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Old 12-08-2012, 09:36 PM   #1
Registered: Dec 2010
Distribution: Scientific Linux Slackware
Posts: 91

Rep: Reputation: 2
Smile how to redirect input and output to one file

Suppose I want to manipulate a file foo, using commands like tr
I enter
tr a b <foo >foo
tr a b <foo | cat >foo
cat foo | tr a b >foo

As you know they all fail as all the contents of foo has been discarded
for the first command I enter, I guess it's because that when the tr command open foo for write, the foo has been overwritten. so the command can read nothing
for two pipelines, I thought it was the same reason.

So, is there a method to accomplish the task I want? or I have to use a tmp file?
Old 12-08-2012, 10:16 PM   #2
Registered: Sep 2003
Posts: 382

Rep: Reputation: 87
You might to trying in terms of your goal. You want to end up with a file named foo, that contains the changed lines of the original file named foo.
That's not the same as saying you have to use a file named foo as both the input and output file in the same command. Instead you could
change your command lines to look like these:

tr a b <foo > ; mv foo

tr a b <foo | cat > ; mv foo

cat foo | tr a b > ; mv foo


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