Why big is bad


If you know me, you know that I don’t like using /bin/bash for scripting. It’s not that hard to write scripts that are portable, and my earlier “10 tips” article might help.

Why don’t I like /bin/bash? There are many reasons, but it’s mostly about size.

drj$ ls -lL $(which sh bash)
-rwxr-xr-x 1 root root 959120 Sep 22 21:39 /bin/bash
-rwxr-xr-x 1 root root 109768 Mar 29 2012 /bin/sh

/bin/bash is nearly 10 times the size of /bin/sh (which in this case, is dash). It’s bigger because it’s loaded with features that you probably don’t need. An interactive editor (two in fact). That’s great for interactive use, but it’s just a burden for non-interactive scripts. Arrays. Arrays are really super useful and fundamental to many algorithms. In a real programming language. If you need arrays, it’s time for your script to grow up and become a program, in Python, Lua, Go, or somesuch.

Ditto job control.
Ditto Extended Regular Expression matching.
Ditto mapfile.
Ditto a random number generator.
Ditto a TCP/IP stack.

You might think that these things can’t harm you if you don’t use them. That’s not true. We have a little bit of harm just by being bigger. When one thing is 10 times bigger than it needs to be, no one will notice. When everything is 10 times bigger than it needs to be then it’s wasteful, and extremely difficult to fix. These features take up namespace. Got a shell script called source or complete? Can’t use it, those are builtins in bash. They slow things down. Normally I wouldn’t mention speed, but 8 years ago Ubuntu switched from bash to dash for the standard /bin/sh and the speed increase was enough to affect boot time. Probably part of the reason that bash is slower is simply that it’s bigger. There are more things it has to do or check even though you’re not making use of those features.

If you’re unlucky a feature you’ve never heard of and don’t use will interact with another feature or a part of your system and surprise you. If you’re really unlucky it will be a remote code exploit so easy to use you can tweet exploits, which is what ShellShock is. Did you know you can have functions in bash? Did you know you can export them to the environment? Did you know that the export feature works by executing the definition of the function? Did you know that it’s buggy and can execute more than bash expected? Did you know that with CGI you can set environment variables to arbitrary strings?

There are lots of little pieces to reason about when considering the ShellShock bug because bash is big. And that’s after we know about the bug. What about all those features of you don’t use and don’t even know about? Have you read and understood the bash man page? Well, those features you’ve never heard of are probably about as secure as the feature that exports functions to the environment, a feature that few people know about, and fewer people use (and in my opinion, no one should use).

The most important thing about security is attitude. It’s okay to have the attitude that a shell should have lots of useful interactive features; it’s arguable that a shell should have a rich programming environment that includes arrays and hash tables.

It’s not okay to argue that this piece of bloatware should be installed as the standard system shell.

2 Responses to “Why big is bad”

  1. Tony Finch Says:

    All good points, but I have a small nitpick. Not all the bloat you listed is optional: POSIX requires job control and vi-style command line editing (and like bash, (d)ash also has an emacs-mode extension).

  2. drj11 Says:

    I didn’t directly argue that a minimal POSIX shell is the answer, but I did imply that, and you’re right to nitpick.

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