| 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 | OSH Quirks
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| 6 | ==========
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| 7 |
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| 8 | This document describes corner cases in OSH.
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| 9 |
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| 10 | Related: [Known Differences](known-differences.html).
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| 11 |
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| 12 | <div id="toc">
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| 13 | </div>
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| 14 |
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| 15 | ## For Bash Compatibility
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| 16 |
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| 17 | ### The meaning of `()` on the RHS
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| 18 |
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| 19 | In Oils, **values** are tagged with types like `Str` and `AssocArray`, as
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| 20 | opposed to the *locations* of values (cells). The construct `()` on the RHS
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| 21 | generates an instance of a distinct type `InitializerList`, which modifies the
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| 22 | content of the LHS that it is assigned to.
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| 23 |
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| 24 | This statement binds an empty indexed array to the name `x`:
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| 25 |
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| 26 | x=() # indexed by integers
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| 27 |
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| 28 | This clears the content of the associative array.
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| 29 |
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| 30 | declare -A x=() # indexed by strings, because of -A
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| 31 |
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| 32 | These assign elements.
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| 33 |
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| 34 | declare -a x=(one two) # set elements
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| 35 | declare -a x=(['k']=v) # set an element to the index $((k))
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| 36 | declare -A x=(['k']=v) # set an element to the key 'k'
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| 37 |
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| 38 | This is not supported:
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| 39 |
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| 40 | declare -A x=(key value) # Error in osh
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| 41 |
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| 42 | When the variable does not exist and a type is not specified, the assignment
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| 43 | creates an indexed array and applies the `InitializerList` to the created
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| 44 | array.
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| 45 |
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| 46 | declare x=(one two) # creates an indexed array
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| 47 | declare x=(['k']=v) # creates an indexed array
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| 48 |
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| 49 | **Quirk (osh <= 0.27.0)**: The construct `()` had an ambiguous type, which was
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| 50 | either `BashArray` or `BashAssoc` depending on its content and the context.
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| 51 | When it's clear from the context, `()` meant an empty **associative** array:
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| 52 |
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| 53 | declare -A x=() # indexed by strings, because of -A
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| 54 |
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| 55 | This was only applied when the array is empty. Otherwise the type was
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| 56 | determined by the literal:
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| 57 |
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| 58 | declare x=(one two) # indexed array
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| 59 | declare x=(['k']=v) # associative array
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| 60 |
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| 61 | These were redundant but OK:
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| 62 |
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| 63 | declare -a x=(one two) # indexed array
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| 64 | declare -A x=(['k']=v) # associative array
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| 65 |
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| 66 | These produced errors:
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| 67 |
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| 68 | declare -A x=(one two) # inconsistent
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| 69 | declare -a x=(['k']=v) # inconsistent
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| 70 |
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| 71 | ## Interactive Shell
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| 72 |
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| 73 | ### With job control, the DEBUG trap is disabled for the last part of a pipeline
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| 74 |
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| 75 | First, some background. These two shell features are fundamentally
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| 76 | incompatible:
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| 77 |
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| 78 | - Job control: e.g. putting a pipeline in a process group, so it can be
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| 79 | suspended and cancelled all at once.
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| 80 | - `shopt -s lastpipe` semantics: the last part of a pipeline can (sometimes) be
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| 81 | run in the current shell.
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| 82 | - [OSH]($xref) uses it by default because it makes `echo hi | read myvar` work. So
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| 83 | [OSH]($xref) is like [zsh]($xref), but unlike [bash](xref).
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| 84 |
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| 85 | As evidence of this incompatibility, note that:
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| 86 |
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| 87 | - [bash]($xref) simply ignores the `shopt -s lastpipe` setting in job control
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| 88 | shells
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| 89 | - [zsh]($xref) doesn't allow you to suspend some pipelines
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| 90 |
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| 91 | ---
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| 92 |
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| 93 | Now that we have that background, note that there's is a **third** feature that
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| 94 | interacts: the `DEBUG` trap.
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| 95 |
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| 96 | [OSH]($xref) emulates the [bash]($xref) `DEBUG` trap, which runs before "leaf"
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| 97 | commands like `echo hi`, `a=b`, etc.
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| 98 |
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| 99 | If we run this trap before the last part of a pipeline, **and** that part is
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| 100 | run in the current shell (`lastpipe`), then the DEBUG trap makes an existing
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| 101 | race condition worse.
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| 102 |
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| 103 | For example, in
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| 104 |
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| 105 | echo hi | cat
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| 106 |
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| 107 | there's nothing stopping `echo hi` from finishing before `cat` is even started,
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| 108 | which means that `cat` can't join the process group of the leader.
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| 109 |
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| 110 | So we simply disable the `DEBUG` trap for the last part of the pipeline, but
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| 111 | **only** when job control is enabled. This won't affect debugging batch
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| 112 | programs.
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| 113 |
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| 114 | Related issues in other shells:
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| 115 |
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| 116 | - bash: <https://superuser.com/questions/1084406/chained-pipes-in-bash-throws-operation-not-permitted>
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| 117 | - fish: <https://github.com/fish-shell/fish-shell/issues/7474>
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| 118 |
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| 119 |
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| 120 | <!--
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| 121 |
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| 122 | ### errexit message and optimized subshells
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| 123 |
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| 124 | For all shells:
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| 125 |
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| 126 | sh -c 'date'
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| 127 |
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| 128 | gets rewritten into:
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| 129 |
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| 130 | sh -c 'exec date'
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| 131 |
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| 132 | That is, they **reuse the parent process**.
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| 133 |
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| 134 | Most shells don't print any diagnostic info when `errexit` is on. However, YSH
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| 135 | does:
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| 136 |
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| 137 | osh -o errexit -c 'false'
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| 138 | [ -c flag ]:1: fatal: Exiting with status 1
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| 139 |
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| 140 | `false` is a builtin rather than an external process, so YSH can print that
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| 141 | message. But when running an external process, the message is lost:
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| 142 |
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| 143 | osh -o errexit -c 'env false'
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| 144 | (silently fails with code 1)
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| 145 | -->
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| 146 |
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| 147 | ## Related
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| 148 |
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| 149 | - The doc on [warts](warts.html) relates to YSH.
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| 150 |
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