| 1 | ---
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| 2 | default_highlighter: oils-sh
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| 3 | ---
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| 4 |
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| 5 | Known Differences Between OSH and Other Shells
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| 6 | ==============================================
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| 7 |
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| 8 | This document is for **sophisticated shell users**.
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| 9 |
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| 10 | You're unlikely to encounter these incompatibilities in everyday shell usage.
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| 11 | If you do, there's almost always a **simple workaround**, like adding a space
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| 12 | or a backslash.
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| 13 |
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| 14 | OSH is meant to run all POSIX shell programs, and most bash programs.
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| 15 |
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| 16 | <!-- cmark.py expands this -->
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| 17 | <div id="toc">
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| 18 | </div>
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| 19 |
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| 20 | <!--
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| 21 | TODO:
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| 22 |
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| 23 | - `` as comments in sandstorm
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| 24 | # This relates to comments being EOL or not
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| 25 |
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| 26 | - Pipelines
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| 27 | - PIPESTATUS only set when a pipeline is actually run.
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| 28 | - zsh-like lastpipe semantics.
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| 29 |
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| 30 | -->
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| 31 |
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| 32 | ## Numbers and Arithmetic
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| 33 |
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| 34 | ### printf '%d' and other numeric formats require a valid integer
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| 35 |
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| 36 | In other shells, `printf %d invalid_integer` prints `0` and a warning. OSH
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| 37 | gives you a runtime error.
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| 38 |
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| 39 | <!-- TODO: Probably should be strict_arith -->
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| 40 |
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| 41 | ### Dynamically parsed command subs disallowed unless `shopt -s eval_unsafe_arith`
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| 42 |
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| 43 | In shell, array locations are often dynamically parsed, and the index can have
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| 44 | command subs, which execute arbitrary code.
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| 45 |
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| 46 | For example, if you have `code='a[$(echo 42 | tee PWNED)]'`, shells will parse
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| 47 | this data and execute it in many situations:
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| 48 |
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| 49 | echo $(( code )) # dynamic parsing and evaluation in bash, mksh, zsh
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| 50 |
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| 51 | unset $code
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| 52 |
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| 53 | printf -v $code hi
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| 54 |
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| 55 | echo ${!code}
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| 56 |
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| 57 | OSH disallows this by default. If you want this behavior, you can turn on
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| 58 | `shopt -s eval_unsafe_arith`.
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| 59 |
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| 60 | Related: [A 30-year-old security problem](https://www.oilshell.org/blog/2019/01/18.html#a-story-about-a-30-year-old-security-problem)
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| 61 |
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| 62 | ## Static Parsing Differences
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| 63 |
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| 64 | This section describes differences related to [static
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| 65 | parsing](http://www.oilshell.org/blog/2016/10/22.html). OSH avoids the
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| 66 | dynamic parsing of most shells.
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| 67 |
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| 68 | (Note: This section should encompass all the failures from the [wild
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| 69 | tests](http://oilshell.org/cross-ref.html?tag=wild-test#wild-test) and [spec
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| 70 | tests](http://oilshell.org/cross-ref.html?tag=spec-test#spec-test).
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| 71 |
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| 72 | ### Strings vs. Bare words in array indices
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| 73 |
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| 74 | Strings should be quoted inside array indices:
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| 75 |
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| 76 | No:
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| 77 |
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| 78 | "${SETUP_STATE[$err.cmd]}"
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| 79 |
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| 80 | Yes:
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| 81 |
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| 82 | "${SETUP_STATE["$err.cmd"]}"
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| 83 |
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| 84 | When unquoted, the period causes an ambiguity with respect to regular arrays
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| 85 | vs. associative arrays. See [Parsing Bash is
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| 86 | Undecidable](https://www.oilshell.org/blog/2016/10/20.html) (2016).
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| 87 |
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| 88 | - [OILS-ERR-101](error-catalog.html#oils-err-101) explains more ways to fix
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| 89 | this.
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| 90 |
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| 91 | ### Subshell in command sub
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| 92 |
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| 93 | You can have a subshell in a command sub, but it usually doesn't make sense.
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| 94 |
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| 95 | In OSH you need a space after `$(`. The characters `$((` always start an
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| 96 | arith sub.
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| 97 |
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| 98 | No:
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| 99 |
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| 100 | $((cd / && ls))
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| 101 |
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| 102 | Yes:
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| 103 |
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| 104 | $( (cd / && ls) ) # Valid but usually doesn't make sense.
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| 105 | $({ cd / && ls; }) # Use {} for grouping, not (). Note trailing ;
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| 106 | $(cd / && ls) # Even better
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| 107 |
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| 108 |
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| 109 | ### Extended glob vs. Negation of boolean expression
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| 110 |
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| 111 | The OSH parser distinguishes these two constructs with a space:
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| 112 |
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| 113 | - `[[ !(a == a) ]]` is an extended glob.
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| 114 | - `[[ ! (a == a) ]]` is the negation of an equality test.
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| 115 |
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| 116 | In bash, the parsing of such expressions depends on `shopt -s extglob`. In
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| 117 | OSH, `shopt -s extglob` is accepted, but doesn't affect parsing.
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| 118 |
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| 119 | ### Here doc terminators must be on their own line
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| 120 |
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| 121 | Lines like `EOF]` or `EOF)` don't end here docs. The delimiter must be on its
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| 122 | own line.
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| 123 |
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| 124 | No:
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| 125 |
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| 126 | a=$(cat <<EOF
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| 127 | abc
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| 128 | EOF)
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| 129 |
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| 130 | a=$(cat <<EOF
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| 131 | abc
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| 132 | EOF # this is not a comment; it makes the EOF delimiter invalid
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| 133 | )
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| 134 |
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| 135 | Yes:
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| 136 |
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| 137 | a=$(cat <<EOF
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| 138 | abc
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| 139 | EOF
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| 140 | ) # this is actually a comment
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| 141 |
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| 142 |
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| 143 | ### Spaces aren't allowed in LHS indices
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| 144 |
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| 145 | Bash allows:
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| 146 |
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| 147 | a[1 + 2 * 3]=value
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| 148 |
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| 149 | OSH only allows:
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| 150 |
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| 151 | a[1+2*3]=value
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| 152 |
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| 153 | because it parses with limited lookahead. The first line would result in the
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| 154 | execution of a command named `a[1`.
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| 155 |
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| 156 | ### break / continue / return are keywords, not builtins
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| 157 |
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| 158 | This means that they aren't "dynamic":
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| 159 |
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| 160 | b=break
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| 161 | while true; do
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| 162 | $b # doesn't break in OSH
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| 163 | done
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| 164 |
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| 165 | Static control flow will allow static analysis of shell scripts.
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| 166 |
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| 167 | (Test cases are in [spec/loop][]).
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| 168 |
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| 169 | ### OSH has more builtins, which shadow external commands
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| 170 |
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| 171 | For example, `append` is a builtin in OSH, but not in `bash`. Use `env append`
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| 172 | or `/path/to/append` if you want to run an external command.
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| 173 |
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| 174 | (Note that a user-defined proc `append` takes priority over the builtin
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| 175 | `append`.)
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| 176 |
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| 177 | ### OSH has more keywords, which shadow builtins, functions, and commands
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| 178 |
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| 179 | In contrast with builtins, **keywords** affect shell parsing.
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| 180 |
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| 181 | For example, `func` is a keyword in OSH, but not in `bash`. To run a command
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| 182 | named `func`, use `command func arg1`.
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| 183 |
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| 184 | Note that all shells have extensions that cause this issue. For example, `[[`
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| 185 | is a keyword in `bash` but not in POSIX shell.
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| 186 |
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| 187 | ## Later Parsing Differences
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| 188 |
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| 189 | These differences occur in subsequent stages of parsing, or in runtime parsing.
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| 190 |
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| 191 | ### Brace expansion is all or nothing
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| 192 |
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| 193 | No:
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| 194 |
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| 195 | {a,b}{ # what does the second { mean?
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| 196 | {a,b}{1...3} # 3 dots instead of 2
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| 197 |
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| 198 | Yes:
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| 199 |
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| 200 | {a,b}\{
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| 201 | {a,b}\{1...3\}
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| 202 |
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| 203 | bash will do a **partial expansion** in the former cases, giving you `a{ b{`
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| 204 | and `a{1...3} b{1...3}`.
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| 205 |
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| 206 | OSH considers them syntax errors and aborts all brace expansion, giving you
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| 207 | the same thing back: `{a,b}{` and `{a,b}{1...3}`.
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| 208 |
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| 209 | ### Brackets should be escaped within Character Classes
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| 210 |
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| 211 | Don't use ambiguous syntax for a character class consisting of a single bracket
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| 212 | character.
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| 213 |
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| 214 | No:
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| 215 |
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| 216 | echo [[]
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| 217 | echo []]
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| 218 |
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| 219 | Yes:
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| 220 |
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| 221 | echo [\[]
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| 222 | echo [\]]
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| 223 |
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| 224 |
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| 225 | The ambiguous syntax is allowed when we pass globs through to `libc`, but it's
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| 226 | good practice to be explicit.
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| 227 |
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| 228 | ### [[ -v var ]] doesn't allow expressions
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| 229 |
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| 230 | In bash, you can use `[[` with `-v` to test whether an array contains an entry:
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| 231 |
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| 232 | declare -a array=('' foo)
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| 233 | if [[ -v array[1] ]]; then
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| 234 | echo 'exists'
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| 235 | fi # => exists
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| 236 |
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| 237 | Likewise for an associative array:
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| 238 |
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| 239 | declare -A assoc=([key]=value)
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| 240 | if [[ -v assoc['key'] ]]
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| 241 | echo 'exists'
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| 242 | fi # => exists
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| 243 |
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| 244 | OSH currently treats these expressions as a string, which means the status will
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| 245 | be 1 (`false`).
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| 246 |
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| 247 | Workaround:
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| 248 |
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| 249 | if [[ "${assoc['key']:+exists}" ]]; then
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| 250 | echo 'exists'
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| 251 | fi # => exists
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| 252 |
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| 253 | In ysh, you can use:
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| 254 |
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| 255 | var d = { key: 42 }
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| 256 | if ('key' in d) {
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| 257 | echo 'exists'
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| 258 | } # => exists
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| 259 |
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| 260 | ## Data Structures
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| 261 |
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| 262 | ### Arrays aren't split inside ${}
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| 263 |
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| 264 | Most shells split the entries of arrays like `"$@"` and `"${a[@]}"` here:
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| 265 |
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| 266 | echo ${undef:-"$@"}
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| 267 |
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| 268 | In OSH, omit the quotes if you want splitting:
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| 269 |
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| 270 | echo ${undef:-$@}
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| 271 |
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| 272 | I think OSH is more consistent, but it disagrees with other shells.
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| 273 |
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| 274 | ### Values are tagged with types, not locations (`declare -i -a -A`)
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| 275 |
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| 276 | Even though there's a large common subset, OSH and bash have a different model
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| 277 | for typed data.
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| 278 |
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| 279 | - In OSH, **values** are tagged with types, which is how Python and JavaScript
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| 280 | work.
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| 281 | - In bash, **cells** (locations for values) are tagged with types. Everything
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| 282 | is a string, but in certain contexts, strings are treated as integers or as
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| 283 | structured data.
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| 284 |
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| 285 | In particular,
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| 286 |
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| 287 | - The `-i` flag is a no-op in OSH. See [Shell Idioms > Remove Dynamic
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| 288 | Parsing](shell-idioms.html#remove-dynamic-parsing) for alternatives to `-i`.
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| 289 | - The `-a` and `-A` flags behave differently. They pertain to the value, not
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| 290 | the location.
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| 291 |
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| 292 | For example, these two statements are different in bash, but the same in OSH:
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| 293 |
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| 294 | declare -A assoc # unset cell that will LATER be an assoc array
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| 295 | declare -A assoc=() # empty associative array
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| 296 |
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| 297 | In bash, you can tell the difference with `set -u`, but there's no difference
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| 298 | in OSH.
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| 299 |
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| 300 | ### Indexed and Associative arrays are distinct
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| 301 |
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| 302 | Here is how you can create arrays in OSH, in a bash-compatible way:
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| 303 |
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| 304 | local indexed=(foo bar)
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| 305 | local -a indexed=(foo bar) # -a is redundant
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| 306 | echo ${indexed[1]} # bar
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| 307 |
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| 308 | local assoc=(['one']=1 ['two']=2)
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| 309 | local -A assoc=(['one']=1 ['two']=2) # -A is redundant
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| 310 | echo ${assoc['one']} # 1
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| 311 |
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| 312 | In bash, the distinction between the two is blurry, with cases like this:
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| 313 |
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| 314 | local -A x=(foo bar) # -A disagrees with literal
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| 315 | local -a y=(['one']=1 ['two']=2) # -a disagrees with literal
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| 316 |
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| 317 | These are disallowed in OSH.
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| 318 |
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| 319 | Notes:
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| 320 |
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| 321 | - The `=` keyword is useful for gaining an understanding of the data model.
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| 322 | - See the [Quirks](quirks.html) doc for details on how OSH uses this cleaner
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| 323 | model while staying compatible with bash.
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| 324 |
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| 325 | ## Assignment builtins
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| 326 |
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| 327 | The assignment builtins are `export`, `readonly`, `local`, and
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| 328 | `declare`/`typeset`. They're parsed in 2 ways:
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| 329 |
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| 330 | - Statically: to avoid word splitting in `declare x=$y` when `$y` contains
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| 331 | spaces. bash and other shells behave this way.
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| 332 | - Dynamically: to handle expressions like `declare $1` where `$1` is `a=b`
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| 333 |
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| 334 | ### `builtin declare x=$y` is a runtime error
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| 335 |
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| 336 | This is because the special parsing of `x=$y` depends on the first word
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| 337 | `declare`.
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| 338 |
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| 339 | ### Args aren't split or globbed
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| 340 |
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| 341 | In bash, you can do unusual things with args to assignment builtins:
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| 342 |
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| 343 | vars='a=b x=y'
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| 344 | touch foo=bar.py spam=eggs.py
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| 345 |
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| 346 | declare $vars *.py # assigns at least 4 variables
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| 347 | echo $a # b
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| 348 | echo $x # y
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| 349 | echo $foo # bar.py
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| 350 | echo $spam # eggs.py
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| 351 |
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| 352 | In contrast, OSH doesn't split or glob args to assignment builtins. This is
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| 353 | more like the behavior of zsh.
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| 354 |
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| 355 | ## Pipelines
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| 356 |
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| 357 | ### Last pipeline part may run in shell process (zsh, bash `shopt -s lastpipe`)
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| 358 |
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| 359 | In this pipeline, the builtin `read` is run in the shell process, not a child
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| 360 | process:
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| 361 |
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| 362 | $ echo hi | read x
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| 363 | $ echo x=$x
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| 364 | x=hi # empty in bash unless shopt -s lastpipe
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| 365 |
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| 366 | If the last part is an external command, there is no difference:
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| 367 |
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| 368 | $ ls | wc -l
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| 369 | 42
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| 370 |
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| 371 | This is how zsh behaves, and how bash (sometimes) behaves with `shopt -s
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| 372 | lastpipe`.
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| 373 |
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| 374 | ### Pipelines can't be suspended with Ctrl-Z
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| 375 |
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| 376 | Because the last part may be the current shell process, the entire pipeline
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| 377 | can't be suspended.
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| 378 |
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| 379 | OSH and zsh share this consequence of the `lastpipe` semantics.
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| 380 |
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| 381 | In contrast, bash's `shopt -s lastpipe` is ignored in interactive shells.
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| 382 |
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| 383 | ### `${PIPESTATUS[@]}` is only set after an actual pipeline
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| 384 |
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| 385 | This makes it easier to check compound status codes without worrying about them
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| 386 | being "clobbered".
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| 387 |
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| 388 | Bash will set `${PIPESTATUS[@]}` on every command, regardless of whether its a
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| 389 | pipeline.
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| 390 |
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| 391 | ## More Differences at Runtime
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| 392 |
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| 393 | ### Alias expansion
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| 394 |
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| 395 | Almost all "real" aliases should work in OSH. But these don't work:
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| 396 |
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| 397 | alias left='{'
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| 398 | left echo hi; }
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| 399 |
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| 400 | (cases #33-#34 in [spec/alias][])
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| 401 |
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| 402 | or
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| 403 |
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| 404 | alias a=
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| 405 | a (( var = 0 ))
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| 406 |
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| 407 | Details on the OSH parsing model:
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| 408 |
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| 409 | 1. Your code is statically parsed into an abstract syntax tree, which contains
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| 410 | many types of nodes.
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| 411 | 2. `SimpleCommand` are the only ones that are further alias-expanded.
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| 412 |
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| 413 | For example, these result in `SimpleCommand` nodes:
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| 414 |
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| 415 | - `ls -l`
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| 416 | - `read -n 1` (normally a builtin)
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| 417 | - `myfunc foo`
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| 418 |
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| 419 | These don't:
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| 420 |
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| 421 | - `x=42`
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| 422 | - `declare -r x=42`
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| 423 | - `break`, `continue`, `return`, `exit` — as explained above, these are
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| 424 | keywords and not builtins.
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| 425 | - `{ echo one; echo two; }`
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| 426 | - `for`, `while`, `case`, functions, etc.
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| 427 |
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| 428 | ### Extended globs are more static like `mksh`, and have other differences
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| 429 |
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| 430 | That is, in OSH and mksh, something like `echo *.@(cc|h)` is an extended glob.
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| 431 | But `echo $x`, where `$x` contains the pattern, is not.
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| 432 |
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| 433 | For more details and differences, see the [Extended Glob
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| 434 | section](word-language.html#extended-glob) of the Word Language doc.
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| 435 |
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| 436 | ### Completion
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| 437 |
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| 438 | The OSH completion API is mostly compatible with the bash completion API,
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| 439 | except that it moves the **responsibility for quoting** out of plugins and onto
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| 440 | the shell itself. Plugins should return candidates as `argv` entries, not
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| 441 | shell words.
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| 442 |
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| 443 | See the [completion doc](completion.html) for details.
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| 444 |
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| 445 | ## Interactive Features
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| 446 |
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| 447 | ### History Substitution Language
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| 448 |
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| 449 | The rules for history substitution like `!echo` are simpler. There are no
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| 450 | special cases to avoid clashes with `${!indirect}` and so forth.
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| 451 |
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| 452 | TODO: Link to the history lexer.
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| 453 |
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| 454 | <!--
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| 455 | TODO: we want to make history more statically parsed. Should test the ZSH
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| 456 | parser.
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| 457 | -->
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| 458 |
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| 459 | ## Links
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| 460 |
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| 461 | - [OSH Spec Tests](../test/spec.wwz/survey/osh.html) run shell snippets with OSH and other
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| 462 | shells to compare their behavior.
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| 463 |
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| 464 | External:
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| 465 |
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| 466 | - This list may seem long, but compare the list of differences in [Bash POSIX
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| 467 | Mode](https://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html).
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| 468 | That page tells you what `set -o posix` does in bash.
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| 469 |
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| 470 |
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| 471 | [spec/command-sub]: ../test/spec.wwz/command-sub.html
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| 472 | [spec/loop]: ../test/spec.wwz/loop.html
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| 473 | [spec/alias]: ../test/spec.wwz/alias.html
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| 474 |
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| 475 |
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