Test condition shell script

Question

So I'm writing a script that has to check if a file exists or not before executing. I have the following code:

if [[ \( -d "DIR2" \) != true ]];
then
    echo "nonexistent dir"
    exit 1
elif [[ \( -d "DIR1" || -f "DIR1" \) != true ]];
then
    echo "nonexistent dir or file"
    exit 1
fi

I seem to be doing something wrong with the brackets since I get the following:

./syncdir.sh: line 11: conditional binary operator expected
./syncdir.sh: line 11: syntax error near `-d'
./syncdir.sh: line 11: `if [[ \( -d "DIR2" \) != true ]];'

I find it odd that bash is expecting a binary operator isnt '!=' one?
Can someone please tell me what I am doing wrong and why?

I really get confused with the brackets, I still haven't got the hang of it.

For conditionals you start here. However, what you are doing is bad practice because your code (even when working) is a perfect race condition. You better simply access the folder and handle the error if the folder does not exist. — hek2mgl
– hek2mgl, Commented Oct 9, 2015 at 23:33
@MiguelM The race condition is this: your test succeeds because the directory exists, then some other process deletes the directory, and then your process tries to use the directory that no longer exists. — William Pursell
– William Pursell, Commented Oct 10, 2015 at 0:02
How should I handle this correctly? Thanks for the previous explanation BTW — Miguel M
– Miguel M, Commented Oct 10, 2015 at 0:15
@MiguelM Like I said, simply access the folder. If the folder does not exist, you'll get an error which you can handle. This would be ok for simple situations. More sophisticated would be to use locks using flock. — hek2mgl
– hek2mgl, Commented Oct 10, 2015 at 1:27

Barmar · Accepted Answer · 2015-10-10 00:45:57Z

2

Your condition syntax is totally wrong. It should be:

if [[ -d "DIR2" ]];

and

if [[ ! ( -d "DIR1" ||  -f "DIR1" ) ]];

If you have parentheses inside a conditional expression, you don't need to escape them (that's only needed if you're using the [ command). And you don't need to compare with true or false.

edited Oct 10, 2015 at 0:45

answered Oct 9, 2015 at 23:47

Barmar

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5 Comments

Miguel M Over a year ago

So I can use parentheses inside [[]] ? But what was causing the error in the first place?

Barmar Over a year ago

The error was because you escaped the parentheses. So it treated them as literal strings, not part of the expression syntax.

Miguel M Over a year ago

Even if I take the () it gives me an error and the solution you gave me also doesn't work :/ but I think I can get it to work. My objective was to check if DIR1 was a file or directory and if DIR2 was only a directory.

sras Over a year ago

@Barmar - is there any specification when to use [ and [[ for conditional expressions ? please advise.

Barmar Over a year ago

[[ is a bash and zsh extension, [ is a POSIX standard. But if you don't need to be portable to all POSIX shells, [[ is generally faster and easier to use. See robots.thoughtbot.com/the-unix-shells-humble-if

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