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Output file redirection

 
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# 1  
Old 04-25-2009
Output file redirection
Suppose I have a file named a
When I write
cat a>a
The following error message is displayed
cat: a: input file is output file
and my file a is truncated to zero size.
Also the exit status of the last command is 1
Can someone tell me what actually happens when I do so?
# 2  
Old 04-25-2009
1. the output file is cleared first.
2. then the cat command can't find the input file, because of step 1
3. the cat command returns an error.

and you file is zero bytes due to step 1.


the more common way of doing things is this:

Code:
cat a > /tmp/tempfile ; mv /tmp/tempfile a

note the use of a temporary file.
 
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