Don't understand why do I get a permission denied error

Question

I have a file hello.c with "hello world" program in C. I also done the following on the shell (UNIX):

$ ls > 1
$ chmod 0 1
$ cc -o hello hello.c
$ chmod 400 hello
$ ./hello > 1

And I got permission denied on 1.

Why I didn't get the permission denied error on hello?

Yes, but the chmod with 400 on hello blocks its permission of execution. So what happen first? hello run on the permission for 1? — Mickey, Jun 21 '14 at 12:07
Debugging step: what do you think `chmod 0 1` does? What do you think `chmod 400 hello` does? — Norman Gray, Jun 21 '14 at 12:08
The evidence of course suggests the permission for 1, but if have a command, how can I know what will happen first? — Mickey, Jun 21 '14 at 12:09
Because if the hello program runs first, I should get the permission denied error on hello. Isn't it? — Mickey, Jun 21 '14 at 12:11
Of course it doesn't matter for me. The question was originally taken from an exam in my university, and I'm solving these exams to understand how the things works. — Mickey, Jun 21 '14 at 12:16

score 1 · Accepted Answer · answered Jun 21 '14 at 12:52

Because when you ask a shell to perform an output redirection for a command, the file to write to is opened by the shell, before invoking the command.

Your shell will:

open 1 for writing
"wire" ./hello stdout to 1
run ./hello

1 has permissions 0o000, so the first operation will fail.

Don't understand why do I get a permission denied error

1 Answers1