Process Termination: Exit Status#

The Shell: A Program Like Any Other Program#

The shell is a program just like any other program (/bin/bash)
Main purpose: start other programs, and report on their exit status

$ grep jfasch /etc/passwd
jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash

Search program grep along $PATH (see Environment Variables)
Start it, passing it two arguments: jfasch, /etc/passwd (Argument Vector (argv)))
Wait for it to terminate
Examine its exit status

$ echo $?            # <-- $? ... exit status of last foreground process (huh?)
0

The Shell: A Programming Language#

$ if grep jfasch /etc/passwd; then echo YAY; else echo NOPE; fi
jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash
YAY

Exit status used as conditional for if (and while)
Functions, recursion, and whatnot
⟶ Full programming language
… albeit a little weird

Exit Status#

An integer in the range 0-255
In the simplest case, a return from the program’s main function is its exit status
Otherwise (exiting deeper in a process’s call chain), see man -s 3 exit

Exit Status: `0` is “OK”#

code/exit-ok.cpp#

#include <stdlib.h>

int main()
{
    exit(0);                                           // <-- 0 is "OK" 
    // return 0;                                       // <-- same as exit(0)
}

In the sunny case, an exit status of zero is returned.
The truth value of zero is true, paradoxically. This makes sense though: there is only one sunny case, but many opportunities to get into trouble.

$ ./exit-ok
$ echo $?
0

Or, programmatically …

$ if ./exit-ok; then echo YAY; else echo NOPE; fi
YAY

Exit Status: `!= 0` is “Not OK”#

code/exit-nok.cpp#

#include <stdlib.h>

int main()
{
    exit(42);                                          // <-- != 0 is "Not OK"
}

In any error case, an exit status of non-zero is returned.
The truth value of non-zero is false. Again, this makes sense because there are possibly many things why a program might fail.

$ ./exit-nok
$ echo $?
42

Or, programmatically …

$ if ./exit-nok; then echo YAY; else echo NOPE; fi
NOPE