Oh, a new Unix shell

Why oh?

Oh is a reimagining of the Unix shell.

Oh provides:

• First-class channels, pipes, environments and functions;
• A list type (no word splitting);
• Rich return values that work with standard shell constructs;
• Support for modularity;
• Lexical scope;
• Exceptions;
• A simplified set of evaluation and quoting rules;
• Kernel-style fexprs (allowing the definition of new language constructs); and
• A syntax that deviates as little as possible from established conventions;

Oh was motivated by the belief that many of the flaws in current Unix shells are not inherent but rather historical. Design choices that are now clearly unfortunate in retrospect have been carried forward in the name of backward compatibility.

Oh's goal is a language that is not only more powerful and more regular but one that respects the conventions established by the Unix shell over the last half-century.

Getting started

Installing

The easiest way to try oh is to download a precompiled binary.

DragonFly BSD

amd64

FreeBSD

386, amd64, arm, arm64

illumos

amd64

Linux

386, amd64, arm, arm64, mips, mips64, mips64le, mipsle, ppc64, ppc64le, riscv64, s390x

macOS

amd64, arm64

OpenBSD

386, amd64, arm, arm64, mips64

Solaris

amd64

You can also build oh from source. With Go 1.16 or later installed, type,

go install github.com/michaelmacinnis/[email protected]

Configuring

When oh starts, it attempts to read a file called .oh-rc in the home directory of the current user. You can override this path by setting the OH_RC environment variable to the full path of an alternative file before invoking oh.

The oh rc file is useful for setting environment variables and defining custom commands. It's also a good place to override oh's default prompt. The command below replaces oh's default prompt method with one that displays the current date.

replace-make-prompt (method (suffix) {
    return `(date)$suffix
})

Oh (thanks to peterh/liner) also provides a searchable command history. By default, this history is stored in a file called .oh-history in your home directory. You can override this by setting the OH_HISTORY environment variable to the full path of an alternative file before invoking oh.

Comparing oh to other Unix shells

Oh is a Unix shell. If you've used other Unix shells, oh should feel familiar. Below are some specific differences you may encounter.

Clobbering

When redirecting output oh will not overwrite an existing file. To force oh to overwrite (clobber) an existing file add a pipe, |, character immediately after the redirection operator. For example,

command >| out.txt

Oh's pipe and redirection syntax is as follows.

* - Used in process substitution.

Command substitution

Many Unix shells support command substitution using the historical backtick syntax,

`command`

or the POSIX syntax,

$(command)

Oh has one syntax for command substitution,

`(command)

This syntax is both nestable and unambiguous.

Here documents

Oh does not have here documents. It does however allow strings to span lines and provides a here command that takes a string argument and can be used to the same effect. For example,

# Build oh for supported BSD platforms
here "
dragonfly amd64
freebsd 386
freebsd amd64
freebsd arm
freebsd arm64
openbsd 386
openbsd amd64
openbsd arm
openbsd arm64
openbsd mips64
" | mill (o a) {
    echo ${o}/${a}
    GOOS=${o} GOARCH=${a} go build -o oh-latest-${o}-${a}
}

Variables

To introduce a new variable, use the define command,

define x 3

To introduce a variable that will be visible to external processes, use the export command,

export GOROOT /usr/local/go

To set the value of an existing variable, use the set command,

set x 4

Variables and implicit concatenation

Like other shells, oh implicitly concatenates adjacent string/symbol values. Unlike other shells, oh allows a larger set of characters to appear in variable names. In addition to letters, numbers, and the underscore character, the following characters,

'!', '%', '*', '+', '-', '?', '[', ']',  and '^' 

can be used in oh variable names. The command,

echo $set!

will cause oh to attempt to resolve a variable called set!. The following characters,

',', '.', '/', ':', '=', '@', and '~'

always result in a symbol of one character. This ensures that commands like,

cd $PWD/$dir

work as expected. When using implicit concatentation, unexpected behavior can be avoided by enclosing variable names in braces.

More detailed comparison

For a detailed comparison to other Unix shells see: Comparing oh to other Unix Shells

Using oh

For more information on using oh, see: Using oh

Contributing to oh

Oh is an ongoing experiment and it needs your help. Try oh. Let me know what works for you and what doesn't.

Pull requests are welcome. For information on contributing, see: CONTRIBUTING

You can also sponsor me through GitHub Sponsors or Patreon.

License

MIT