UNIX for Dummies Questions & Answers

Whatever is running in the foreground gets interrupted by ctrl-c. when sleep is running, it gets interrupted and not your shell.

read is a shell builtin, not an external command, so when it's waiting on read, your shell is still in the foreground.

Okay -- but I still don't get why the message isn't displayed. Other folks can get it. Any idea why not me? My book says it should display the message and go right back to the loop.

Sorry if it's a basic question, but I'm still new to this.

No they don't, not when their shell's not the foreground process. When you're running an external command, your shell isn't the foreground process, so the signal doesn't get delivered to it. It gets delivered to the foreground process instead, in this case, sleep, which causes it to quit and return a non-zero error code:

Code:

$ sleep 30
^C
$ echo $?
130
$

This is because the process isn't actually killed by the shell -- it's killed directly, by you. The terminal device translates control-C directly into SIGINT for you. What the 'foreground process' is isn't a property of the shell, either -- it's a property of the terminal itself, so it always knows where to send SIGINT direct.

This is true only if your shell is the foreground process. If it's in the middle of running any externals, your shell's not in the foreground.

I'm sorry to be so dense corona, but can you tell me what is meant by external command, foreground process, etc. I'm not sure the book is clear on this.

An external command is one that's not built into the shell. It exists as an actual file somewhere, and when you run it, it creates a whole new process.

Code:

$ whereis -b cat
cat: /bin/cat
$ /bin/cat file.txt
contents of file
$ whereis read
read:
$ read VAR
qwertyuiop
$ echo $VAR
qwertyuiop
$

builtins, on the other hand, are commands the shell just automatically knows how to do. There's no 'read' program on the system, but 'read' works anyway, putting the qwertyuiop typed on the keyboard into the variable VAR.

So when you run an external command, it's run seperately, and it has control of the terminal until it's done. When you run an internal command, no programs are run, the shell just does what you tell it internally.

Another important builtin is one you probably already know -- 'cd'. Consider how external programs are run -- they have a separate process, their own environment. If cd wasn't a builtin it couldn't possibly work, because it'd create a new process, change that process' directory, quit, and return to your shell, leaving your shell unchanged.