In this tutorial, you will learn how to use sed, a stream editor, for modifying and substituting text in shell scripts. The sed command is a powerful tool that manipulates and transforms text, making it an invaluable resource for scripting and programming.
Throughout this tutorial, we will:
- Understand the basic syntax of sed
- Learn how to perform a simple text substitution
- Explore more complex text manipulations
Prerequisites: Basic knowledge of Linux command line is beneficial.
sed uses a simple syntax: sed 's/old/new/' filename. This command replaces the first occurrence of the word "old" with "new" in each line of the file "filename".
Here are some useful tips:
- To replace all occurrences in a line, add g at the end: sed 's/old/new/g' filename
- To replace on a specific line number, add the line number before s: sed '5s/old/new/' filename
- To replace within a range of lines, specify the range before s: sed '5,10s/old/new/' filename
Example 1: Simple text substitution
echo "Hello world" | sed 's/world/universe/'
This echo command outputs "Hello world", which is then piped into sed. sed replaces "world" with "universe". The output is "Hello universe".
Example 2: Replace all occurrences
echo "Hello world world" | sed 's/world/universe/g'
In this case, sed replaces both occurrences of "world" with "universe", resulting in "Hello universe universe".
Example 3: Replace on a specific line number
sed '2s/foo/bar/' filename
This replaces the first occurrence of "foo" with "bar" on the second line of the file.
In this tutorial, we have covered:
- The basic syntax of sed
- How to substitute text with sed
- How to perform more complex text manipulations
As next steps, you may want to learn about more advanced sed features such as pattern deletion and multi-line transformations. You can find more information in the sed documentation here.
Exercise 1:
Replace "apple" with "banana" in the following string:
"apple is red"
Exercise 2:
Replace the third occurrence of "cat" with "dog" in the following string:
"cat cat cat cat cat"
Exercise 3:
Replace "foo" with "bar" only on lines 10 to 20 in a given file.
Solutions:
1. echo "apple is red" | sed 's/apple/banana/' - This will output "banana is red".
2. echo "cat cat cat cat cat" | sed 's/cat/dog/3' - This will output "cat cat dog cat cat".
3. sed '10,20s/foo/bar/' filename - This will replace "foo" with "bar" only on lines 10 to 20 in the file.
Remember, practice is crucial for mastering sed. Try to incorporate it in your scripts and see the magic it can perform. Good luck!