commit: 3c9c906ebec8d63884108618cc2f8322a1712e12
parent b60422c99c896535dd1d43b19874baeaf7d5dbd1
Author: Drew DeVault <sir@cmpwn.com>
Date: Mon, 31 Jul 2023 12:44:49 +0200
The rc shell and its excellent handling of whitespace
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+---
+title: The rc shell and its excellent handling of whitespace
+date: 2023-07-31
+---
+
+*This blog post is a response to Mark Dominus' "[The shell and its crappy handling of whitespace](https://blog.plover.com/Unix/whitespace.html)"*.
+
+I've been working on a shell for Unix-like systems called
+[rc](https://git.sr.ht/~sircmpwn/rc), which draws heavily from the Plan 9 shell
+[of the same name](http://man.9front.org/1/rc). When I saw Mark's post about the
+perils of whitespace in POSIX shells (or derived shells, like bash), I thought
+it prudent to see if any of the problems he outlines are present in the shell
+I'm working on myself. Good news: they aren't!
+
+Let's go over each of his examples. First he provides the following example:
+
+```
+for i in *.jpg; do
+ cp $i /tmp
+done
+```
+
+This breaks if there are spaces in the filenames. Not so with rc:
+
+```
+% cat test.rc
+for (i in *.jpg) {
+ cp $i subdir
+}
+% ls
+a.jpg b.jpg 'bite me.jpg' c.jpg subdir test.rc
+% rc ./test.rc
+% ls subdir/
+a.jpg b.jpg 'bite me.jpg' c.jpg
+```
+
+He gives a similar example for a script that renames jpeg to jpg:
+
+```
+for i in *.jpeg; do
+ mv $i $(suf $i).jpg
+done
+```
+
+This breaks for similar reasons, but works fine in rc:
+
+```
+% cat test.rc
+fn suf(fname) {
+ echo $fname | sed -e 's/\..*//'
+}
+
+for (i in *.jpeg) {
+ mv $i `{suf $i}.jpg
+}
+% ls
+a.jpeg b.jpeg 'bite me.jpeg' c.jpeg test.rc
+% rc ./test.rc
+% ls
+a.jpg b.jpg 'bite me.jpg' c.jpg test.rc
+```
+
+There are other shells, such as fish or zsh, which also have answers to these
+problems which don't necessarily call for generous quoting like other shells
+often do. rc is much simpler than these shells. At the moment it clocks in at
+just over 3,000 lines of code, compared to fish at ~45,000 and zsh at ~144,000.
+Admittedly, it's not done yet, but I would be surprised to see it grow beyond
+5,000 lines for version 1.0.[^1]
+
+[^1]: Also worth noting that these line counts are, to some extent, comparing
+ apples to oranges given that fish, zsh, and rc are written respectively in
+ C++/Rust, C, and Hare.
+
+The key to rc's design success in this area is the introduction of a second
+primitive. The Bourne shell and its derivatives traditionally work with only one
+primitive: strings. But command lines are made of *lists* of strings, and so a
+language which embodies the primitives of the command line ought to also be able
+to represent those as a first-class feature. In traditional shells a list of
+strings is denoted inline with the use of spaces within those strings, which
+raises obvious problems when the members themselves contain spaces; see Mark's
+post detailing the errors which ensue. rc adds lists of strings as a formal
+primitive alongside strings.
+
+```
+% args=(ls --color /)
+% echo $args(1)
+ls
+% echo $args(2)
+--color
+% echo $#args
+3
+% $args
+bin dev home lost+found mnt proc run srv swap tmp var
+boot etc lib media opt root sbin storage sys usr
+% args=("foo bar" baz)
+% touch $args
+% ls
+ baz 'foo bar'
+```
+
+Much better, right? One simple change eliminates the need for quoting virtually
+everywhere. Strings can contain spaces and nothing melts down.
+
+Let me run down the remaining examples from Mark's post and demonstrate their
+non-importance in rc. First, regarding $\*, it just does what you expect.
+
+```
+% cat yell.rc
+#!/bin/rc
+shift
+echo I am about to run $* now!!!
+exec $*
+% ls *.jpg
+'bite me.jpg'
+% ./yell.rc ls *.jpg
+I am about to run ls bite me.jpg now!!!
+'bite me.jpg'
+```
+
+Note also that there is no need to quote the arguments to "echo" here. Also note
+the use of shift; $\* includes $0 in rc.
+
+Finally, let's rewrite Mark's "lastdl" program in rc and show how it works fine
+in rc's interactive mode.
+
+```
+#!/bin/rc
+cd $HOME/downloads
+echo $HOME/downloads/`{ls -t | head -n1}
+```
+
+Its use at the command line works just fine without quotes.
+
+```
+% file `{lastdl}
+/home/sircmpwn/downloads/test image.jpg: JPEG image data, JFIF standard 1.01,
+aspect ratio, density 1x1, segment length 16, baseline, precision 8,
+5000x2813, components 3
+```
+
+Just for fun, here's another version of this rc script that renames files with
+spaces to without, like the last example in Mark's post:
+
+```
+#!/bin/rc
+cd $HOME/downloads
+last=`{ls -t | head -n1}
+if (~ $last '* *') {
+ newname=`{echo $last | tr ' \t' '_'}
+ mv $last $HOME/downloads/$newname
+ last=$newname
+}
+echo $HOME/downloads/$last
+```
+
+The only quotes to be found are those which escape the wildcard match testing
+for a space in the string.[^2] Not bad, right? Like Plan 9's rc, my shell
+imagines a new set of primitives for shells, then starts from the ground up and
+builds a shell which works better in most respects while still being very
+simple. Most of the problems that have long plagued us with respect to sh, bash,
+etc, are solved in a simple package with rc, alongside a nice interactive mode
+reminiscent of the best features of fish.
+
+rc is a somewhat complete shell today, but there is a bit more work to be done
+before it's ready for 1.0, most pressingly with respect to signal handling and
+job control, alongside a small bit of polish and easier features to implement
+(such as subshells, IFS, etc). Some features which are likely to be omitted, at
+least for 1.0, include logical comparisons and arithmetic expansion (for which
+/bin/test and /bin/dc are recommended respectively). Of course, rc is destined
+to become the primary shell of the [Ares operating system](https://ares-os.org)
+project that I've been working on, but I have designed it to work on Unix as
+well.
+
+Check it out!
+
+[^2]: This is a bit of a fib. In fact, globbing is disabled when processing the
+ args of the ~ built-in. However, the quotes are, ironically, required to
+ escape the space between the \* characters, so it's one argument rather than
+ two.
