[SOLVED] if [ ! -a foo ] does not do what it's expected to do?

stf92 · 04-30-2014, 06:18 AM

Hi:

Code:

$ cat t1.sh
#!/bin/sh

set -e
set -x
if [ ! -a /mnt/cd0 ] ; then
	mkdir /mnt/cd0
fi
$ ./t1.sh
+ '[' '!' -a /mnt/cd0 ']'
+ mkdir /mnt/cd0
mkdir: cannot create directory '/mnt/cd0': File exists

If I use -d instead of -a then it doesn't execute 'mkdir /mnt/cd0', as expected. But the bash man page says:

Code:

      -a file
              True if file exists.

/mnt/cd0 is a directory. But then it is a file! And then t1.sh should not have executed 'mkdir /mnt/cd0'. But it does. What am I missing here?
How do you explain this?

Ser Olmy · 04-30-2014, 07:03 AM

It seems bash has some very peculiar ideas about the nature of directories. The output from this script only makes sense in a particular universe:

Code:

#!/bin/bash

mkdir /tmp/tea

if [ -a /tmp/tea ]; then
  echo You have tea.
fi

if [ ! -a /tmp/tea ]; then
  echo You have no tea.
fi

rmdir /tmp/tea

As you said, the results are specific to the -a operator; the -d operator produces the expected result(s), and so does the -a operator if you put it inside double brackets ([[ / ]]).

This has to be a bug.

bigearsbilly · 04-30-2014, 07:12 AM

you may just as well use mkdir -p

no point in you checking then the kernel checking again.

rtmistler · 04-30-2014, 07:38 AM

I agree with the suggestion to use:

Code:

mkdir -p <dir-path-and-name>

However I also feel that for things like temporary mount points that it's best to create that as well as remove it so as to not leave temporary unused files lying around. Further, there is a test concern to consider which is if that directory does exist, then is it already a mount for some other reason, so as a result the script should consider making that determination, failing, or trying to use an alternate mount point so as to not cause undesired system problems.

ntubski · 04-30-2014, 09:30 AM

I think is a quirk due to -a having 2 meanings for the test (aka "[") builtin:

Code:

$ help test
...
      -a FILE        True if file exists.
      EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
      STRING         True if string is not empty.
      -e FILE        True if file exists.

[ ! -a /tmp/tea ] is true because "!" is "true" (it's a nonempty string), and "/tmp/tea" is "true" for the same reason. Note that -e means the same as unary -a, and doesn't have this problem. Also you can put the "!" outside of the test:

Code:

! [ -a /tmp/tea ]

stf92 · 04-30-2014, 07:00 PM

Not remembering what the logical negation operator was, I looked in the internet and saw it was used this way:

Code:

if [ ! -<some primary> <filename> ]

But

Code:

if ! [ -<unary primary> <filename> ]

should be preferred I think, because it makes it clear what the scope of '!' is. In the case of '-a' if forces bash to interpret it as a unary operator. Thanks for the feedback.

Ser Olmy · 04-30-2014, 07:11 PM

Quote:

Originally Posted by ntubski

I think is a quirk due to -a having 2 meanings for the test (aka "[") builtin:

Of. Course. Putting the ! operator before the -a basically turned the test into:

Code:

if [ foo -a bar ]