In Ubuntu (and most Linux systems), grep is a highly practical text search tool. It helps us quickly find content containing specific keywords in files or filter information from command outputs. Even if you’re new to Linux, you can easily get started with it.
I. What is grep?¶
In simple terms, grep stands for “Global Regular Expression Print.” Its core function is to search for matching lines in text. You can think of it as a “text filter” that precisely locates information using keywords.
II. Basic Usage: Simplest Text Search¶
Syntax¶
grep "keyword" filename
Example¶
Suppose you have a file named test.txt with the following content:
I love Ubuntu!
Ubuntu is great.
Learning Linux is fun.
ubuntu is user-friendly.
To find lines containing “ubuntu”, run:
grep "ubuntu" test.txt
Output:
I love Ubuntu!
ubuntu is user-friendly.
Note: By default, grep is case-sensitive (i.e., “Ubuntu” and “ubuntu” are treated as different). We’ll cover case-insensitive search in the parameters section below.
III. Common Parameters: Make Search More Flexible¶
grep offers many parameters to customize search rules. Here are the most commonly used ones for beginners:
1. Ignore Case (-i)¶
When the case of the keyword is uncertain (e.g., “Ubuntu” vs. “ubuntu”), use the -i parameter to ignore case.
grep -i "ubuntu" test.txt
Output:
I love Ubuntu!
ubuntu is user-friendly.
(The -i flag treats “Ubuntu” and “ubuntu” as matching.)
2. Show Line Numbers (-n)¶
To find out which line a keyword is on, use the -n parameter.
grep -n "is" test.txt
Output:
2:Ubuntu is great.
3:Learning Linux is fun.
(Line numbers are shown before each matching line for easy location.)
3. Invert Match (-v)¶
To exclude lines containing the keyword and only show non-matching content, use -v.
grep -v "is" test.txt
Output:
I love Ubuntu!
(The original file has “is” in lines 2 and 3, so only line 1 is printed.)
4. Show Only Matches (-o)¶
If you only want to see the keyword itself (not the entire line), use -o.
grep -o "Ubuntu" test.txt
Output:
Ubuntu
ubuntu
(Only the matched “Ubuntu” and “ubuntu” are displayed, one per line.)
5. Count Matching Lines (-c)¶
To quickly count how many lines contain the keyword, use -c.
grep -c "Ubuntu" test.txt
Output:
2
(There are 2 lines containing “Ubuntu” in the original file.)
IV. Advanced Tips: Make grep More Powerful¶
1. Recursive Directory Search (-r)¶
To search all files in a folder and its subdirectories, use -r (recursive).
grep -r "error" ./my_project # Search all files in the ./my_project directory
Parameter Note: ./my_project specifies the target directory, and . represents the current directory.
2. Search Multiple Files¶
To search multiple files at once, simply list the filenames:
grep "python" file1.txt file2.txt file3.txt
grep will search each file and display the filename along with matching lines.
3. Use with Pipes (|)¶
grep can work with other commands to filter outputs. For example, list all files and filter those containing “txt”:
ls | grep "txt"
Principle: The output of ls is piped to grep, which searches for “txt” in the list.
V. Summary¶
grep is the most basic and practical text search tool in Ubuntu. Master these key points to get started quickly:
- Basic usage: grep "keyword" filename
- Core parameters: -i (ignore case), -n (show line numbers), -v (invert match), -r (recursive search)
- Common scenarios: Quickly locate file content, filter command outputs, and count matching lines
Practice combining different parameters, and you’ll find grep significantly improves text processing efficiency! For further learning, you can explore regular expressions (e.g., matching emails or IPs), but the basic usage already solves most daily needs.