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 Forum index » Taking the Puppy out for a walk » Misc
Unix Trivia
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rufwoof


Joined: 24 Feb 2014
Posts: 3725

PostPosted: Thu 07 Nov 2019, 23:06    Post subject:  Unix Trivia  

Why is the dd command called "dd" ?

.
.
.

dd = convert and copy command, but as c compile (cc) was already used it was decided to shift up the alphabet for the 'convert and copy' command.

Second video https://catonmat.net/why-unix-commands-are-short 30:40

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Last edited by rufwoof on Fri 08 Nov 2019, 06:37; edited 1 time in total
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jafadmin

Joined: 19 Mar 2009
Posts: 1258

PostPosted: Thu 07 Nov 2019, 23:36    Post subject:  

It actually comes from the IBM Job Control Language for batch processing.
The following example, using JCL, DD is used to copy a file on OS/360:
Code:

//IS198CPY JOB (IS198T30500),'COPY JOB',CLASS=L,MSGCLASS=X
//COPY01   EXEC PGM=IEBGENER
//SYSPRINT DD SYSOUT=*
//SYSUT1   DD DSN=OLDFILE,DISP=SHR
//SYSUT2   DD DSN=NEWFILE,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(40,5),RLSE),
//            DCB=(LRECL=115,BLKSIZE=1150)
//SYSIN  DD DUMMY
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mistfire

Joined: 04 Nov 2008
Posts: 1424
Location: PH

PostPosted: Fri 08 Nov 2019, 05:44    Post subject:  

DD

DUPLICATE
DISK
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rufwoof


Joined: 24 Feb 2014
Posts: 3725

PostPosted: Fri 08 Nov 2019, 06:36    Post subject:  

Code:
# dd --help

Quote:
Usage: dd [OPERAND]...
or: dd OPTION
Copy a file, converting and formatting according to the operands.
.
.

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Burn_IT


Joined: 12 Aug 2006
Posts: 3702
Location: Tamworth UK

PostPosted: Fri 08 Nov 2019, 11:31    Post subject:  

jfadmin No it doesn't.

The DD statement in JCL stands for Data Definition and has nothing whatsoever to do with copying.

The program IEBGENER is the thing that does the copying.
SYSUT1 is the source file
and SYSUT2 is the target file
in your example.

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musher0

Joined: 04 Jan 2009
Posts: 15041
Location: Gatineau (Qc), Canada

PostPosted: Fri 08 Nov 2019, 11:55    Post subject:  

Bump one letter up in the Latin alphabet? I see:
dd = donvert and dopy. Laughing
whatever that means! Twisted Evil

Following a similar reasoning, conky is actually a donkey!!! Laughing Hee-haw!

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jafadmin

Joined: 19 Mar 2009
Posts: 1258

PostPosted: Fri 08 Nov 2019, 18:33    Post subject:  

Burn_IT wrote:
jfadmin No it doesn't.

The DD statement in JCL stands for Data Definition and has nothing whatsoever to do with copying.

The program IEBGENER is the thing that does the copying.
SYSUT1 is the source file
and SYSUT2 is the target file
in your example.


https://en.wikipedia.org/wiki/Dd_(Unix)#History
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p310don

Joined: 19 May 2009
Posts: 1502
Location: Brisbane, Australia

PostPosted: Fri 08 Nov 2019, 19:13    Post subject:  

I read somewhere it was data duplicate(or)
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Burn_IT


Joined: 12 Aug 2006
Posts: 3702
Location: Tamworth UK

PostPosted: Fri 08 Nov 2019, 19:32    Post subject:  

Read what I said!
I said IN JCL

The dd statement in JCL is used to link a logical filename in a program to a physical data stream somewhere in the real world.
It has nothing to do with what that data is used for, though it may tell the operating system what state to leave that data in when it is finished with it.

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jafadmin

Joined: 19 Mar 2009
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PostPosted: Fri 08 Nov 2019, 22:52    Post subject:  

Burn_IT wrote:
Read what I said!
I said IN JCL

The dd statement in JCL is used to link a logical filename in a program to a physical data stream somewhere in the real world.
It has nothing to do with what that data is used for, though it may tell the operating system what state to leave that data in when it is finished with it.


http://ibmmainframes.com/about9267.html


.
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Burn_IT


Joined: 12 Aug 2006
Posts: 3702
Location: Tamworth UK

PostPosted: Mon 11 Nov 2019, 09:58    Post subject:  

And your point is??
In all those examples there is a program involved to do the copying.

Even if you do want to use JCL to do things like allocate space for a file or catalogue a file, you still have to have a "PGM" statement (an executable program).
The most used program for this is IEFBR14 which consists of just a return statement.

In those examples the program is the sort utility.

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01101001b


Joined: 08 Mar 2017
Posts: 124
Location: Buenos Aires, Argentina

PostPosted: Wed 25 Mar 2020, 20:21    Post subject:  

mistfire wrote:
DD

DUPLICATE
DISK

Somewhere I read it as "disk dump" Wink
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Burn_IT


Joined: 12 Aug 2006
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Location: Tamworth UK

PostPosted: Wed 25 Mar 2020, 20:35    Post subject:  

deleted as a repeat
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01micko


Joined: 11 Oct 2008
Posts: 8787
Location: qld

PostPosted: Thu 26 Mar 2020, 00:11    Post subject:  

Digressing somewhat but still on OT..

How many know what the .tar extension stands for?

Tape Archive.

Of course if you are over 35 or so and in IT or are a coder you knew this, no? Cool

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Leon

Joined: 22 Jun 2005
Posts: 268

PostPosted: Thu 26 Mar 2020, 05:21    Post subject:  

01micko wrote:
How many know what the .tar extension stands for?

Tape Archive.

Of course if you are over 35 or so and in IT or are a coder you knew this, no? Cool

Or at least 35 or so ago starting in IT and has inserting tapes in tape drives of PDP (Programmed Data Processor)...

Very Happy
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