sed — stream text substitution

💬 "sed command"

sed processes text as a stream to perform substitution and deletion. It is often used for batch changes to configuration files.

-i modifies files directly; to avoid mistakes, it is best to use a backup extension as well.

• Concept: Replaces or deletes matching parts of an input stream.
• s///: The basic form of the substitution command.
• -i: Modifies files directly.
• -n + p: Prints only specific lines.

Commonly used options

3 essential command combinations

• sed -i.bak 's/10.0.0.5/10.0.0.9/g' /etc/app/config.yml — Batch-change IPs in a configuration (with backup)
• sed -n '/^server_name/p' /etc/nginx/nginx.conf — Extract only specific configuration lines
• sed '/^#/d;/^$/d' /etc/app/config.yml | head — Remove comments and blank lines for viewing

💬 "Create an example that includes how to undo this operation"

Here are common situations on production servers, organized step by step.

Step 1 · Check the current value

Check the target string before changing it.

grep -n '10.0.0.5' /etc/app/config.yml

Step 2 · Replace (with backup)

Use -i.bak to keep the original while replacing.

sed -i.bak 's/10.0.0.5/10.0.0.9/g' /etc/app/config.yml

Step 3 · Verify

Check the result and confirm the service reflects it.

grep -n '10.0.0.9' /etc/app/config.yml && systemctl restart myapp

In practice, this is how it is used

1. Replace configuration IPs

Scenario — Change IPs in the configuration due to a database server migration.

sed -i.bak 's/10.0.0.5/10.0.0.9/g' /etc/app/config.yml

Result — All references are changed to the new IP, and the original remains in .bak.

Key point — Check the number of matches with grep before replacing to avoid omissions or over-replacement.

2. Compare settings by removing comments

Scenario — You want to see only the settings that actually apply.

sed '/^#/d;/^$/d' /etc/nginx/nginx.conf | head -30

Result — You see the actual settings with comments and blank lines removed.

Key point — Because the original is not modified, you can check safely.

Things to watch out for

• -i modifies files directly. Always include a backup extension.
• If the string to replace contains /, change the delimiter (e.g., s|a|b|g).
• Regular expression special characters need to be escaped.

Actual execution output

This is the result of batch-replacing configuration IPs with a backup and verifying them.

!sed command execution result: batch configuration replacement

Actual production server

Source (HTML): https://games.aidebatehub.com/programs/linux/automation/sed.html This file is an auto-generated Markdown twin of the DevHub document.