RE: what is &&

[CARTER-HITCHIN, David, GBM]

> What is the difference between:
> cd dir; do_smth

cd into dir then run do_smth; if 'cd dir' fails, then run do_smth anyway

> and
> cd dir && do_smth

cd into dir then run do_smth; if 'cd dir' fails then don't run do_smth

It's a logic control statement meaning AND so

cmd1 AND cmd1

is evaluated by the shell for 'truth'.  If the first statement is false
(i.e. cmd1 fails), then this short-circuits the logic as the shell doesn't
need to know the truth/falsehood of the second 'statement' (cmd2), so it
doesn't evaluate it.  So 'a && b' is equivalent to 'if a then b'.  || (or)
has similar uses.  See the man page for the shell you are using for more
details, e.g. 'man ksh' on my system has some info about this idiom.

HTH,



David Carter-Hitchin.
--
Royal Bank of Scotland
Interest Rate Derivatives IT
135 Bishopsgate
LONDON EC2M 3TP

Tel: +44 (0) 207 085 1088


***********************************************************************************
The Royal Bank of Scotland plc. Registered in Scotland No 90312. Registered 
Office: 36 St Andrew Square, Edinburgh EH2 2YB. 
Authorized and regulated by the Financial Services Authority 
 
This e-mail message is confidential and for use by the 
addressee only. If the message is received by anyone other 
than the addressee, please return the message to the sender 
by replying to it and then delete the message from your 
computer. Internet e-mails are not necessarily secure. The 
Royal Bank of Scotland plc does not accept responsibility for 
changes made to this message after it was sent. 

Whilst all reasonable care has been taken to avoid the 
transmission of viruses, it is the responsibility of the recipient to 
ensure that the onward transmission, opening or use of this 
message and any attachments will not adversely affect its 
systems or data. No responsibility is accepted by The Royal 
Bank of Scotland plc in this regard and the recipient should carry 
out such virus and other checks as it considers appropriate. 
Visit our websites at: 
http://www.rbos.com
http://www.rbsmarkets.com 
********************************************************************************