AWK as a filter (reading input from the Terminal)

Filter commands can take their input from stdin instead of reading it from the file. We can omit giving input filenames at the command line while executing the awk program, and simply call it from the Terminal as:

$ awk  'program'

In the previous example, AWK applies the program to whatever you type on the standard input, that is, the Terminal, until you type end-of-file by pressing Ctrl + D, for example:

$ awk  '$2==50{ print }'
apple 50
apple 50
banana 60
litchi 50
litchi 50
mango 55
grapes 40
pineapple 60
........

The line that contains 50 in the second field is printed, hence it's repeated twice on the Terminal. This functionality of AWK can be used to experiment with AWK; all you need is to type your AWK commands first, then type data, and see what happens next. The only thing you have to take care of here is to enclose your AWK commands in single quotes on the command line. This prevents the shell expansion of special characters, such as $, and also allows your program to be longer than one line.

Here is one more example in which we take input from the pipe and process it with the AWK command:

$ echo -e "jack \nsam \ntarly \njerry" | awk '/sam/{ print }' 

On executing this code, you get the following result:

sam

We will be using examples of executing the AWK command line on the Terminal throughout the book for explaining various topics. This type of operation is performed when the program (AWK commands) is short (up to a few lines).