IFS null is not the same as unset IFS?

Question

I read a great question on here called Understanding IFS. I was surprised because the answers and comments quote POSIX which states that IFS= is not the same as unsetting IFS. If you unset IFS, apparently the default value is used. If you make IFS null instead, there is no splitter. I knew I had seen a different take on that, and I found this in my bookmarks:

Bourne Shell Programming

$IFS

The very first statement in your script should be

IFS=

which resets the input field separator to its default value. Otherwise, you inherit $IFS from the user, who may have set it to some bizarre value in order to make sh parse strings differently from the way you expect, and induce weird behavior.

So was that true some time ago or is the author just wrong?

Solution

The answers you found on Stack Exchange are right and this tutorial is wrong. You can experiment by yourself or look it up in the standard. An unset IFS is equivalent to setting it to the default value of space-tab-newline, while an empty IFS effectively turns off field splitting.

You can consult Sven Mascheck's page on IFS about historical implementations. A few historical shells didn't like unset IFS, and a very old version of ksh treated it like an empty IFS but all modern shells and most old shells treat an unset IFS like the default value.

You should not start your script with IFS= unless you want to turn off field splitting (which can be a reasonable decision — but note that you still need to put double quotes around substitutions to avoid globbing, unless you turn that off with set -f too). To reset the default value, use unset IFS. It's debatable whether this is useful at the beginning of a script; there are plenty of other bad things such as a dodgy PATH that the caller can do to make your script go wrong.

This tutorial also advises to reset PATH. This is usually bad advice. In most cases, you cannot predict what the correct search path is, but the user knows. How do you know whether /usr/local/bin or /home/bob/bin contains bug-fixed versions of utilities on an ancient unix where the ones in /usr/bin are buggy? Do you really want to embed all the logic to figure out whether to put /usr/xpg6/bin ahead of /bin? At what position you want /usr/gnu/bin? Do not reset PATH unless your script targets a specific system.

I haven't read this tutorial, but I did check one thing: it doesn't tell you right from the start to always put double quotes around variable substitutions and command substitutions. So I do not think this tutorial is a good one.

Comments

[Gilles 'SO- stop being evil']

@Melab unset IFS does not reset IFS to its default value, it unsets it. But an unset IFS has the same effect as IFS set to the default value.