Re: sometimes surprised/confused

[Lianheng Tong]

Hi Datakanja,

I am not entirely sure what you are trying to do, but the arguments

this should not happen

are already separated per default POSIX parsing. You do not need --colsep.

Thus

parallel --dry-run -n 2 echo {} ::: this should not happen

correctly produces on my macbook, with 2 processors:

echo this should

echo not happen

as the maximum number of arguments assigned to echo will be 2, so the first two arguments are sent to processor 1 and the next two are sent to processor 2.

If you want each processor to echo one argument at a time, then do:

parallel --dry-run -n 1 echo {} ::: this should not happen

which then gives:

echo this

echo should

echo not

echo happen

This is what you want to do right?

I hope this will be helpful.

Best regards,

Lianheng

================================================

Lianheng Tong                                    Tel: +44 79 1758 3822  

Room S4.02, Strand Building           Fax: +44 20 7848 2420  

Department of Physics                       lianheng.tong@kcl.ac.uk

Kings College London                                  

Strand, London WC2R 2LS, U.K.                         

================================================

On 6 Jan 2014, at 11:06, ChessDoter wrote:

Hi Lianheng,

thanks for avoiding calling me PEBE. ;-)

Ok, i do see now, that the ` has to be escaped, like

echo this should not happen | xargs -n 1 echo | parallel --dry-run -n 2 echo \`echo {}\`

Would you please provide a pointer to the solution of the --colsep problem? i wasnt able to get rid of the xargs step for that on my own.

Best regards
Datakanja

Am 06.01.2014 11:42, schrieb Lianheng Tong:

Hi,

I think the problem you have encountered is a problem associated with a possible misunderstanding how POSIX command lines are interpreted by the system, rather than a particular problem with parallel.

Best regards,

Lianheng

================================================

Lianheng Tong                      Tel: +44 79 1758 3822

Room S4.02, Strand Building           Fax: +44 20 7848 2420

Department of Physics lianheng.tong@kcl.ac.uk <mailto:lianheng.tong@kcl.ac.uk>

Kings College London

Strand, London WC2R 2LS, U.K.

================================================

On 6 Jan 2014, at 10:35, ChessDoter wrote:

hello,

far from being a power user with linux...

... i happen to play with parallel...

and get confused/surprised at times. Like so:

echo this should not happen | xargs -n 1 echo | parallel --dry-run -n 2 echo `echo {}`

First, i was not able to separate the 4 words into different arguments with --colsep ' ', so that is why i used xargs in the mix.

Then, i did not expect --dry-run to allow for execution of the embedded `echo` command.

Ok, i can imagine scenarios, when this comes in handily...

My current conclusion is: be VERY careful with this powerful tool, test thoroughly before execution but even then... (can i trust it?) beware of unexpected side-effects.

just my 2 cents