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Full Discussion: Sed command help (appending)
Top Forums Shell Programming and Scripting Sed command help (appending) Post 302685649 by Don Cragun on Monday 13th of August 2012 07:34:56 AM
Old 08-13-2012
Quote:
Originally Posted by RudiC
Please enlighten me: why not sed?
If /etc/group occupies more than one disk block, it would be safer to use:
Code:
sed "/^$Grp:/s/$/,$Usr/" /etc/group > tmpfile
mv tmpfile /etc/group

to avoid the possibility of /etc/group being corrupt if someone tries to authenticate during the time it takes the "w" command to complete in the ed script I gave, but slufoot80 explicity said:
Quote:
I need to append a user to the end of a group in the /etc/group file
not that he wanted to a add user to the end of a group in a copy of the /etc/group file.

Using the code above, you also have to be sure that tmpfile isn't an existing file and can't be created (hi-jacked) by a hacker between the sed and the mv. Note that you to run this in /etc; if tmpfile is on a different file system than /etc, the mv won't be an atomic operation any more than the w command in ed would have been.
 

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TMPFILE(3)								 1								TMPFILE(3)

tmpfile - Creates a temporary file

SYNOPSIS
resource tmpfile (void ) DESCRIPTION
Creates a temporary file with a unique name in read-write (w+) mode and returns a file handle . The file is automatically removed when closed (for example, by calling fclose(3), or when there are no remaining references to the file handle returned by tmpfile(3)), or when the script ends. For details, consult your system documentation on the tmpfile(3) function, as well as the stdio.h header file. RETURN VALUES
Returns a file handle, similar to the one returned by fopen(3), for the new file or FALSE on failure. EXAMPLES
Example #1 tmpfile(3) example <?php $temp = tmpfile(); fwrite($temp, "writing to tempfile"); fseek($temp, 0); echo fread($temp, 1024); fclose($temp); // this removes the file ?> The above example will output: writing to tempfile SEE ALSO
tempnam(3), sys_get_temp_dir(3). PHP Documentation Group TMPFILE(3)
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