Re: pipelines and &&

[John Riddoch <jr@scms.rgu.ac.uk>]

> I would very much like to know how sublists and pipelines are done:
> I read the following without understanding how to do it:
> 
> "If two pipelines are separated by ``&&'', the second
> pipeline is executed only if the first is successful (returns a zero
> value).  If two pipelines are separated by ``||'', the second is
> executed only if the first is unsuccessful (returns a nonzero value).
> Both operators have equal precedence and are left associative."

This has nothing to do with pipes; the term is probably somewhat confusing.

A better example would be:

cp file file.bak || echo "file not backed up"

If the file could not be backed up (eg, the file does not exist or the user 
does not have write permission in the directory), then the error message is 
shown.  Alternatively:

cp file file.bak && echo "file backed up"

|| and && are alternatives to testing $?
eg:

cp file file.bak
if [ $? = 0 ]; then echo "file backed up"; done

is equivalent to the second example above.  However, || and && are shorter and 
can be slightly more understandable.

--
John Riddoch	Email: jr@scms.rgu.ac.uk	Telephone: (01224)262721
Room C6, School of Computer and Mathematical Science
Robert Gordon University, Aberdeen, AB25 1HG
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