Shell Programming and Scripting

Hi,
I know that export command is used to set environment variable.

Can you please explain what ":--100" in the below command is doing?

Code:

export ROWSTOLOAD=${ROWSTOLOAD:--100}

Thanks,
Gangadhar

This can be divided into 2 parts

Code:

export ROWSTOLOAD=${ROWSTOLOAD:--100}

If "ROWSTOLOAD" is defined then

Code:

export ROWSTOLOAD=${ROWSTOLOAD}

else set it to

Code:

export ROWSTOLOAD=-100

In general if a variable is needed to be exported but you don't know if its already defined then you use the above syntax to define it to a default value in case it is not set.
Suppose you have variable "VAR" and not sure if it already had some value, but you want it to be defaulted to "300" if it's not defined, then you use the command

Code:

export VAR=${VAR:-300}

EDIT: aster007 explains it better!

Does it work ??

From what I know [very little], it shouldn't.

The parenthetical ${text} is a character interpretation, and
The parenthetical $(num) is a numerical interpretation.

AlphaLexman is correct - This won't work with numbers.
If "ROWSTOLOAD" is null, then the output in this case will be "{ROWSTOLOAD:--100}" instead of -100

This syntax is legal and it should work fine in ksh, bash, or even the old Bourne shell.

Code:

$ ROWSTOLOAD=""
$ export ROWSTOLOAD=${ROWSTOLOAD:--100}
$ env | grep ROWS
ROWSTOLOAD=-100
$

edit ---

Well the ROWSTOLOAD=${ROWSTOLOAD:--1} is valid in the Bourne shell. But I just remembered that the Bourne shell does not allow you to set a value in the export statement like that. But you can use the obscure feature of redefining the environment for a single command by prepending assignment and express it like this:

Code:

ROWSTOLOAD=${ROWSTOLOAD:--1}  export ROWSTOLOAD

And a test...

Code:

$
$ ROWSTOLOAD=""
$ export ROWSTOLOAD
$ env | grep ROWS
ROWSTOLOAD=
$ ROWSTOLOAD=${ROWSTOLOAD:--100}  export ROWSTOLOAD
$ env | grep ROWS
ROWSTOLOAD=-100
$

That was with ksh. This syntax really should be universal.

Thanks everyone for the explanations. It helped and I got it

Regards,
Gangadhar