Introduction to grep command
grep is a command-line tool in Linux used for searching a pattern of characters in a specific file. That pattern is called the regular expression. grep stands for Global Regular Expression Print. It prints all lines containing the pattern in a file. grep command is a useful tool to search through large text files.
Different examples to use grep command
The simple grep command requires two arguments: pattern to search and file name. grep is a case sensitive tool, you have to use correct case when searching through grep commands. It prints the whole line that contain matching patterns until the line breaks occur. If there are no any matching patterns, it prints nothing.
In this article, you will learn to use grep commands using different options to search a pattern of characters in files.
1. Use grep command to search a file
This is the most basic grep command to find a pattern of characters in a specific file.
$ grep pattern file_name
Sample Output:
You can also enclose a pattern in single or double inverted commas. It is useful when there are multiple words to search.
$ grep 'pattern' file_name
OR
$ grep "pattern" file_name
Sample Output:
2. Search multiple files using grep command
grep command can search through multiple files in a single line of code. To do so, you have to separate file names with a space. It prints every lines that contain pattern along with a file name.
$ grep pattern file_name1 file_name2 file_name3
Sample Output:
3. Perform case sensitive search using grep command
This is an important command in grep which allows you to search for strings pattern case insensitively. It prints the matched pattern having both lowercase and uppercase letters.
$ grep -i pattern file_name
Sample Output:
4. grep command to search whole words (exact word) only
Normally, grep prints every matching characters in a file. But with the help of this command, it only prints if the whole words are matched. When the whole word is not matched, it prints nothing.
$ grep -w pattern file_name
Sample Output:
5. Count the number of lines using grep command
grep -c command counts the number of lines that contain matching pattern in a file and prints it. It does not count the number of matches in a file.
$ grep -c pattern file_name
Sample Output:
6. Inverse the search in grep command
grep -v command can be used to inverse the search and print all lines that do not contain the matching pattern.
grep -v pattern file_name
Sample Output:
Note: In the above output, blank lines are also printed because they also do not contain 'com'.
7. grep command to print line number
grep -n prints the line number of the pattern that is matched. It is very useful command to locate characters in large files.
$ grep -n pattern file_name
Sample Output:
8. Print only the matched pattern with grep command
grep -o command prints only the matched pattern instead of the whole line. Normally, grep command prints the whole line that contain pattern till the line breaks.
$ grep -o pattern file_name
Sample Output:
9. Search all files in directory using grep command
grep command allows you to search all files in the current directory using asterisk(*). It does not search files that are in the sub-directories.
$ grep pattern *
Sample Output:
10. grep command to search in directories and sub-directories
This command searches the matches in all files in the current directory including its sub-directories. It also prints the exact path for the file in sub-directories.
$ grep -r pattern *
Sample Output:
11. grep command to print list of matching files only
grep -l command prints the file names only that contain the matching patterns instead of printing the whole line. It is a useful command when you want to know file names only.
$ grep -l pattern *
Sample Output:
Note:Â You can combine options in grep command to get the desired result. In the above output, -r and * are used to search files in the current directory and its sub-directories.
12. Print files name having unmatched patterns using grep command
This is just the opposite version of previous command. You can print the names of file that do not contain the matching patterns using grep -L command.
$ grep -L pattern *
Sample Output:
13. Stop reading a file after NUM matching lines with grep command
grep -m command prints the limited number of line that contains the matching patterns. grep command normally prints all matched patterns in a file.
It takes a number(NUM) as an argument along with it to print NUM lines. The first NUM lines with the match will only be printed.
$ grep -mNUM pattern file_name
Sample Output:
Note: If there are matched patterns in multiple files, it will print N lines of each file.
14. Take pattern from file using grep command
grep -f command allows you to take pattern from file. It takes pattern from each line.
grep -f pattern_file file_name
Sample Output:
15. Print filename along with the match in grep command
grep -H command prints the every line with file name that contain the matching patterns. By default, grep command only prints file names if there are multiple files.
$ grep -H pattern file_name
Sample Output:
16. Hide filename of the matched pattern with grep command
grep -h command hides the file name in the output. grep command normally shows file names when there are matched patterns in multiple files. You can use any commands that search in multiple files.
$ grep -h pattern file_name1 file_name2 file_name3
Sample Output:
17. Print lines before or after pattern match
You can print the lines that come before and after the matching patterns.
grep -A nprints the n lines after the match.grep -B nprints the n lines before the match.grep -C nprints the n lines before and after the match.
They also print the lines containing the match.
$ grep -A n pattern file_name
$ grep -B n pattern file_name
$ grep -C n pattern file_name
Sample Output:
18. Search lines that start with pattern using grep command
You can search specific lines that start with a pattern using grep command. It does not print other lines that contain the matching pattern elsewhere.
$ grep ^pattern file_name
Sample output:
19. grep command to search lines that end with matching pattern
You can also search specific lines that end with a pattern using grep command. You have to put dollar sign($) in the end of pattern.
$ grep pattern$ file_name
Sample Output:
Note: Any character at the end counts including spaces.
20. Search for multiple patterns with single command using grep command
grep -e command allows you to use multiple patterns at once. -e option indicates an expression in the grep command. Suppose, you need to search three different patterns in a file then you can use:
$ grep -e pattern1 -e pattern2 -e pattern3 file_name
Sample Output:
Conclusion
These are the most important grep commands that contain different options available. By using these commands, you can find the specific characters in large files easily. You can also combine different options in grep command to create variations in your search. So, grep is a powerful tool that can be used in the terminal of Linux systems.
What's Next
You can explore Easy regex to grep exact match with examples
Further Reading
