Shell Programming and Scripting

program file contains:
echo $2 $1
input.in file contains:
a b

When doing:

Code:

./program < input.in > output.out

The output.out was empty
However, when doing:
./program `cat input.in` > output.out
The output.out has the expected output which was b a .

I feel so confused about that. What is the difference?

Quite simple: "$1" (2, 3, ...) are variables which are filled with COMMANDLINE ARGUMENTS, whereas what you are redirecting to/from is INPUT and OUTPUT respectively. These are simply two different things. If you tell someone to count all persons coming through door 1 and there will be 100 persons coming through door 2 he will still come up with 0.

What your program does is: take the first two arguments and print them to "stdout" in reversed order. Nothing at all is done with what comes from "stdin". Therefore the output file is empty: you passed no commandline arguments, therefore "$1" and "$2" are empty and this emptyness is written to the file "output.out".

Why your second try worked is simple: "cat input.in" is a useless use of cat, but will print the contents of input.in at the commandline, because it is surrounded by backticks (which should not be used either). Therefore what you wrote was equivalent to

Code:

./program a b > output.out

because this is what the shell will evaluate the whole backticks-construction to. This will fill the commandline variables "$1" and "$2" with some value ("a" and "b") and therefore something gets printed to "output.out" - just as i said above.

I hope this helps.

bakunin

Wow, that's excellent! Now I understand the differences. But, how should I modify my program to get the standard input from input.in?

Change the contents of program from:

Code:

echo $2 $1

to:

Code:

read a1 a2 junk; echo $a2 $a1

or, if the contents of input.in could contain characters with special meaning to the shell (including backslash (\) or starting with a minus sign (-):
IFS="" read -r a1 a2 junk; printf "%s %s\n" "$a2" "$a1"

Will this work for you?

Content of program:
awk '{print $2" " $1}'

then you execute it like..

./program < input.in > output

This will output as..

Code:

cat output
b a