Powerful grep command

There are a lot of explanations about the grep command on the Internet, and they are basically a rough translation of the man command. To be honest, I will forget it after reading it! Personally, I think the best way to learn commands is to simulate the real environment, write down some command combinations that may be used, and then slowly digest them. At least that's how I came here. At the beginning, it was basically rote memorization, and then I slowly expanded my horizons. As time went by, I accumulated more. Okay, let’s get to the point and talk about the awesome grep command.

grep is the abbreviation of general regular expression parser. We can simply understand it as a search command in the Linux system. Next, let’s go directly to the example:

1. Search for the specified string

In the /etc/passwd file, search for the string guolei:

grep 'guolei' /etc/passwd

Note that the quotation marks for guolei can be omitted. But if there are spaces in the search string or you use regular expressions, you need to add them.

2. Search for the specified string in multiple files

In the current directory, search for files containing the string guolei:

grep -r guolei *

Note: -r is the abbreviation of recursive, which means recursive search.

In the .java file in the current directory, search for files containing the string guolei:

grep -r guolei *.java

Sometimes, our search results may be more, we can combine the less command to display the results

grep -r guolei *.java | less

or there are more search results, We only need to list the file names:

grep -rl guolei *.java

There is another requirement that is more common. We often want to find files containing a specified string in a certain directory. Note that the above command cannot be searched recursively. For example, we want to recursively search for all .java files in the current directory that contain the string guolei:

find . -type f -name *.java -exec grep -il guolei {} \;

3. Ignore case when searching

When searching for guolei, ignore case:

grep -ri guolei *

Note :-i is the abbreviation of Ignore case, which means ignoring case.

4. List the line number in the search results

In the search results, list the line number where the string appears:

grep -rn guolei *.java

Note that -n is the abbreviation of number, which means the line number.

5. Reverse search

In actual development, there is another situation that is more common. We need to search for files that do not contain a certain string in a certain directory:

grep -riv guolei * | less

Note: -v is reverse The abbreviation of means reverse. The above example searches for files that do not contain guolei in the current directory.

6. Use grep in pipelines

We often also use grep in pipeline commands, this is the most common. For example, we want to search for the mysql process in the current system:

ps -ef | grep mysql

or list files ending with html in the current directory:

ls | grep 'html$'