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pipes and streams and one file

Hi,
I'm writing little Sed wrapper. It's task is to replace text. I have to override original file.

so my first try was:

Code:

#!/bin/bash

sed s/$1/$2/g <$3 >$3

and i got empty file. Hire is my first question: why for commend like this:

Code:

./my_script foo bar file

and file like this

Code:

foo

foo

foo

file is empty instead of one "bar" word?

Anyway I fixed this with:

Code:

#!/bin/bash

sed s/$1/$2/g <$3|cat >$3

And It works! But I'm not sure why? I think I should get only one bar word because when first line is outputted the input file is maked.

The second approach works but maybe I should better use tmp file?

Sorry for my poor English. And TIA for answers.

yes it's better to use a temp file in this case. when you use > it wipes the file you are writing to and since you were reading and writing from the same file the contents were getting wiped before you did anything with them. The pipe to cat only worked because the file was small. If it had been larger than one buffered read I'd bet you see similar problems to your first attempt.

And in addition to the temp-files idea:

newer GNU seds (> 3) allow in-file changes.

Code:

sed -i.bak s/$1/$2/g $3

with an optional back-up file created.

Cheers,
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