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Global Shell Options: Turning OSH into YSH

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

shopt --set strict_backslash  # YSH style
shopt --set ysh:upgrade       # A whole group of options
set -o errexit                # Bourne shell style

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

For example, YSH doesn't have word splitting on whitespace. Instead, it use Simple Word Evaluation. (Blog: Oil Doesn't Require Quoting Everywhere). (Until 2023, YSH was called the "Oil language".)

This isn't the only use for options, but it's an important one.

What Every User Should Know (2 minutes)

When you run bin/osh, the option groups strict:all and ysh:upgrade are "canned settings" that relieve you of having to know about dozens of shell options.

Running bin/ysh is equivalent to using shopt --set ysh:all in bin/osh.

Let's look at three examples.

Strict

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 

Upgrade

If you want to upgrade a script, and don't care about running under other shells, use this:

# Start enabling YSH syntax and semantics
shopt --set ysh:upgrade

This second line may break a few things, but is designed to be an easy upgrade. See What Breaks When You Upgrade to YSH.

YSH

If you're writing a new script, you can use bin/ysh to get all enhancements. Typically you use a shebang line like this:

#!/usr/bin/env ysh

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

Using Shell Options

There are several different ways of using shell options.

Preferred Style

YSH 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 YSH:

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 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 YSH option
osh +O strict_tilde -c 'shopt -p'  # turn off YSH 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
shopt -p ysh:upgrade          # print options in the given group

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

Kinds of Options, With Examples

Option groups like ysh:upgrade are baked into the interpreter. What follows is an informal list of kinds of options, which are different categorization:

• Groups: How much of YSH 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 aggressive

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 "YSH option": by setting it, you're using parts of YSH.

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 YSH 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.

List of Options

Selected Options

strict_arith. Strings that don't look like integers cause a fatal error in arithmetic expressions.

strict_argv. Empty argv arrays are disallowed (because there's no practical use for them). For example, the second statement in x=''; $x results in a fatal error.

strict_array. No implicit conversions between string an array. In other words, turning this on gives you a "real" array type.

strict_control_flow. break and continue outside of a loop are fatal errors.

simple_eval_builtin. The eval builtin takes exactly one argument. It doesn't concatenate its arguments with spaces, or accept zero arguments.

strict_word_eval. More word evaluation errors are fatal.

• String slices with negative arguments like ${s: -1} and ${s: 1 : -1} result in a fatal error. (NOTE: In array slices, negative start indices are allowed, but negative lengths are always fatal, regardless of strict_word_eval.)
• UTF-8 decoding errors are fatal when computing lengths (${#s}) and slices.

For options affecting exit codes, see the error handling doc.

Complete List

See the Chapter on Global Shell Options in the reference.

FAQ: Aren't Global Variables Bad?

Options are technically globals, but YSH controls them in 2 ways:

1. It has scoped mutation with Ruby-like blocks.
   • Example: shopt --unset errexit { false }
2. Like all Bourne shells, YSH uses process-based concurrency. It doesn't have shared memory.

Related Documents

• Up: Interpreter State, which is under construction