[SOLVED] Bash question about slackbuild doinst.sh

phalange · 08-16-2019, 01:30 PM

Sorry if this is elementary, but I want to clearly understand what this is doing:

Code:

if [ -x /usr/bin/update-desktop-database ]; then
  /usr/bin/update-desktop-database -q usr/share/applications >/dev/null 2>&1
fi

if [ -x /usr/bin/update-mime-database ]; then
  /usr/bin/update-mime-database usr/share/mime >/dev/null 2>&1
fi

I understand that

Code:

/dev/null 2>&1

is saying, "ignore the Standard Error and Standard Out for the two programs respectively", but under what condition? And why? Is "-x" debug?

Thanks.

montagdude · 08-16-2019, 01:36 PM

"if [ -x file ]; then" means that the command inside the if statement will occur if file exists and is executable. The ">/dev/null 2>&1" just means that the output of the command will be sent to nowhere.

ehartman · 08-16-2019, 01:38 PM

Quote:

Originally Posted by phalange

Sorry if this is elementary, but I want to clearly understand what this is doing:

Is "-x" debug?

See "man test" too: -x means: if file "exist AND is executable", so it tests if the two update programs can be executed and if so it executes them, without showing any output (or errors).

phalange · 08-16-2019, 02:44 PM

Ok, so -x we test if database program is present and is executable. If yes, we update it. If no, we ignore it. Neither route raises an error or shows output. Is this accurate?

And just for my own clarity, in what scenarios would I NOT use this doinst.sh script? It seems pretty useful across the board.

Didier Spaier · 08-16-2019, 04:09 PM

Quote:

Originally Posted by phalange

Ok, so -x we test if database program is present and is executable. If yes, we update it. If no, we ignore it. Neither route raises an error or shows output. Is this accurate?

Yes

Quote:

And just for my own clarity, in what scenarios would I NOT use this doinst.sh script? It seems pretty useful across the board.

In no scenario. You won't need these updates if you do never choose an application to start based on a .desktop file's content or a mime type, but even in this case running the doinst.sh is harmless.

orbea · 08-16-2019, 04:42 PM

I just want to point out that '-x' is not true with only files, it will also be true with directories that are executable (755). It would probably be safer to check both '-f' and '-x' to make sure its an executable file, but in practice it mostly doesn't matter.

phalange · 08-16-2019, 09:06 PM

Excellent, thanks to all above for the helpful insights.

ehartman · 08-17-2019, 04:42 AM

Quote:

Originally Posted by orbea

it will also be true with directories that are executable (755).

All directories are executable (or rather searchable, that's what the "x" means for directory files), so -e (exists) and -x are indistinguable for directories (or should be).
You cannot use ls or such (no wildcards like * or ? either) on directories that are NOT searchable (although you can open files IN it of which you already know the exact filename, that only needs the directory to be readable).

orbea · 08-17-2019, 06:58 AM

Quote:

Originally Posted by ehartman

All directories are executable (or rather searchable, that's what the "x" means for directory files), so -e (exists) and -x are indistinguable for directories (or should be).
You cannot use ls or such (no wildcards like * or ? either) on directories that are NOT searchable (although you can open files IN it of which you already know the exact filename, that only needs the directory to be readable).

If you want to navigate a directory it needs to be executable, but there is no reason someone can't change the permissions for a directory with chmod where -e would match, but -x wouldn't. I think its best to use both -f and -x for executable files and -d for directories, but as I already said -x is usually enough for executable files.