What is Sed?
Sed is a text stream editor that performs basic text transformations on an input stream (a file or input from a pipeline) based on a set of specified commands. It’s particularly useful for tasks like find-and-replace operations, text substitution, line deletion or insertion, and more.
Basic Syntax
The basic structure of a Sed command is as follows:
sed ‘expression’ file
– Expression: The action or set of commands you want to perform on the input.
– File: The input file on which Sed should operate. If not specified, Sed reads from standard input.
Sed Examples
1. Find and Replace:
Let’s say you have a text file named `document.txt`, and you want to replace all occurrences of “old” with “new”.
sed ‘s/old/new/g’ document.txt
2. Delete Lines:
Suppose you want to remove lines containing a specific keyword, such as “obsolete”.
sed ‘/obsolete/d’ document.txt
3. Numbering Lines:
You can use Sed to add line numbers to a file.
sed = document.txt | sed ‘N;s/\n/\t/’
4. Appending and Inserting Lines:
Adding content at the beginning or end of a file can be achieved with Sed.
sed ‘1i This is the first line’ document.txt
sed ‘$a This is the last line’ document.txt
Automation with Sed: Script Example
Here’s a simple script named `process_logs.sh` that uses Sed to extract IP addresses from a log file:
#!/bin/bash
log_file=”access.log”
sed -nE ‘s/^([0-9]+\.[0-9]+\.[0-9]+\.[0-9]+).*/\1/p’ $log_file
Make the script executable (`chmod +x process_logs.sh`), and running it will extract and print IP addresses from the `access.log` file.
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