If you look closely, I think you'll find that that is "if { command }" (with no expr); the exit status of command, in this case, is the expression.

ongoto already explained most of what is going on in the code above. From your comments above, I get the feeling that you don't understand how fork() works. If we look at the code:

Code:

    if(fork()) /*parent*/ wait(&ccode);
    else { /*child*/
        doex(1);
        goto err;
    }
    while((a=nxtarg()) && (!eq(a,"}")));
    return(ccode? 0 : 1);

What you have to understand is that after a successful call to fork(), it essentially returns twice; once in the parent process and once in the child process. In the parent process (the one that called fork()), the return code is never zero; if it is positive, it is the process ID of the newly created child process, and if it is negative, it indicates that the fork was unsuccessful and errno will indicate what error occurred (in this case there is no child process).

In the child process, fork() always returns zero.

So the code shown in green above, is only executed in the child process and the code shown in red is only executed in the parent process. Presumably, the function doex() in the child will parse the expression between the { and }, evaluate it, and then branch to the code at the label err which should terminate the child process with an exit code indicating whether the expression evaluated to true or false.

The wait(&ccode) in the parent waits for the child to exit and saves the exit status from the child in the variable ccode. Then the while loop in the parent skips over the expression that was evaluated by the child and returns true or false depending on the exit status of the child.

Note also that exit code 0 conventionally indicates success and a non-zero exit code indicates a failure. So, the command:

Code:

test -r file

produces exit status zero if file is readable and produces a non-zero exit status if file can't be found or is not readable. As ongoto explained, the ternary operator converts the exit code convention (0 for success, non-zero for failure) into the C convention (1 for true, 0 for false).