sh-expand

A python library for expanding shell ASTs that can be used to construct sound analyses for shell scripts (after having expanded the AST).

Design

There are two implementations of shell expansion:

1. expand.py does an in-python expansion of simple & safe AST nodes. It doesn't support bashisms.
2. bash_expand.py expands by conservatively echoing safe commands to a bash process containing the current shell state.

Both, when called via expand_command, recurse down into the words of a shasta AST node to expand, leave the word as is, or raise an ExpansionError. Examples:

• Process substitutions are unsafe.
• Simple variable substitutions are safe.
• Assignments are considered unsafe, since in Pash they indicate an unparallelizable region.

expand.py interprets the ArgChars libdash parses (ex: for variable and process substitutions) to check and expand words.

Libbash does not parse ArgChars, so bash_expand.py instead conservatively checks for the presence of potentially expandable or unsafe literal characters (see should_expand_var) before expanding.

Not having proper ArgChar parsing causes false positive unsafe errors, so expand.py should eventually be extended to support bash-only AST nodes, and libbash and shasta extended to parse ArgChars.