I frequently use Perl’s in place file editing from the command line. What I didn’t consider until it bit me today is that the file ownership can change using this method.

Here’s the original file, owned by tomcat_6 and only readable by user and group.

$ ls -l web.xml
-rw-rw---- 1 tomcat_6 tomcat 49384 Jul 10 11:38 web.xml

I belong to the tomcat group, so have write permissions to the file. The enclosing directory is also tomcat group writable. The importance of this is noted below.

Using the ‘perl pie’ one-liner to make an in place edit if the file:

$ perl -p -i -e 's;<session-timeout>\d+</session-timeout>;\
<session-timeout>1440</session-timeout>;' web.xml

Now the file is owned by me and my default group.

$ ls -l web.xml
-rw-rw---- 1 crashing daily 49384 Jul 10 21:55 web.xml

Most critically, now the file is no longer readable by the tomcat processes. This little change prevented my Tomcat server from starting. Ouch.

sed is a little nicer. It changes the owner but not the group.

$ sed -i 's;<session-timeout>.*</session-timeout>;\
<session-timeout>1445</session-timeout>;g' web.xml
$ ls -l web.xml 
-rw-rw---- 1 crash tomcat 49385 Jul 10 22:44 web.xml

Neither the Perl nor the sed one-liners work if the directory is not writable because Perl and sed require unlinking the original file and replacing it with a new version.

The winner for both maintaining file ownership and working if the directory is not writable is ed.

$ ed - "web.xml" <<EOF
,s;<session-timeout>[[:digit:]]*</session-timeout>;\
<session-timeout>1440</session-timeout>;
w
EOF

ed truly does an in place edit. Nice. If only I could remember the syntax.

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