NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/comp/unix/question/14753

Text File | 1992-12-16 | 1.5 KB | 50 lines

Newsgroups: comp.unix.questions
Path: sparky!uunet!zaphod.mps.ohio-state.edu!news.acns.nwu.edu!casbah.acns.nwu.edu!navarra
From: [e-mail] (John Navarra)
Subject: cat a > a , getting it straight
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Organization: Northwestern University, Evanston Illinois.
Date: Thu, 17 Dec 1992 02:08:05 GMT
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I have a question on shell redirection. If I do
$ cat a > a

I get a warning message from cat saying that the input file is the same as
the output. As I understand it, when a shell opens a file for redirection,
it truncates its length to zero. Thus, you wind up cat'ing a zero length
file to a zero-length file. Result: your original file was blown away.
Similarly, if I do
$ wc a > a
$ cat a
0       0       0 a

wc gives me the result I expect.

Now, if I do:
$ cat a
file a
$ cat a >> a
cat: input a is output
$ cat a
file a        #why aren't there two lines of 'file a'?
$ wc a >> a
$ cat a
file a
       1       2       7 a

Why does appending a to a with cat not work but wc a >> a does produce
the output from wc that is expected?

confused just a little,
-tms

--
You can get further with a kind word | You can get further with a kind word
and a gun than a kind word alone.    | and a phaser than a kind word and a gun.
          --al capone                |           -- John Navarra
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