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.
Roughly speaking, shells treat arithmetic like "macro processing", while OSH treats it more like part of a programming language.
Despite these differences, OSH is very compatible with existing shell scripts.
Arithmetic is statically parsed, so expressions like $(( 1 $op 2 )) fail with
a parse error. Use an explicit eval for these rare use cases.
Strings like '' or 'foo' aren't coerced to 0 in arithmetic contexts.
OSH produces a fatal error unless you opt out with shopt -u strict-arith.
(This is the only strict option that's on by default.)
In other shells, printf %d invalid_integer prints 0 and a warning. OSH
gives you a runtime error.
This section describes differences related to static parsing. OSH avoids the dynamic parsing of most shells.
(Note: This section should encompass all the failures from the wild tests and spec tests.
Strings should be quoted inside array indices:
No:
"${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.
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 ;
$(cd / && ls) # Even better
[[ !(a == a) ]] is an extended glob.[[ ! (a == a) ]] is the negation of an equality test.In bash the rules are more complicated, and depend on shopt -s extglob. The
extglob setting does nothing in OSH.
Lines like EOF] or EOF) don't end here docs. The delimiter must be on its
own line.
No:
a=$(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
This means that they aren't "dynamic":
b=break
while true; do
$b # doesn't break in OSH
done
Static control flow will allow static analysis of shell scripts.
(Test cases are in spec/loop).
Bash allows:
a[1 + 2 * 3]=value
OSH only allows:
a[1+2*3]=value
because it parses with limited lookahead. The first line would result in the
execution of a command named a[1.
These differences occur in subsequent stages of parsing, or in runtime parsing.
No:
x=abc >out.txt
x=${y} >out.txt
x=$((1 + 2)) >out.txt
# This is the only one that makes sense, but is still disallowed.
x=$(echo hi) >out.txt
Yes:
x=$(echo hi >out.txt)
The first three constructs don't make sense, and the fourth has a clearer alternative spelling, so OSH disallows such redirects altogether.
That is, they can't be variables themselves.
No:
declare "$1"=abc
Yes:
declare x=abc
NOTE: This restriction will probably be relaxed.
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}.
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.)
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.
In rare cases, OSH processes backslashes within backticks differently than other shells. However there are two workarounds that are compatible with every shell.
No:
`echo \"` # is this a literal quote, or does it start a string?
Yes:
$(echo \") # $() can always be used instead of ``.
# There's no downside to the more modern construct.
`echo \\"` # also valid, but $() is more readable
Notes:
Almost all "real" aliases should work in OSH. But these don't work:
alias left='{'
left echo hi; }
(cases #33-#34 in spec/alias)
or
alias a=
a (( var = 0 ))
Details on the OSH parsing model:
SimpleCommand are the only ones that are further alias-expanded.For example, these result in SimpleCommand nodes:
ls -lread -n 1 (normally a builtin)myfunc fooThese don't:
x=42declare -r x=42break, continue, return, exit — as explained above, these are
keywords and not builtins.{ echo one; echo two; }for, while, case, functions, etc.Most shells split the entries of arrays like "$@" 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.
errexit while it's temporarily disabledIn all shells, checks for non-zero status are disabled in these situations:
if, while, and until constructs!|| and && except the last.Now consider this situation:
errexit is onset -o errexit to turn it back on.This is a fatal error in OSH. Other shells delay the restoration of errexit
until after the temporary disablement.
Good articles on errexit:
OSH also has strict-errexit to fix two issues with bash's behavior:
errexit should be inherited inside $(), so errors aren't ignored. OSH
behaves like dash and mksh, not bash.local foo=... should be fatal. In other shells, local is
a command rather than a keyword, which means local foo=$(false) behaves
differently than than foo=$(false).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 OSH manual for details.
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.
External:
set -o posix does in bash.