[TriLUG] Basic bash shell scripting question

[Jeremy Portzer]

On Thu, 2003-05-29 at 09:40, Ryan Leathers wrote:
> Hi gang. I am writing an application in Java, part of which needs to add
> the user name of a user logging onto a Linux host to a plain text list
> of user names.  (don't worry - this is not required or intended to be
> secure). The user will launch this application from their login script.
> It occurred to me that the simplest way to do this was to use JNI to
> drop to a bash shell and then mess with the file as needed in Java later
> on.  I suppose I could just launch the script independently of the Java
> app and get the same thing... but thats not really the point.
> 
> I have rarely done shell scripting over the years and am looking for
> some advice.  The lines below will do what I want, but how can I make
> this more elegant?  Can I accomplish this without creating deleting and
> renaming as seen below?   
> 
> whoami > name
> cat checked.txt name > checked.new
> rm checked.txt
> rm name
> mv checked.new checked.txt
> 

You can do all of that in one command:
	whoami >> checked.txt
The double >> will open the file in "append" mode, which won't overwrite
the existing file, so there's no need for the file juggling you show
above.

Note that doing something like this is not multiuser safe -- if this
file is written to at the same time by multiple people, you might have
unintended consequences.  But since you say it's not supposed to be
"secure" maybe that won't be a problem.

Also, it might be clearer to simply open the file in append mode
directly from the Java application.  I'm not familiar with Java, but
opening a file for appending is a basic operation that ought to be
straight-forward in any language; hopefully someone else here will post
how to do that.

--Jeremy

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| Jeremy Portzer       jeremyp at pobox.com       trilug.org/~jeremy     |
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