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OSH Quirks

This document describes corner cases in OSH.

Related: Known Differences.

For Bash Compatibility

The meaning of () on the RHS

In Oils, values are tagged with types like Str and AssocArray, as opposed to the locations of values (cells). The construct () on the RHS generates an instance of a distinct type InitializerList, which modifies the content of the LHS that it is assigned to.

This statement binds an empty indexed array to the name x:

x=()  # indexed by integers

This clears the content of the associative array.

declare -A x=()  # indexed by strings, because of -A

These assign elements.

declare -a x=(one two)  # set elements
declare -a x=(['k']=v)  # set an element to the index $((k))
declare -A x=(['k']=v)  # set an element to the key 'k'

This is not supported:

declare -A x=(key value)  # Error in osh

When the variable does not exist and a type is not specified, the assignment creates an indexed array and applies the InitializerList to the created array.

declare x=(one two)  # creates an indexed array
declare x=(['k']=v)  # creates an indexed array

Quirk (osh <= 0.27.0): The construct () had an ambiguous type, which was either BashArray or BashAssoc depending on its content and the context. When it's clear from the context, () meant an empty associative array:

declare -A x=()  # indexed by strings, because of -A

This was only applied when the array is empty. Otherwise the type was determined by the literal:

declare x=(one two)  # indexed array
declare x=(['k']=v)  # associative array

These were redundant but OK:

declare -a x=(one two)  # indexed array
declare -A x=(['k']=v)  # associative array

These produced errors:

declare -A x=(one two)  # inconsistent
declare -a x=(['k']=v)  # inconsistent

Interactive Shell

With job control, the DEBUG trap is disabled for the last part of a pipeline

First, some background. These two shell features are fundamentally incompatible:

• Job control: e.g. putting a pipeline in a process group, so it can be suspended and cancelled all at once.
• shopt -s lastpipe semantics: the last part of a pipeline can (sometimes) be run in the current shell.
  • OSH uses it by default because it makes echo hi | read myvar work. So OSH is like zsh, but unlike bash.

As evidence of this incompatibility, note that:

• bash simply ignores the shopt -s lastpipe setting in job control shells
• zsh doesn't allow you to suspend some pipelines

Now that we have that background, note that there's is a third feature that interacts: the DEBUG trap.

OSH emulates the bash DEBUG trap, which runs before "leaf" commands like echo hi, a=b, etc.

If we run this trap before the last part of a pipeline, and that part is run in the current shell (lastpipe), then the DEBUG trap makes an existing race condition worse.

For example, in

echo hi | cat

there's nothing stopping echo hi from finishing before cat is even started, which means that cat can't join the process group of the leader.

So we simply disable the DEBUG trap for the last part of the pipeline, but only when job control is enabled. This won't affect debugging batch programs.

Related issues in other shells:

• bash: https://superuser.com/questions/1084406/chained-pipes-in-bash-throws-operation-not-permitted
• fish: https://github.com/fish-shell/fish-shell/issues/7474

Related

• The doc on warts relates to YSH.