What exactly is $?

1. Two ways a process can be terminated

A process can be terminated in two different ways.

• the process calls system call _exit(exitCode)
• the process receives a specific signal, like SIGKILL

2. $? is a number represents the terminated child's status

The shell waits for its child processes and put the collected status of the terminated child into $? variable.

Based on how the child was terminated, $? has a different value.

• If the child was terminated by calling _exit(exitCode), $? = exitCode.
• If the child was terminated by a signal, $? = 128 + signalNo

The signalNo is great than 1, so if the child was terminated by a signal, $? > 128.

e.g.

$ cat test.c

#include <unistd.h>

int main(void)

{

    _exit(2);

}

$ gcc test.c

$ ./a.out

$ echo $?

2

$ sleep 100

(Press Ctr + C)

$ echo $?

130

3. $? is confusing when it's great than 128

e.g if  $? is 130, can you infer how the child was terminated?

Actually, two cases could cause such a result.

• The child called _exit(130)
• The child was terminated by signal SIGINT (the signal no is 2)

To make $? less confusing, applications should NOT exit with a code greater than 128. So developers must bear in mind when writing your new apps.