io redirection

[amishera]

I really dont understand how this double redirection works:

ls > dirlist 2>&1

directs both standard output and standard error to the file dirlist,

how?? if the final output is redirected to dirlist why is it not in the end? like

2>&1 >dirlist

on the other hand

dirlist 2>&1

seems to be concatenating both dirlist and 2 just like

cat file1 file2

Someone please elaborate. If possible with the step by step flow.

Thanks in advance.

[vsemaska]

Look at the manpage for bash (or whatever shell you're using) and find the section on redirection. In the bash manpage it says:

Note that the order of redirections is significant. For example, the
command

ls > dirlist 2>&1

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

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard
error was duplicated as standard output before the standard output was
redirected to dirlist.

[Cabhan]

Well first, we need to figure out exactly what we're typing:

In Bash, typing N> FILE has the effect of directing output stream number N to FILE. If N is unspecified, it defaults to '1', standard output. So standard output is IO stream 1, and standard error is IO stream 2.

So we have some command:

command_foo

This command will be executed, and it will by default have its IO streams point wherever its parents do. In this case, the parent is the shell, so it points at the terminal. Now let's make a little change:

command_foo > file

Now command_foo has its standard output point to file, and its standard error point to wherever its parent's standard error points (again, in this case, the terminal). Finally, we have:

command_foo > file 2>&1

Now command_foo has its standard output point to file, and its standard error point to wherever its standard output (IO stream 1) points.

Make sense?


As a side note, this "> DEST 2>&1" construct is so common that more recent Bashes have a special notation for it. If you use "&> DEST", then both standard output and standard error will point to DEST.