shell script - how hard can it be

[Adam Noble]

Hi,

I simply want to use an if statement to see whether I should send the output of a command to my output file.

I'm using the following and only want to send the output to the file if the 1st 2 characters begin HP.

if [ $revision = "HP*" ];then

The above however doesn't work how do I do this.

[Peter Godron · Accepted solution]

Adam,
how about:
if [ `expr substr "$revision" 1 2` = "HP" ]
then
.
.
fi

[Ralph Grothe]

It very much depends on the shell you are doing this in.
For instance shells such as Bash offer an expansion syntax which doesn't even require some external filter commands and yet let you extract a certain substring.
However, with HP posix shell you could do this
(but there are many other solutions conceivable)

(( $(expr "$revision" : ^HP) == 2 )) && echo do something

[Ralph Grothe]

Sorry, I made a mistake.
The correct terminology I was referring to when talking about Bash was "Parameter Substitution" and not "expansion".
But this is due to my English non-nativeness, I suppose.
And here's what I meant in Bash:

$ OS=$(uname -s)
$ [[ ${OS:0:2} = Li ]] && echo "we are running Linux"
we are running Linux

[Adam Noble]

thanks it clearly wasn't as straight forward I was using ksh and your solution worked. Thanks to you both

[Dennis Handly]

You really want to use pattern matching:
if [[ "$revision" = HP* ]]; then

As in Ralph's case, you need [[ ]]. Don't quote the HP*.