How to Use the grep Command in Linux?

The `grep` command in Linux is a useful tool when you want to search for specific patterns in text files, code, logs, or even system files. For example, you can find a particular word or a phrase. You need not manually look through pages of code or logs for that. Also, you don’t have to waste time scrolling through tons of information.

`grep` scans the content and helps you quickly locate exactly what you want.

You can search through large files or even multiple files at once. Moreover, this command is a great help when troubleshooting or debugging something in your system. All in all, you get a magnifying glass for text in the form of ‘grep’.

Let us now understand how to use it practically in your projects.

The basic format of the grep command

As discussed, the grep command is a tool in Linux with which you need to find a certain string or text quickly within massive files. The basic syntax for using grep is very simple. It looks like this –

$ grep [options] pattern [file...]

Here,

The ‘pattern’ is the text you are trying to find
[file…] refers to the file or files in which you want to search.

Let’s look at the example of one of the most common uses of grep, i.e. to search through files. Run the below command to search for the matching results related to the ‘Cantech’ string in the ‘file.txt’ file –

$ grep "Cantech" file.txt

This command can be used with various options that modify how the search works. Let’s discuss them with Examples.

‘grep’ Command Options with Examples

You can use the -i option to make the search case-insensitive. So, it does not matter if the text is uppercase or lowercase.
Example – The below command shows all the lines in file.txt where the word “Cantech” appears; adding the -i option ignores case sensitivity –
```
$ grep -i "Cantech" file.txt
```
You can reverse the search, too. You can use the -v option to show all lines that do not contain the pattern or string.
For example, the below command gives all lines that do not have the string Cantech.
```
$ grep -v "Cantech" file.txt
```
The -c option will count the number of lines that match your pattern.
For example, the below command will count the number of lines with Cantech text.
```
$ grep -c "Cantech" file.txt
```
Use the -l option to get the list of directory files matching a certain file name order.
For example – List files with the string ‘Hi’ in the working directory.
```
$ grep -l "Hi" *.txt
```
Similarly, -n shows the line numbers with matching results
Example – the below command shows all lines matching the ‘Cantech’ string with respective line numbers.
```
$ grep -n "Cantech" file.txt
```
The -r option searches recursively in directories.
Example- find ‘Hi’ in all files in a certain directory and its sub-directories.
```
$ grep -r "Hi" /path/to/directory
```
Tailor your search to match whole words only with the -w option.
E.g. find the string Cantech as a whole word in file.txt
```
$ grep -w "Cantech" file.txt
```
Other options –
-x option will help you to match a full line.
Working with logs or large text files? You can use context options like -A, -B, and -C to display additional lines after, before or around your match, respectively.
** You can use more command options with the ‘man grep’ command. You will get the grep manual page with this.

Regular Expressions (Regex)

You can make the grep command more powerful using regular expressions. Regular expressions (regex) help in defining complex search patterns. You can find patterns that match a specific format and not just a single word or phrase. This is useful when scanning structured data where specific values appear at fixed positions.

=> caret symbol (^) helps search for a word that appears at the beginning of a line. So, grep looks only at the start of each line.

For example, get all lines that start with “Cantech” with the below command.:

$ grep "^Cantech" file.txt

=> Similarly, you can find something that appears at the end of a line with the dollar symbol ($). The below Example command will match all lines that end with the word “Hosting.”.

$ grep "Hosting.$" file.txt

=> grep can also search for numbers within a file. Thus, you need not type each number manually, instead, you can use [0-9] to match any digit between 0 and 9. For example, the below command will find and display all lines that contain at least one digit. –

$ grep "[0-9]" file.txt

Advanced Uses of grep

Now, let’s start combining different options and use ‘grep’ for more complex searches. The below instances will show the relat power of ‘grep’ –

=> You can find multiple words in a file using the -e option that defines multiple search terms. Thus, it becomes easier to search for variations of names or keywords. For example, the below command will display lines that have “Cantech” or “Canttech” in them –

$ grep -e "Cantech" -e "Canttech" file.txt

=> Sometimes, you can filter out specific details from another command’s output. For that, you can combine grep with other commands. For instance, to filter all running processes and get results for a specific process (in the below case it is process_abc), you can use:

$ ps aux | grep "process_abc"

This will show only the processes that contain the given name. This is a quick way to check if a particular process is running on your system.

=> Use grep command with the find command to search for a specific string inside all files within a directory. For example, the below command will go through each file in the given directory and look for occurrences of the word “cantech”. It helps when you are unsure about which file contains the data you need.

$ find /path/to/directory -type f -exec grep "cantech" {} +

grep to Display Context

When just seeing the matching line is not enough, and you may want to see the surrounding lines for better understanding, the -C option helps. The below command shows the three lines before and after the matching text. Thus, it provides a few extra lines around each match. This is great for searching through logs or configuration files.

$ grep -C 3 "Cantech" file.txt

Searching Inside Compressed Files

Using grep, you can search for a specific word in compressed files without extracting them first. This tool works just like grep but is meant for .gz files. For example, the below can search inside compressed logs or archived files without needing to extract them.

$ zgrep "pattern" file.gz

Excluding Certain Files

You may want to ignore certain types of files when you are searching in directories. You can use the –exclude option for this. This is useful when working in directories that contain unnecessary files that you do not want in your search results.

For example, you are searching through a folder but do not want to include log files. Thus, the below command ensures that all .log files are ignored while searching.

$ grep --exclude="*.log" -r "pattern" /path/to/directory

Conclusion

We discussed the primary grep command usage. Thus, it is evident that it is not just a simple search tool; it is much more than that. It is an essential part of working efficiently in a Linux environment. You can search and manipulate files and directories for text and strings efficiently with it.