AWK script input

Question

#!/bin/bash -f

awk '

BEGIN {
    print "type a number";
}
{
    printf "The square of %d is %d\n", $1, $1*$1;
    #print "The square of ", $1, " is ", $1*$1;
    print "type another number\n";
}
END {
    print "Done"
}'

Can anyone explain how this program takes input? I tried searching on the web but in vain. Grateful for any explanation given. Thanks.

input is CLI. Paste that in terminal, hit Enter: everything you tipe into stdin will be fed to the script when hitting Enter again. — Daemon Painter
– Daemon Painter, Commented May 12, 2020 at 13:21
If you're doing nothing but calling awk in your script, you might as well start it with a #!/usr/bin/awk -f (adjust for correct location of awk) shebang line and drop the single quotes; see gnu.org/software/gawk/manual/gawk.html#Executable-Scripts — Benjamin W.
– Benjamin W., Commented May 12, 2020 at 13:59
Awk will continue reading input and looping until end-of-file (ctrl-d in bash). — stark
– stark, Commented May 12, 2020 at 14:10

Quasímodo · Accepted Answer · 2020-05-12 16:02:35Z

2

The program takes input from stdin. So, after making it executable with chmod +x scriptname, you can either start it with ./scriptname and answer to each prompt, or redirect a file's content to stdin. For example, suppose this is numbers.txt:

4
7
1

Then execute the script this way:

$ ./scriptname < numbers.txt
type a number
The square of 4 is 16
type another number

The square of 7 is 49
type another number

The square of 1 is 1
type another number

Done

Why does that awk script read standard input? According to awk POSIX specification, section STDIN:

The standard input shall be used only if no file operands are specified, or if a file operand is '-', or if a progfile option-argument is '-';

Notice that awk was provided no file operand, so stdin is used.

edited May 12, 2020 at 16:02

answered May 12, 2020 at 13:56

Quasímodo

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2 Comments

FNF Over a year ago

Why does the program takes input from stdin and how is the program reading it? This might seem like a weird question but i want to understand the workings of the code.

Quasímodo Over a year ago

@FNF I've added the why. How is a deeper and totally different question that would be out of place in the answer. I refer you to How to signal the end of stdin input and How does a program detect the end of stdin input?, they may be illuminating.

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