[olug] Unix Tip: EAT YOUR PERL PIE

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			      UNIX GURU UNIVERSE 
			         UNIX HOT TIP

			Unix Tip 2177 - December 17, 2002

		    http://www.ugu.com/sui/ugu/show?tip.today

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EAT YOUR PERL PIE

Mom always sed, "eat your Perl pie"!

A common task is to substitute 
text strings within one or 
more files.  The sed string 
substitution command is often 
used to accomplish this, where 
sed is passed a file name, the
string substitutions are written 
to another file, and then that 
file is copied over the first 
file to effect the changes
desired in the original.

This approach has some inherent 
problems.  The command syntax is 
lengthy and cumbersome, a 
secondary file is created
which involves additional disk 
I/O, and when that file is 
copied or moved back over the 
first one, permissions problems 
and file ownership problems can 
be created.  If your umask 
doesn't match the permissions of 
the original file, then the 
permissions of the final modified 
file will be different than those 
of the original.  Likewise, 
default ownerships and group 
names are imposed on the temporary 
copy, and if one is not careful, 
when the temporary copy is written 
back to the original file, these 
can destroy the original file's 
ownership and group information.

A much better way to accomplish 
text substitution within a file or 
group of files is to use the 
"perl pie" approach, as shown here 
on file foo:

perl -p -i -e 's/original text string/replacement string' foo

The above command will replace the 
first instance of "original text 
string" with "replacement string" 
in file foo.  If you want to 
perform this globally within the 
file, add the /g global specifier 
at the back end, as follows:

perl -p -i -e 's/original text string/replacementstring/g' foo

To act on several files in the 
same directory, change the file 
specification as needed, by 
specifying foo*, *, or whatever 
you need.

Note that perl uses the exact 
same syntax for the actual string 
substitution portion of the command 
as sed does, which makes the command 
syntax easy to remember. However, 
perl performs the entire operation 
on the file without the use of any
secondary files needing to be 
created, which eliminates the extra 
disk I/O and the potential 
permissions and ownership issues.

 

This tip generously supported by: jem at postfive.rose.hp.com




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