Difference between revisions of "Talk:Terminal Process Management"

Latest revision as of 13:17, 9 January 2019

redirect dev null variants

nohup gedit &> /dev/null

is POSIX syntax and is the same as:

nohup gedit &
> /dev/null

That is run nohup gedit in background and then do a > /dev/null redirection without running a command.

nohup gedit >& /dev/null

is not POSIX syntax and is the csh way to redirect both stdout and stderr to /dev/null. csh doesn't have the 2>&1 operator as found in Bourne, so it's the only way csh has to redirect stderr.

zsh (as often) also provides with the csh syntax, but it also supports the x>&y fd duplication operator of the Bourne shell, which means there's a conflict there.

ls >&file

redirects ls's stdout and stderr to file, but if the file is 2, you've got a problem as

ls >&2

means redirect stdout to the resource pointed to by fd 2 (dup(2, 1)). So you need to write it:

ls >& ./2

if you wanted to redirect both the stdout and stderr of ls into a file called 2 in the current directory; or use the standard syntax.

bash initially did no understand >&, but it introduced the &> operator instead for that, breaking POSIX compliance in the process (though it's unlikely a script would use cmd &> xxx).

ksh copied that operator in ksh93t+ in 2009, mksh in R35 in 2008 (disabled in posix mode) but not >&.

bash added support for >& in 2.05.

busybox sh added support for both &> and >& in 1.13 (2008).

Neither >& nor &> as meaning redirect stdout and stderr are POSIX/Bourne.

If you want to redirect both stdout and stderr portably, the syntax is

cmd > file 2>&1