Test command - Two square brackets

UNIX for Beginners Q & A UNIX for Dummies Questions & Answers
1

Hello,
Can someone please explain to me the following line,
[[ -z $(echo $c | tr -d "\015") ]] && break

I do not understand why two test square brackets are used.

Thanks,
Shantanu

---------- Post updated at 03:38 PM ---------- Previous update was at 03:35 PM ----------

And, also why there's a $ before (echo $c | tr -d "\015")
Thanks,
Shantanu

2

There may be other differences, but as far as I know [[ ... ]] is the same as [ ... ] except [[ ... ]] does not allow wildcard expansion.

The result of the command inside $( ) will be tested.

3

Thanks Jsmithstl,
But, $ is used for variable subsitution. How is the use of $ in this expression related to that? Could you please explain more?

Thanks,
Shantanu

4

[[ -z $(echo $c | tr -d "\015") ]] && break

The line means ..
echo $c
--> Just echo value of variable c

echo $c | tr -d "\015
--> value of variable c should if it contains special character "\015" it is something like line feed then delete it (Tr = translate )

$(echo $c | tr -d "\015")
---> Now this become a variable ... So suppose I say check if $VAR is good or I can put $(SOMETHING) then SOMETHING will be evaluated.

[[ -z $(echo $c | tr -d "\015") ]]
--> This is more like if [ SOME CONDITION ] then kind of ... shortcut .. Usually if people like to keep their job and dont want to loose to junior people ... Just kidding :slight_smile:
--> So if you check what does -z mean from internet it says
-z = string is null, that is, has zero length
So What we have check if the result of evaluated variable is null or not ..

Now to the last part ..

[[ -z $(echo $c | tr -d "\015") ]] && break

--> && mean Anding .. Logical and .. or read in this case as THEN for if .. so it will break if condition in the if test is true ..
Now break is means break some loop and get out ... So this might be inside some loop ... Also you can have your own function here called ..

Added this
|| -> ORing if you want else part also ..

If you need more command to be excuted in then and also in OR ... you can write ..

[[ -z ... ]] && ( SOME COMMAND ) || ( SOME OTHER COMMANDS)

You can also write above as
VAR=$(echo $c | tr -d "\015")
if [ -z $VAR ];
then
break
fi

Please Note: all the above is true BASH scripting only ...

5

For most recent shells [ is a builtin command (it's a synonym for the "test" builtin).
Some shells like ksh93, zsh and bash support the shell keyword [[ which is supposed to be an enhanced version of the test command:
it supports a more friendly syntax for expressions with logical relations via the && and || operators, the variables inside are not subject to word splitting and pathname expansion (a.k.a. globbing) etc.

As far as the above example is concerned the command substitution $(command[s]) should be quoted if you use the test builtin command instead of the [[ ... ]] compound command.

$() is the newer supported by POSIX syntax for command substitution (the older one `...` is harder to read and less conducive to nesting).

6

Thanks all
Shantanu