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WHy the double square brackets?

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Top Forums Shell Programming and Scripting WHy the double square brackets?
# 8  
Old 12-08-2008
Quote:
Originally Posted by methyl
#!/bin/ksh
if [ ! -s valid_users.tmp -a ! -s invalid_users.tmp ]
then
exit
fi

Why are we back to -s?
What is the -a for?

Why is there no &&?

I am a little confused.
# 9  
Old 12-08-2008
Quote:
Originally Posted by methyl
#!/bin/ksh
if [ ! -s valid_users.tmp -a ! -s invalid_users.tmp ]
then
exit
fi
Why is there no double bracket?
What is the -a for and why is it not there for second file?

If I sound confused and need an explanation well I am!

Sorry about the double posting. I thought that the first one did not go through....
# 10  
Old 12-08-2008
Quote:
Originally Posted by Ikon
Something wrong somewhere else in your script it seems.

Actually why are you using -s ? -e should be used.

-e file = True if the file exists.
-s file = True if the file exists and is a socket.
-e does not work it seems. -s does!
# 11  
Old 12-08-2008
Single brackets.
Please see "man test" for a good explanation about testing file types.
Single square bracket syntax "[ expr ]" is exactly the same as "test expr".
In many shells "test" is actually a shell builtin.
Within "test" the -a is the logical AND operator (as distinct from -o which is the logical OR operator).


Now for double brackets!

There is a formal unix shell syntax involving "[[ expression ]]" which has much overlap with "test". There is a good explanation in "man ksh" . The term between the brackets is a Conditional Expression which is also explained at great length in "man ksh" - including the use of double-ampersand for logical AND "&&", and double pipe for logical OR "||".

Personally I only find use for "[[ expression ]]" in "while" loops. Others may differ.
while [[ expression ]]
do
done
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