Grep Command in Unix // Linux: Filter and Pipe in Unix // Linux

You can connect two commands together so that the output of a program is the input of the next program. Two or more commands connected in this way form a pipe .

To create a pipe, place a vertical bar (|) on the command line between the two commands.

When a program receives input from another program, performs several operations on that input, and writes the output to the output, it is referred to as a filter .

The grep command in Unix / Linux

The grep program finds a file or files for the lines that have a certain pattern. Its syntax is:

 $ grep pattern file ( s ) 

The name grep derives from the ed command (UNIX line editor), g / re / p means "globally search for a regular expression and print all lines containing it" (search global for a regular expression and print all lines containing it)."

A regular expression is either some plain text or special characters used for pattern connections.

The simplest use of grep is to search for a template containing a single word. It can be used in a pipe so that only those lines of input files contain a given string sent to the standard output. If you don't give grep a file name to read, it reads the standard input, this is how all filters work.

 $ ls - l | grep "Aug" - rw - rw - rw - 1 john doc 11008 Aug 6 14 : 10 ch02 - rw - rw - rw - 1 john doc 8515 Aug 6 15 : 30 ch07 - rw - rw - r - 1 john doc 2488 Aug 15 10 : 51 intro - rw - rw - r - 1 carol doc 1605 Aug 23 07 : 35 macros $ 

There are different functions that you can use in parallel with the grep command:

Function Description -v Print all lines without matching the given pattern. -n Print matched lines and its line numbers. -l Print only the file name with matching lines. -c Only print the number of matched lines. -i Matches either with uppercase or lowercase letters.

Now, we use a regular expression that tells the grep command to find lines with "carol", followed by no more or more characters summarized in a regular expression as a "*" asterisk, then followed by "Aug".

Here we are using the -i function to have a case-sensitive search:

 $ ls - l | grep - i "carol. * aug" - rw - rw - r - 1 carol doc 1605 Aug 23 07 : 35 macros $ 

Sort command in Unix / Linux

The sort command sorts the lines of text in alphabetical or numerical order. The example below ranks the lines in a food file.

 $ sort food Afghani Cuisine Bangkok Wok Big Apple Deli Isle of Java Mandalay Sushi and Sashimi Sweet Tooth Tio Pepe 's Peppers $ 

Sort , by default, sorts the lines of text in alphabetical order. There are many functions that control this alignment:

Function Description -n Sort by numeric order, skip spaces and tabs. -r Inverse order of arrangement. -f Arrange uppercase and lowercase letters together. + x Ignore the first x fields when sorting.

There may be more than two commands connected in a pipe . The pipe example uses grep , we can sort files in a different way by editing the order of size in August.

The following pipe contains the commands: ls, grep, and sort :

 $ ls - l | grep "Aug" | sort + 4n - rw - rw - r - 1 carol doc 1605 Aug 23 07 : 35 macros - rw - rw - r - 1 john doc 2488 Aug 15 10 : 51 intro - rw - rw - rw - 1 john doc 8515 Aug 6 15 : 30 ch07 - rw - rw - rw - 1 john doc 11008 Aug 6 14 : 10 ch02 $ 

This pipe sorts all the files in your directory edited in order of size in August and prints them to the terminal screen. Function + 4n ignores 4 fields (fields that are distinguished by spaces), then sorts the lines in numerical order.

The pg commands and more in Unix / Linux

A long output often overflows the screen, but if you run text through more or pg as a filter, the display will stop after each screen is full of text.

We assume that you have a long directory list. To make it easier to read this sorted list, output the output via the more command as follows:

 $ ls - l | grep "Aug" | sort + 4n | more - rw - rw - r - 1 carol doc 1605 Aug 23 07 : 35 macros - rw - rw - r - 1 john doc 2488 Aug 15 10 : 51 intro - rw - rw - rw - 1 john doc 8515 Aug 6 15:30 ch07 - rw - rw - r - 1 john doc 14827 Aug 9 12 : 40 ch03 . . . - rw - rw - rw - 1 john doc 16867 Aug 6 15 : 56 ch05 - More - ( 74 %) 

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