Re: don't just seek to the next line if the script has been edited

[Greg Wooledge]

On Fri, Jun 07, 2013 at 09:09:53PM +0800, jidanni@jidanni.org wrote:
> Well OK but sometimes a script could be running for years, during which
> any change to a file will result in bash executing random bytes...

This is why you don't edit an installed script in-place.  Instead, you
move it aside, or remove it, or hardlink it, or do some other fancy thing.


# This is the naive way.  It is bad because an already-running shell reading
# the script as input may get garbled text.  Also, there is a period of
# vulnerability where the new script is only partially written to disk;
# anyone running the script at that time may just get part of it.  Finally,
# the old script is not preserved, so if the OS crashes during the copy,
# you have neither the old one nor the new one.

cp myscript /usr/local/bin


# This looks better at first glance, and it dodges the problems of mixing
# old and new text in an already-running shell, but it leaves a period of
# vulnerability where there's no installed script, or where there's a
# partially installed script, and it doesn't prevent loss of the old script
# if the OS crashes during the copy.  So, this is still bad.

rm /usr/local/bin/myscript && cp myscript /usr/local/bin


# This one is only slightly better.  It preserves the old script in case
# the OS crashes during the copy, which allows for manual rollback, but
# it still leaves that vulnerable period where someone could invoke the
# script while it's only partially copied.

mv /usr/local/bin/myscript /usr/local/bin/myscript.bak &&
cp myscript /usr/local/bin


# This is better.  The mv is atomic, so there is no period of vulnerability
# during which the script is either missing or partially installed.

cp myscript /usr/local/bin/myscript.tmp &&
mv /usr/local/bin/myscript.tmp /usr/local/bin/myscript


# Alternative, maintaining several installed versions on disk simultaneously.
# The installed "script" is really a symlink.  The ln is atomic, so this
# also avoids all the issues, as long as you never use the same version
# number more than once.

cp myscript /usr/local/bin/myscript-some.version.number &&
ln -sf myscript-some.version.number /usr/local/bin/myscript