dash-bash-diffs.md

Differences between Dash and Bash frontends

Shasta was designed for libdash, with libbash support added later. Both json_to_ast (for libdash) and bash_to_shasta_ast (for libbash) contain to_ast_node(s) functions, which each take a parsed, untyped AST and convert it to a shasta AST as defined in ast_node. Thr transformation is direct; bash is assumed to be a superset of dash. This is not strictly true, since both have some minor divergences from the POSIX spec, but is good enough.

The following fields of AstNodes are only used for Bash scripts:

• RedirectionNode and *RedirNode: The fd field is either ('var', filename) or ('fixed', fd). Dash only uses the latter form; the first exists to support bashisms like exec {fd} > log.txt, which open the file and assigns fd the new file descriptor.
• FileRedirNode: 'ReadingString' subtype. Handles here-strings.
• BackgroundNode: The after_ampersand field. Handles an edge case with heredocs.

The pretty printer uses the various nobraces and semicolon fields to determine whether braces and semicolons are printed. In dash, this doesn't matter, but in bash they can lead to a syntax error. For example:

( { echo hi; echo bye } )

is invalid.

The following AstNodes are only used for Bash scripts:

• SelectNode
• ArithNode
• CondNode
• ArithForNode
• CoprocNode
• TimeNode
• SingleArgRedirNode
• GroupNode