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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).

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

x=()  # indexed by integers

Quirk: When it's clear from the context, () means an empty associative array:

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

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

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

Redundant but OK:

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

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.