CODEWITHSUNDEEP
bash
Donbhupi
Donbhupi

Reputation: 229

Bash variable assignment explanation {var:=0} {var:-0}?

I'm a bash amateur reading up books and manuals for learning further. I came across following script in a book that calculates total size and shows the use of command substitution. I could not understand the parts that contain {totalsize:=0}, {size:-0}, {totalsize-unset}. Could somebody please explain?

At first sight, it looked like an array but what do the operators := and :- do here and totalsize was the variable containing the values but when I echo ${totalsize-unset}, it does return the sum of filesizes in PWD. Is -unset some kind of built-in ?

$ while read perms links owner group size month day time file
> do
>  printf "%10d %s\n" "$size" "$file"
>  totalsize=$(( ${totalsize:=0} + ${size:-0} ))
> done < <(ls -l *)
$ echo ${totalsize-unset}

Upvotes: 4

Views: 3403

Answers (1)

Rob Kielty
Rob Kielty

Reputation: 8162

(Ok let's be fair it's a contrived example to demonstrate Process Substitution with no warning about the security risks highlighted here. Unfortunate.)

In Bash, variables can be manipulated or expanded according to the syntax described here :

http://www.tldp.org/LDP/abs/html/parameter-substitution.html#PARAMSUBREF

The intention in the above script is simply to assign zero to totalsize if it does not have a value. totalsize will not have a value during the first iteration of the loop. zero will be used in the calculation instead.

There are a miriad of ways of achieving the same result. It would make more sense to use the du -h command.

Run man du first and understand what this command does (It estimates files space usage)

Upvotes: 4

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