How do I suppress stderr warning messages from a command inside command substitution?

Question

touch ~/deleted/$(echo "directory_"$(readlink -f foo)|tr '/' '\') 2> /dev/null

where foo is a directory

It's reading the full pathname of foo and creating it as a file replacing all '/' with '\' and putting a directory_ in front so the directory /home/test/foo would create a file directory_\home\test\foo

It does exactly what I want except a warning keeps printing tr: warning: an unescaped backslash at end of string is not portable

a. I don't know what that means b. I would like it to not show

I tried to redirect stderr to /dev/null but it doesn't work. How do I suppress it?

Answer 1

The error message means that you have an unescaped backslash. In the *nix world, special characters are 'escaped' by adding a backslash before them. That means that they're interpreted as simple characters and not special ones. For example:

$ printf 'foo\nbar'
foo
bar

The \n is a special character that means newline. But if we escape the slash we get:

$ printf 'foo\\nbar'
foo\nbar

So, you can get rid of it either by escaping the backslash:

touch ~/deleted/"$(echo "directory_$(readlink -f -- "$1")"|tr '/' '\\')

Or by redirecting the error output of tr (not of touch which is what you were doing):

touch ~/deleted/"$(echo "directory_$(readlink -f -- "$1")"|tr '/' '\' 2> /dev/null)

However, I cannot stress enough that it is a Very Bad Idea® to create file names with backslashes. Really. They will be hell to parse and deal with later on. Why not use another character?

Answer 2

Some expansions happen before redirection so you have to place it directly against your tr command:

touch ~/deleted/$(echo "directory_"$(readlink -f foo)|tr '/' '\' 2> /dev/null)

Answer 3

Use single quotes with double back slash, and you will not get any error.

touch ~/deleted/$(echo "directory_"$(readlink -f foo)|tr '/' '\\')