Re: [PLUG] sed question

[Bill Jonas <bill@billjonas.com>]

On Tue, Nov 20, 2001 at 12:44:47PM -0500, Jason Wertz wrote:
> am I missing a command line arg or is my use of the command wrong?

It's actually a shell issue, not sed.

The way that redirection works is that when you enter the command, the
specified file is opened for writing right away, including truncation is
you specify > instead of >>.  The command does not complete first.  So
basically, if you do something like 'sed s/class=\"bodytext\"//g
computing.php > computing.php', computing.php is truncated and opened
for writing, then the sed command is run.  Since the file now has no
contents, the sed command will exit, and you're left with a zero-length
file.

The shell doesn't know how big the output is going to be.  Consider the
extreme case.  Imagine if you ran that command on a 1.5GB file; the
shell would be trying to buffer all that data in memory before writing
it to disk.

In a book I read (it might have been _Programming in the UNIX
Environment_), a shell-script solution was presented to allow this sort
of thing without having to make a temporary file either before or after.
I don't have the book in front of me (it was borrowed), but I can try
recall it if anyone's interested.

-- 
Bill Jonas    *    bill@billjonas.com    *    http://www.billjonas.com/

Developer/SysAdmin for hire!   See http://www.billjonas.com/resume.html

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