Search text strings within files

The grep command is used to search plain-text data. It can search a single file or a whole directory of files for lines that contain the specified string. If a match is found, the line containing the string is printed on the screen. The syntax of the grep command is:

grep [OPTIONS] PATTERN FILE

Here is an example. Let’s say that we have a text file called textfile.txt with the following content:

grep example file

To search textfile.txt for each occurence of the word text, we can use the following command:

grep command

Two occurences of the word text have been found in textfile.txt. The lines containing the word text have been printed on the screen.

Here is another example. To search the /home/pi/files/ directory (the working directory, in this case) for each occurence of the word text, we can use the following command:

grep directory command

As you can see from the output of the ls command, three files are stored in the /home/pi/files/ directory: file.txt, new-file.txt, and textfile.txt. The grep command has searched the whole working directory (hence the ./* string) and found and printed three matches of the word text.

The grep command is often used with commands that produce a lot of output, so you can sift through the output more easily. For example, to list all processes on the system, you can use the ps -A command:

ps a example

This command produces a lot of output – hundreds of lines of text, in fact. We can use the grep command along with ps -A to find the specific process we are looking for. For example, to search for a process started by the nano text editor, we can use the ps -A | grep nano command:

ps a grep example

Now we got only a single line of output – the line that contains the word nano. Note that the pipe (|) was used to send the output of the ps -A command to the grep command.

By default, the grep command searches the text in the case-sensitive way. To do case-insensitive searches, use the -i flag with the command.

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Geek University 2019