Is there an idiom that folds the no-files case into the files-exist case so
they can be handled with a single mechanism?

$ bash --version
GNU bash, version 3.1.17(1)-release (i486-pc-linux-gnu)
Copyright (C) 2005 Free Software Foundation, Inc.

$ ls

$ chmod a-x *
chmod: cannot access `*': No such file or directory

$

I could do this

$ chmod a-x * 2>/dev/null

$

but that just hides the error; there are still two cases (the exit status tells
the tale). I could do this

$ chmod a-x * 2>/dev/null ; :

$

but that's more hiding. This works

$ files=$(find . -type f)

$ [ -z "$files" ] || chmod a-x $files

$

and is, I suppose, good documentation, but it's too much machinery. This
problem is similar to handling zero-iteration loops in programs, but it's
irratating that

$ for i in * ; do chmod a-x $i ; done
chmod: cannot access `*': No such file or directory

$

doesn't work. Anything else?

R. Clayton wrote:
> Is there an idiom that folds the no-files case into the files-exist case so
> they can be handled with a single mechanism?
>
> $ bash --version
> GNU bash, version 3.1.17(1)-release (i486-pc-linux-gnu)
> Copyright (C) 2005 Free Software Foundation, Inc.
>
> $ ls
>
> $ chmod a-x *
> chmod: cannot access `*': No such file or directory
>
> $
>
> I could do this
>
> $ chmod a-x * 2>/dev/null
>
> $
>
> but that just hides the error; there are still two cases (the exit status tells
> the tale). I could do this
>
> $ chmod a-x * 2>/dev/null ; :
>
> $
>
> but that's more hiding.

The "problem" in your example is that chmod(1) requires files to operate
on; a simple chmod a-x without arguments won't work, even if the shell
file globbing mechanism would produce an empty argument list instead of
taking the * literally.

> This works
>
> $ files=$(find . -type f)
>
> $ [ -z "$files" ] || chmod a-x $files
>
> $

You could do

set -- *
[ -f "$1" ]

but I am not sure whether you like that since it does not avoid the test
operation that you seem to dislike.

> and is, I suppose, good documentation, but it's too much machinery. This
> problem is similar to handling zero-iteration loops in programs, but it's
> irratating that
>
> $ for i in * ; do chmod a-x $i ; done
> chmod: cannot access `*': No such file or directory
>
> $
>
> doesn't work. Anything else?

A while/read loop

find . -type f | while read -r f ; do chmod a-x "$f" ; done


Janis

R. Clayton wrote:

> $ chmod a-x *
> chmod: cannot access `*': No such file or directory

As Janis said, the error is due to chmod /always/ expecting an argument.
However, you might find the nullglob feature of bash useful sometimes.

$ ls
$ echo *
$ *
$ shopt -s nullglob
$ echo *

$

Note, however, that this does not solve the problem for all those situations
where an argument is expected anyway, like eg chmod:

$ chmod a-x *
chmod: missing operand after `a-x'
Try `chmod --' for more information.

This is something that cannot really be avoided, AFAIK.

--
All the commands are tested with bash and GNU tools, so they may use
nonstandard features. I try to mention when something is nonstandard (if
I'm aware of that), but I may miss something. Corrections are welcome.

The "problem" in your example is that chmod(1) requires files to operate on;
a simple chmod a-x without arguments won't work, even if the shell file
globbing mechanism would produce an empty argument list instead of taking the
* literally.

True, but I was whining more about misbehaving glue code. Here's another
irritating one what doesn't work:

$ find . -type f | xargs chmod a-x
chmod: missing operand after `a-x'
Try `chmod --' for more information.

$

although there is a gnu extension that makes it behave:

$ find . -type f | xargs --no-run-if-empty chmod a-x

$

R. Clayton wrote:
> The "problem" in your example is that chmod(1) requires files to operate on;
> a simple chmod a-x without arguments won't work, even if the shell file
> globbing mechanism would produce an empty argument list instead of taking the
> * literally.
>
> True, but I was whining more about misbehaving glue code.

> Here's another irritating one what doesn't work:

Yes, but actually it's the same case as upthread; if the list provided
by find is empty chmod won't get the arguments it expects.

Janis

>
> $ find . -type f | xargs chmod a-x
> chmod: missing operand after `a-x'
> Try `chmod --' for more information.
>
> $
>
> although there is a gnu extension that makes it behave:
>
> $ find . -type f | xargs --no-run-if-empty chmod a-x
>
> $