grep command to find file contents

grep command to find file contents

grep is a command commonly used on the Linux command line to find and filter the contents of text files. The simplest usage is:

grep apple fruitlist.txt

Find the content containing apple in the fruitlist.txt file.

If you want to ignore case, you can use the -i parameter:

grep -i apple fruitlist.txt

If you want to search all files in the directory, including subdirectories, and display the line in the results No., you can use the following command:

grep -nr apple *

grep’s syntax supports regular expressions. Regular expressions are a bit complicated and will be explained later. The following are some useful parameters:

-A num, --after-context=num: Output the num lines after the matching line in the result at the same time

-B num, - -before-context=num: Also output num lines before the matching line in the result. Sometimes we need to display several lines of context.

-i, --ignore-case: Ignore case

-n, --line-number: Display line number

-R, -r, -- recursive: recursively search subdirectories

-v, --invert-match: output no matching lines

We can make grep more powerful through pipeline operations, pipeline operations It is to use the output of the previous command as the input of the next command, thereby combining many simple commands to complete complex functions. For example, if we want to find lines containing apple, but want to filter out pineapple, we can use the following command:

grep apple fruitlist.txt | grep -v pineapple

If we want to save the search results, we can redirect the standard output of the command to File:

grep apple fruitlist.txt | grep -v pineapple > apples.txt

The difference between the redirection symbol> and the pipe operation symbol| is that the redirection is followed by a file, and no more files or commands can be followed after it; while the pipe operation is followed by Orders can be continued indefinitely. If you want to append to a file, use >>. Pipeline operation is a philosophy of the Linux command line. It is one of the few technologies in computer technology that has been used for decades. Through pipeline operations, one line of commands can complete text processing functions that thousands of lines of programs under Windows cannot.

Recommended learning: Linux operating system tutorial

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