Redirection using file descriptors - specific example

Question

I am reading this documentation which gives the following examples about redirection and file descriptors.

ls > dirlist 2>&1

will direct both standard output and standard error to the file dirlist, while the command

ls 2>&1 > dirlist

will only direct standard output to dirlist. This can be a useful option for programmers.

Are these examples the wrong way round? It seems to me the second example "will direct both standard output and standard error to the file dirlist" while the first example "will only direct standard output to dirlist".

If I'm wrong about this (...probably...) can someone explain the logic of these 2 examples clearly?

Answer 1

First of all, you need to understand the n>&m syntax. this is a duplication operation, meaning copy m to n. check out section "3.6.8 Duplicating File Descriptors".

I had the same doubt as you when I studied the article. So I can try to explain it to you:

ls > dirlist 2>&1 

with the above command, you first redirect the output to dirlist by > dirlist. 2>&1 means copy file descriptor 1 to 2, in other words make fd2 a copy of fd1. Now fd1 is already pointed to dirlist, making fd2 a copy of fd1 means fd2 also points to dirlist. As a result, fd1->dirlist, fd2->dirlist.

ls 2>&1 > dirlist   

with this, you first make fd2 a copy of fd1 which is still the terminal, so fd2->terminal. then you redirect fd1 to dirlist. as a result, fd2 -> terminal, fd1->dirlist.

Answer 2

The directions are processed left to right.

ls > dirlist 2>&1

makes the shell which will eventually turn itself into ls, do the following system calls (error checking omitted for brevity).

/* Handle > dirlist */
int temp_fd = open("dirlist",O_WR);  /* Open dirlist for output */
dup2(temp_fd, 1); /* Make file descriptor 1 (stdout) point to dirlist */
close(temp_fd);  /* Don't need this other file descriptor for dirlist */
/* Handle 2>&1 */
dup2(1,2); /* Make fd 2 be a copy of fd 1, which points to dirlist */

So first fd 1 is changed, and then fd 2 is changed.

Conversely ls 2>&1 > dirlist does

/* Handle 2>&1 */
dup2(1,2); /* Make fd 2 be a copy of fd 1, the original stdout */
/* Handle > dirlist */
int temp_fd = open("dirlist",O_WR);  /* Open dirlist for output */
dup2(temp_fd, 1); /* Make file descriptor 1 (stdout) point to dirlist */
close(temp_fd);  /* Don't need this other file descriptor for dirlist */

A different way to look at it is as just assignments. Originally fd1=initial_stdout, then

/* ls > dirlist 2>&1 */
fd1=to_dirlist
fd2=fd1 (i.e. to_dirlist)

/* ls 2>&1 > dirlist */
fd2=fd1 (i.e. initial_stdout)
fd1=to_dirlist.

Or you can say copy by value rather than copy by reference.