mjhjmtu

How do I use GNU touch to update a file called -?

How do I use GNU cat to display a file called -?

I'm running:

% cat --version | head -n1
cat (GNU coreutils) 8.29
% touch --version | head -n1
touch (GNU coreutils) 8.29

Firstly, touch:

% touch -
% ls -l
total 0
% touch -- -
% ls -l -- -
ls: cannot access '-': No such file or directory

Ok, I'll give up on creating a file with touch. Let's create it with date instead:

% date > -
% ls -l -
-rw-r--r-- 1 ravi ravi 29 Sep 8 19:54 -
%

Now, let's try to cat it:

% cat -
% # I pressed ^D
% cat -- -
% # Same again - I pressed ^D

I know I can work around with:

% > -

and

% cat < -

But why don't these GNU utils support the convention that -- means that everything following is treated as a non-option?

How do I use these tools in the general case, for example I have a variable with the contents -?

Use an explicit path to the file:

touch ./-
cat ./-

GNU touch treats a file operand of - specially:

A FILE argument string of - is handled specially and causes touch to
change the times of the file associated with standard output.

For cat, the POSIX standard specifies that a file operand - should be interpreted as a request to read from standard input.

The double-dash convention is still in effect, but it's not for signalling the end of arguments but the end of options. In neither of these cases would - be taken as an option (a lone - can not be an option) but as an operand ("file name argument").

Regarding your last question:

To protect the contents of a variable against being interpreted as a set of options when using it as

utility "$variable"

use

utility -- "$variable"

Note that if the utility is cat, sed, awk, paste, sort and possibly a few others (or GNU touch), and $variable is -, this will still cause the utility to do its special processing since, as said above, - is not an option. Instead, make provisions so that filenames, if they may start with or are equal to -, are preceded by a path, for example ./ for files in the current working directory.

A good habit is to use

for filename in ./*; do

rather than

for filename in *; do

for example.