# grep command
### The Linux `grep` command
The `grep` command is a very useful tool. When you master it, it will help you tremendously in your day to day coding.
> If you're wondering, `grep` stands for *global regular expression print*.
You can use `grep` to search in files, or combine it with pipes to filter the output of another command.
For example here's how we can find the occurences of the `document.getElementById` line in the `index.md` file:
```bash
grep -n document.getElementById index.md
```
Using the `-n` option it will show the line numbers:
```bash
grep -n document.getElementById index.md
```
One very useful thing is to tell grep to print 2 lines before and 2 lines after the matched line to give you more context. That's done using the `-C` option, which accepts a number of lines:
```bash
grep -nC 2 document.getElementById index.md
```
Search is case sensitive by default. Use the `-i` flag to make it insensitive.
As mentioned, you can use grep to filter the output of another command. We can replicate the same functionality as above using:
```bash
less index.md | grep -n document.getElementById
```
The search string can be a regular expression, and this makes `grep` very powerful.
Another thing you might find very useful is to invert the result, excluding the lines that match a particular string, using the `-v` option:
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