Trying to Understand Behavior of `Test ` in Bash

Question

Suppose I have this simple C program (test.c):

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
     exit (1);
}

Obviously, the exit code of this program is 1:

$ gcc test.c
$ ./a.out
$ echo $?
1

But when I run test ./a.out, the result of the test doesn't match the exit code:

$ test ./a.out
$ echo $?
0

So what is actually being tested? Why is the result of the test 0?

Accepted Answer

test is a Bash built-in, often invoked by the alternative name [.

The last command (test ./a.out) exits with status 0 indicating success because test ./a.out checks whether ./a.out as a string has one or more characters in it (is not an empty string), and because it isn't an empty string, returns success or 0. The test ./a.out command line does not execute your a.out program — as you could see by printing something from within your program.

As written, your program doesn't need the <stdio.h> header or the arguments to main() — it should be int main(void). You could lose <stdlib.h> too if you use return 1; instead of exit(1);:

int main(void)
{
    return 1;
}

To use the exit status in an if condition in the shell, just use it directly:

if ./a.out ; then
    echo Success
else
    echo Failure
fi

Rule of Thumb: Don't call C programs test because you will be confused sooner or later — usually sooner rather than later.

Answer 2

Your C program returns "1" to the shell (I'd prefer"return()" over exit()", but...)
If you wanted to actually run "a.out" in conjunction with the "*nix" test command, you'd use syntax like:
```
`./a.out` # classic *nix
```
or
```
$(./a.out) # Bash
```
If you did that, however, "test" would read the value printed to "stdout", and NOT the value returned by your program on exit.
You can read more about test here:
Here is an example:

C program:

#include <stdio.h>

int main (int argc, char *argv[]) {
  printf("%d\n", argc);
  return 2;
}

Shell script:

echo "Assign RETVAL the return value of a.out:"
./a.out RETVAL=$? echo " " RETVAL=$RETVAL

echo "Assign RETVAL the value printed to stdout by a.out:"
RETVAL=$(./a.out) echo " " RETVAL=$RETVAL

echo "Turn an 'trace' and run a.out with 'test':" 
set -x 
if [ $(./a.out) -eq 1 ]; then
  echo "One" 
else
  echo "Not One"
fi

Example output:

paulsm@vps2:~$ ./tmp.sh
Assign RETVAL the return value of a.out:
1
  RETVAL=2
Assign RETVAL the value printed to stdout by a.out:
  RETVAL=1
Turn an 'trace' and run a.out with 'test':
+++ ./a.out
++ '[' 1 -eq 1 ']'
++ echo One
One

ALSO:

A couple of points that have already been mentioned:

a. return 1 is generally a better choice than exit (1).

b. "test" is probably a poor name for your executable - because it collides with the built-in "test" command. Something like "test_return" might be a better choice.

Trying to Understand Behavior of `Test ` in Bash

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Chloe Bennett

Trying to Understand Behavior of `Test ` in Bash

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Chloe Bennett