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Known Differences Between OSH and Other Shells

This document is for sophisticated shell users.

You're unlikely to encounter these incompatibilities in everyday shell usage. If you do, there's almost always a simple workaround, like adding a space or a backslash.

OSH is meant to run all POSIX shell programs, and most bash programs.

Numbers and Arithmetic

printf '%d' and other numeric formats require a valid integer

In other shells, printf %d invalid_integer prints 0 and a warning. OSH gives you a runtime error.

Dynamically parsed command subs disallowed unless shopt -s eval_unsafe_arith

In shell, array locations are often dynamically parsed, and the index can have command subs, which execute arbitrary code.

For example, if you have code='a[$(echo 42 | tee PWNED)]', shells will parse this data and execute it in many situations:

echo $(( code ))  # dynamic parsing and evaluation in bash, mksh, zsh

unset $code

printf -v $code hi

echo ${!code}

OSH disallows this by default. If you want this behavior, you can turn on shopt -s eval_unsafe_arith.

Related: A 30-year-old security problem

Parsing Differences

This section describes differences related to static parsing. OSH avoids the dynamic parsing of most shells.

(Note: This section should encompass all the failures from the wild tests and spec tests.

Strings vs. Bare words in array indices

Strings should be quoted inside array indices:

No:

"${SETUP_STATE[$err.cmd]}"

Yes:

"${SETUP_STATE["$err.cmd"]}"

When unquoted, the period causes an ambiguity with respect to regular arrays vs. associative arrays. See Parsing Bash is Undecidable.

Subshell in command sub

You can have a subshell in a command sub, but it usually doesn't make sense.

In OSH you need a space after $(. The characters $(( always start an arith sub.

No:

$((cd / && ls))

Yes:

$( (cd / && ls) )   # Valid but usually doesn't make sense.
$({ cd / && ls; })  # Use {} for grouping, not ().  Note trailing ;
$(cd / && ls)       # Even better

Extended glob vs. Negation of boolean expression

The OSH parser distinguishes these two constructs with a space:

• [[ !(a == a) ]] is an extended glob.
• [[ ! (a == a) ]] is the negation of an equality test.

In bash, the parsing of such expressions depends on shopt -s extglob. In OSH, shopt -s extglob is accepted, but doesn't affect parsing.

Here doc terminators must be on their own line

Lines like EOF] or EOF) don't end here docs. The delimiter must be on its own line.

No:

a=$(cat <<EOF
abc
EOF)

a=$(cat <<EOF
abc
EOF  # this is not a comment; it makes the EOF delimiter invalid
)

Yes:

a=$(cat <<EOF
abc
EOF
)  # this is actually a comment

Spaces aren't allowed in LHS indices

Bash allows:

a[1 + 2 * 3]=value

OSH only allows:

a[1+2*3]=value

because it parses with limited lookahead. The first line would result in the execution of a command named a[1.

break / continue / return are keywords, not builtins

This means that they aren't "dynamic":

b=break
while true; do
  $b  # doesn't break in OSH
done

Static control flow will allow static analysis of shell scripts.

(Test cases are in spec/loop).

OSH has more builtins, which shadow external commands

For example, append is a builtin in OSH, but not in bash. Use env append or /path/to/append if you want to run an external command.

(Note that a user-defined function append takes priority over the builtin append.)

OSH has more keywords, which shadow builtins, functions, and commands

In contrast with builtins, keywords affect shell parsing.

For example, func is a keyword in OSH, but not in bash. To run a command named func, use command func arg1.

Note that all shells have extensions that cause this issue. For example, [[ is a keyword in bash but not in POSIX shell.

More Parsing Differences

These differences occur in subsequent stages of parsing, or in runtime parsing.

Brace expansion is all or nothing

No:

{a,b}{        # what does the second { mean?
{a,b}{1...3}  # 3 dots instead of 2

Yes:

{a,b}\{
{a,b}\{1...3\}

bash will do a partial expansion in the former cases, giving you a{ b{ and a{1...3} b{1...3}.

OSH considers them syntax errors and aborts all brace expansion, giving you the same thing back: {a,b}{ and {a,b}{1...3}.

Tilde expansion and Brace expansion don't interact

In bash, {~bob,~jane}/src will expand the home dirs of both people. OSH doesn't do this because it separates parsing and evaluation. By the time tilde expansion happens, we haven't evaluated the brace expansion. We've only parsed it.

(mksh agrees with OSH, but zsh agrees with bash.)

Brackets should be escaped within Character Classes

Don't use ambiguous syntax for a character class consisting of a single bracket character.

No:

echo [[]
echo []]

Yes:

echo [\[]
echo [\]]

The ambiguous syntax is allowed when we pass globs through to libc, but it's good practice to be explicit.

Data Structures

Arrays aren't split inside ${}

Most shells split the entries of arrays like "$@" and "${a[@]}" here:

echo ${undef:-"$@"}

In OSH, omit the quotes if you want splitting:

echo ${undef:-$@}

I think OSH is more consistent, but it disagrees with other shells.

Values are tagged with types, not locations (declare -i -a -A)

Even though there's a large common subset, OSH and bash have a different model for typed data.

• In OSH, values are tagged with types, which is how Python and JavaScript work.
• In bash, cells (locations for values) are tagged with types. Everything is a string, but in certain contexts, strings are treated as integers or as structured data.

In particular,

• The -i flag is a no-op in OSH. See Shell Idioms > Remove Dynamic Parsing for alternatives to -i.
• The -a and -A flags behave differently. They pertain to the value, not the location.

For example, these two statements are different in bash, but the same in OSH:

declare -A assoc     # unset cell that will LATER be an assoc array
declare -A assoc=()  # empty associative array

In bash, you can tell the difference with set -u, but there's no difference in OSH.

Indexed and Associative arrays are distinct

Here is how you can create arrays in OSH, in a bash-compatible way:

local indexed=(foo bar)
local -a indexed=(foo bar)            # -a is redundant
echo ${indexed[1]}                    # bar

local assoc=(['one']=1 ['two']=2)
local -A assoc=(['one']=1 ['two']=2)  # -A is redundant
echo ${assoc['one']}                  # 1

In bash, the distinction between the two is blurry, with cases like this:

local -A x=(foo bar)                  # -A disagrees with literal
local -a y=(['one']=1 ['two']=2)      # -a disagrees with literal

These are disallowed in OSH.

Notes:

• The = keyword is useful for gaining an understanding of the data model.
• See the Quirks doc for details on how OSH uses this cleaner model while staying compatible with bash.

Assignment builtins

The assignment builtins are export, readonly, local, and declare/typeset. They're parsed in 2 ways:

• Statically: to avoid word splitting in declare x=$y when $y contains spaces. bash and other shells behave this way.
• Dynamically: to handle expressions like declare $1 where $1 is a=b

builtin declare x=$y is a runtime error

This is because the special parsing of x=$y depends on the first word declare.

Args aren't split or globbed

In bash, you can do unusual things with args to assignment builtins:

vars='a=b x=y'
touch foo=bar.py spam=eggs.py

declare $vars *.py       # assigns at least 4 variables
echo $a       # b
echo $x       # y
echo $foo     # bar.py
echo $spam    # eggs.py

In contrast, OSH doesn't split or glob args to assignment builtins. This is more like the behavior of zsh.

Pipelines

Last pipeline part may run in shell process (zsh, bash shopt -s lastpipe)

In this pipeline, the builtin read is run in the shell process, not a child process:

$ echo hi | read x
$ echo x=$x
x=hi  # empty in bash unless shopt -s lastpipe

If the last part is an external command, there is no difference:

$ ls | wc -l
42

This is how zsh behaves, and how bash (sometimes) behaves with shopt -s lastpipe.

Pipelines can't be suspended with Ctrl-Z

Because the last part may be the current shell process, the entire pipeline can't be suspended.

OSH and zsh share this consequence of the lastpipe semantics.

In contrast, bash's shopt -s lastpipe is ignored in interactive shells.

${PIPESTATUS[@]} is only set after an actual pipeline

This makes it easier to check compound status codes without worrying about them being "clobbered".

Bash will set ${PIPESTATUS[@]} on every command, regardless of whether its a pipeline.

More Differences at Runtime

Alias expansion

Almost all "real" aliases should work in OSH. But these don't work:

alias left='{'
left echo hi; }

(cases #33-#34 in spec/alias)

or

alias a=
a (( var = 0 ))

Details on the OSH parsing model:

1. Your code is statically parsed into an abstract syntax tree, which contains many types of nodes.
2. SimpleCommand are the only ones that are further alias-expanded.

For example, these result in SimpleCommand nodes:

• ls -l
• read -n 1 (normally a builtin)
• myfunc foo

These don't:

• x=42
• declare -r x=42
• break, continue, return, exit — as explained above, these are keywords and not builtins.
• { echo one; echo two; }
• for, while, case, functions, etc.

Extended globs are more static like mksh, and have other differences

That is, in OSH and mksh, something like echo *.@(cc|h) is an extended glob. But echo $x, where $x contains the pattern, is not.

For more details and differences, see the Extended Glob section of the Word Language doc.

Completion

The OSH completion API is mostly compatible with the bash completion API, except that it moves the responsibility for quoting out of plugins and onto the shell itself. Plugins should return candidates as argv entries, not shell words.

See the completion doc for details.

Interactive Features

History Substitution Language

The rules for history substitution like !echo are simpler. There are no special cases to avoid clashes with ${!indirect} and so forth.

TODO: Link to the history lexer.

Links

• OSH Spec Tests run shell snippets with OSH and other shells to compare their behavior.

External:

• This list may seem long, but compare the list of differences in Bash POSIX Mode. That page tells you what set -o posix does in bash.