: How do I use the `-e` option in sed?

A: The -e option in sed is used to specify a sed command to be executed.

Q: What are some common sed commands?

A: Some common sed commands include:

s/old_string/new_string/: Replaces the old string with the new string.
s/old_string/new_string/g: Replaces all occurrences of the old string with the new string.
s/${old_string}$/new_string/: Replaces the old string with the new string, using a capture group to reference the old string.
s/${old_string}$/new_string/g: Replaces all occurrences of the old string with the new string, using a capture group to reference the old string.

Q: What are some common sed options?

A: Some common sed options include:

-n: Suppresses the output of the sed command.
-r: Enables extended regular expressions.
-E: Enables extended regular expressions.
-i: Edits the input file in place.
-e: Specifies a sed command to be executed.

Q: How do I troubleshoot sed commands?

A: To troubleshoot sed commands, you can use the following steps:

Check the syntax of the sed command.
Check the regular expressions used in the sed command.
Check the capture groups used in the sed command.
Test the sed command on a small input file.
Test the sed command on a larger input file.

Q: How do I optimize sed commands?

A: To optimize sed commands, you can use the following steps:

Use regular expressions to match and manipulate text.
Use capture groups to capture and reference specific parts of a string.
Use the -n option to suppress the output of the sed command.
Use the -r or -E option to enable extended regular expressions.
Use the -i option to edit the input file in place.

Q: How do I use sed with other tools?

A: Sed can be used with other tools such as grep, awk, and perl. You can use sed to preprocess the input file for these tools, or to postprocess the output of these tools.

Q: How do I learn more about sed?

A: To learn more about sed, you can use the following resources:

The sed manual page.
The sed documentation.
Online tutorials and courses.
Books and articles about sed.