How do I return from an awk script back to the command line?

Question

I have a simple awk script, test1.awk:

{
    x=temp;
    print x;
}

When I run it from the Command Prompt with:

awk -v temp=hello -f test1.awk

I get just a cursor.  I hit Enter, it writes “hello” and gives another cursor, hit Enter again and it writes “hello” again, etc.  Just goes in an endless loop requiring input from the user, and never comes back to the command prompt.

D:\>awk -v temp=hello -f test1.awk

hello

hello

hello

hello
^C
D:\>

How do I get it write “hello” (without requiring user input) and then stop?

Answer 1

Awk reads from stdin. When you run awk as you are doing above, the awk script is going to process stdin. You did not pass it in any file to process. If you have a file, test.in for example, you can run your awk program as follows:

awk -v temp=hello -f test1.awk test.in

The awk script will process test.in and exit.

Hope this helps.

Answer 2

To avoid a pointless input file, you can put the commands in the END clause:

END { x=temp; print x; }

Then call the awk script with /dev/null for input, as suggested by Lewis M:

awk -v temp=hello -f test1.awk </dev/null

Or equivalently in one line:

awk -v temp=hello 'END {x=temp; print x;}' </dev/null

Answer 3

You say you want to run some awk statement(s) once, without requiring user input, and then stop and return to the command prompt.  And in your case you are talking about the Command Prompt (cmd.exe in Windows), and not a shell (like bash).

1. Matthias Gralle’s answer,

   awk -v temp=hello 'END {x=temp; print x;}' < /dev/null

   will work in Unix / Linux, and maybe even in a Unix-like shell in Windows, but not in cmd.exe.  Windows doesn’t have a /dev/null, per se.  But Windows does have a null device, called simply NUL.  (You may add a colon, resulting in NUL:, if you want; it’s optional.  And, as is typical in Windows, this is case-insensitive, so you can say nul.)  So

   awk -v temp=hello 'END {x=temp; print x;}' < nul

   will probably work.

   You don’t say what flavor of awk you have on your Windows machine.  I have Cygwin, and it turns out that I can do

   C:\Users\scott> \cygwin64\bin\gawk -v temp=hello 'END {x=temp; print x;}' /dev/null
   hello

   The difference is that, when you say < filename, the command processor (cmd.exe, in your case) processes the filename, but, if you leave out the < and say just filename, the program processes it.  And it turns out that Cygwin programs interpret /dev/null as being the null device, even if they’re being run from Command Prompt.

   Since I don’t know what flavor of awk you have on your Windows machine, I don’t know whether this will work for you.

2. Matthias came very close to giving you a really good answer.  As Lewis M explained, awk processes input.  A typical awk program looks something like

   BEGIN { something to do (once) at the beginning of the program }
         { something to do on every line of input }
   /foo/ { something to do on every line of input that contains “foo” }
         { something else to do on every line }
               ︙
   END   { something to do (once) at the end of the program }

   where all of these parts are optional.  (And, as you demonstrated in your question, code blocks (a.k.a. actions) can be broken up into multiple lines.  And short things can be put together on the same line.)  Matthias’s idea is to put your statement(s) into the END code block, and then to provide a null input, so awk will start, execute your statement(s) once, and then exit.  Not that there’s anything wrong with that.™  But it turns out that, if your program consists of a BEGIN code block, and nothing else, then awk executes your statement(s) (once) without even trying to read any input.  So you can say

   awk -v temp=hello 'BEGIN {x=temp; print x;}'

   and not need to worry about providing / specifying any input.

3. Here’s an example of a trivial awk program that’s not too trivial:

   awk '{print "Hello", $0}'

   If you type this and then hit Enter, it will say Hello.  If you type world, it will say Hello world.  If you type Kitty, it will say Hello Kitty©.  As you know, you can get back to your command prompt by hitting Ctrl+C, which basically means “interrupt / stop processing”.  But a “nicer” thing to do is to hit Ctrl+D, which means “this is the end of the input”.  This is standard on Unix & Linux, but I can do the following (where the bold italic text is what I typed):

   C:\Users\scott> \cygwin64\bin\gawk '{print "Hello", $0} END {print "Good bye"}'
   sailor
   Hello sailor
   stranger
   Hello stranger
   Ctrl+D
   Good bye

   whereas, if I hit Ctrl+C, the END code doesn’t run.

   Again, if you’re using something other than Cygwin, I don’t know whether this will work for you.

---

Throughout the above examples, anything that I put between single quotes ('…') can be put into a file, which you then (as you know) invoke with -f filename.  On Unix, Linux and Cygwin, I prefer to put my awk programs into shell scripts, like

#!/bin/sh
awk '{print "Hello", $0}' "$@"

rather than use .awk files.  The "$@" lets you pass arguments to awk.  You might be able to do that in a Windows batch file with $*, but things are never easy in Windows.