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Global Shell Options: Turning OSH into Oil

This document describes global shell options, which look like this:

shopt --set strict_backslash  # Oil style
set -o errexit                # Bourne shell style

They can affect parsing or execution, and are used to gradually turn the OSH language into the Oil language.

For example, Oil doesn't have word splitting on whitespace, use Simple Word Evaluation by default. (Blog: Oil Doesn't Require Quoting Everywhere).

This isn't the only use for them, but it's one of the main uses.

Quick Start

The option groups strict:all and oil:basic are "canned settings" that relieve you of having to know about dozens of shell options.

If you put this line at the top of your shell script, it will still run under other shells, but OSH will act as sort of a "runtime linter":

# Abort on more errors, but fixes will still be compatible
shopt -s strict:all 2>/dev/null || true 

If you don't care about running under other shells, use this:

# Start enabling Oil syntax and semantics
shopt --set oil:basic

This second line may break a few things, but is designed to be an easy upgrade. See Shell Language Deprecations.

Use bin/oil for a brand new Oil script, opting into all enhancements. Your shebang line might be #!/usr/bin/env oil. This is the equivalent of shopt --set oil:all.

That's all most users need to know. For more details, see the wiki page: Gradually Upgrading Shell to Oil.

FAQ: Aren't Global Variables Bad?

• Oil adds scope with Ruby-like blocks.
• Like all Bourne shells, Oil uses process-based concurrency. It doesn't have shared memory.

Usage

There are several different ways of using shell options.

Preferred Style

Oil has long flags for readability, which are preferred:

shopt --set errexit
shopt --unset errexit

It also allows scoped options:

shopt --unset errexit {
  false    # non-zero status ignored
  ls /bad
}
false  # original setting restored

Bourne Shell Style

For compatibility, these styles works in Oil:

set -e          # abort script on non-zero exit exit code
set +e          # turn it off

set -o errexit  # a more readable version of the above
set +o errexit 

Bash-style option with shopt:

shopt -s nullglob  # turn it on
shopt -u nullglob  # turn it off

Setting Options Via Command Line Flags

You typically invoke the shopt builtin at the the top of a script, but you can also set options at the command line:

osh -O errexit -c 'shopt -p -o'  # turn on Bourne option
osh +O errexit -c 'shopt -p -o'  # turn off Bourne option

osh -O strict_tilde -c 'shopt -p'  # turn on Oil option
osh +O strict_tilde -c 'shopt -p'  # turn off Oil option

Inspecting Option State

Shell has many ways to do this, like:

set -o                      # print all Bourne shell options
shopt -p                    # print all bash options
shopt -p nullglob failglob  # print selected options

TODO: Oil should enable shopt --print for all options. It should have a flat list.

Option Groups Are Named

To let you turn them all on or off at once.

List of Option Groups

• strict:all: Help you find bugs. Do NOT break things to improve style.
• oil:basic: Allow using Oil features that are unlikely to break something, or have an easy fix (example: @foo -> '@foo', and () -> forkwait). Again, do NOT break things to improve style.
• oil:all: Allow even more Oil features. And also break things to improve style. (Example: simple_eval_builtin).

TODO: Do we need simple:all?

Example: oil:all

Runnig bin/oil is equivalent to

shopt --set oil:all

It turns on:

• errexit, nounset (sh modes to get more errors)
• pipefail and inherit_errexit (bash modes to get more errors)
• Oil modes:
  • simple_word_eval (subsumes nullglob that strict:all includes)
  • command_sub_errexit
  • strict_* (strict_array, etc.)
  • parse_* (parse_at, etc.)

Kinds of Options, With Examples

This is NOT FORMAL like GROUPS. GROUPS AND Kinds are different!

They are orthogonal axes.

• Groups: how much of Oil do you want to use?
• Kinds: Does this option affect parsing behavior, runtime behavior, or something else?

Naming Conventions

• parse_*: Change parsing.
  • enable new features: parse_at, parse_equals.
  • turn off to reject bad or old code: parse_backticks, parse_backslash, parse_dollar.
• strict_*: Fail at runtime instead of ignoring the bug like bash.
  • ${#s} on invalid unicode is a runtime error.
  • ~typo is a runtime error.
• simple_*: Break things to improve style.
  • simple_eval_builtin, simple_echo.
  • simple_word_eval is the most aggresive

Strict Options Produce More Errors

These options produce more programming errors. Importantly, the resulting program is still compatible with other shells.

For example, shopt -s strict_array produces runtime errors when you confuse strings and arrays. After you fix these problems, your program will still run correctly under bash.

In contrast, if you set shopt -s simple_word_eval (an option that doesn't start with strict_), the semantics of your program have changed, and you can no longer run it under other shells. It's considered an "Oil option": by setting it, you're using parts of the Oil language.

Parse Options Change Syntax

Options that affect parsing start with parse_. For example, shopt -s parse_at enables splicing with the @ character:

var words = %(ale bean)
write -- @words
# =>
# ale
# bean

and inline function calls:

write -- @split('ale bean')
# =>
# ale
# bean

As another example, shopt --set parse_brace takes over the { } characters. Specifically, it does three things:

1. Allow builtins like cd to take a block (discussed in a Zulip thread)
2. Control flow like if, case, for, and while/until, use curly brace delimiters instead of then/fi, do/done, etc. See below.
3. To remove confusion, braces must be balanced inside a word. echo foo{ is an error. It has to be echo foo\{ or echo 'foo{'.
   • In a correct brace expansion, they're always balanced: {pea,coco}nut
   • This is so that the syntax errors are better when you forget a space.

Here's idiomatic Oil syntax after parse_brace:

cd /tmp {
  echo $PWD
}

if test -d foo {
  echo 'dir'
} elif test -f foo {
  echo 'file'
} else {
   echo 'neither'
}

# Single line statements are supported:
if test -d / { echo 'dir' } else { echo 'nope' }

while true {
  echo hi
  break
}

# Loop over words
for x in ale bean *.sh {
  echo $x
}

# Replace 'in' with {, and 'esac' with }
case $x {
  *.py)
    echo python
    ;;
  *.sh)
    echo shell
    ;;
}

What's the motivation for this? Mainly familiarity: I hear a lot of feedback that nobody can remember how to write if statements in shell. See The Simplest Explanation of Oil.

Runtime Options Change Behavior

• simple_echo. Changes the flags accepted by the echo builtin, and style of flag parsing. See the Builtins > echo below.
• simple_word_eval. Word evaluation consists of one stage rather than three:
  • No word splitting or empty elision. (In other words, arity isn't data-dependent.)
  • Static globbing, but no dynamic globbing. (In other words, data isn't re-parsed as code.)
  • This option is intended to be implemented by other shells.

TODO: copy examples from spec tests

echo $dir/*.py

• command_sub_errexit. A error in a command sub can cause the parent shell to exit fatally. Also see inherit_errexit and strict_errexit.

Complete List of Options

These documents have a short description of each option:

• OSH Help Topics > option
• Oil Help Topics > option

(TODO: longer descriptions.)

Related Documents

• Up: Interpreter State, which is under construction)