Redirecting output after a "cat" command

Question

I first encountered this with "do shell script" in AS, and the issue also occurs in Terminal.
Basically, I want to get contents from a file, make some changes via "sed ," and return the changed version to the same file -- but the file always winds up empty. Without the redirect, the commands work as expected.
For example, in Terminal:
cat --path to some file-- | sed 's/this/that/g'
results in the expected changes (returned to Terminal)
but
cat --path to some file-- | sed 's/this/that/g' > path to some (same) file
results in a blank file
(The "sed" command isn't even necessary for this to happen; just trying to return the output of the "cat" command to the file results in a blank file.)
I've certainly been able to accomplish this sort of thing with commands other than "cat." Is this something that's not possible with "cat" or do I have the syntax wrong?
(Of course, I can get around this by following up with an "echo" command but, in "do shell script," that requires more code.)
Thanks in advance for your comments.

Answer 1

HI osimp,
   I've always thought that the "sed 's/this/that/g' < input > output" syntax looked cool because you can sort of see the flow of information. However, the end of the command is simply a pair of redirect directives. The first tells the shell to open the file and pipe the contents into the standard input stream of sed. Then the output of sed is piped into the output file.
   The command I was proposing, "sed 's/this/that/g' input > output", is simpler in that it tells sed to read the file directly rather than asking the shell to read it.
   You said above that you were interested in storing the result in a variable. That could be done simply with a command like:
OUTPUT=$( sed 's/this/that/g' input )
For many types of content this would work nicely but the shell will try to expand the output so plenty of things can cause errors. I was going to wait to post the above until I discovered a way around that expansion but I haven't had enough time to do that so I'm posting it and I'll see if I can improve it later.
   I know some about AppleScript and have written several scripts that combine capabilities of the shell with those of AppleScript. There are probably more people here that know AppleScript here than there are shell scripters on the AppleScript Discussion. You should try posting the whole question over here as well as over there. However, try and see if you can get the help of Deivy over there. He knows a lot about interactions between AppleScript and the shell.
Gary
~~~~
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