product/bashlex

Python parser for bash

bashlex - Python parser for bash

bashlex is a Python port of the parser used internally by GNU bash.

For the most part it's transliterated from C, the major differences are:

1. it does not execute anything
2. it is reentrant
3. it generates a complete AST

Installation:

$ pip install bashlex

Usage

$ python
>>> import bashlex
>>> parts = bashlex.parse('true && cat <(echo $(echo foo))')
>>> for ast in parts:
...     print ast.dump()
ListNode(pos=(0, 31), parts=[
  CommandNode(pos=(0, 4), parts=[
    WordNode(pos=(0, 4), word='true'),
  ]),
  OperatorNode(op='&&', pos=(5, 7)),
  CommandNode(pos=(8, 31), parts=[
    WordNode(pos=(8, 11), word='cat'),
    WordNode(pos=(12, 31), word='<(echo $(echo foo))', parts=[
      ProcesssubstitutionNode(command=
        CommandNode(pos=(14, 30), parts=[
          WordNode(pos=(14, 18), word='echo'),
          WordNode(pos=(19, 30), word='$(echo foo)', parts=[
            CommandsubstitutionNode(command=
              CommandNode(pos=(21, 29), parts=[
                WordNode(pos=(21, 25), word='echo'),
                WordNode(pos=(26, 29), word='foo'),
              ]), pos=(19, 30)),
          ]),
        ]), pos=(12, 31)),
    ]),
  ]),
])

It is also possible to only use the tokenizer and get similar behaviour to shlex.split, but bashlex understands more complex constructs such as command and process substitutions:

>>> list(bashlex.split('cat <(echo "a $(echo b)") | tee'))
['cat', '<(echo "a $(echo b)")', '|', 'tee']

..compared to shlex:

>>> shlex.split('cat <(echo "a $(echo b)") | tee')
['cat', '<(echo', 'a $(echo b))', '|', 'tee']

The examples/ directory contains a sample script that demonstrate how to traverse the ast to do more complicated things.

Limitations

Currently the parser has no support for:

• arithmetic expressions $((..))
• the more complicated parameter expansions such as ${parameter#word} are taken literally and do not produce child nodes

Debugging

It can be useful to debug bashlex in conjunction to GNU bash, since it's mostly a transliteration. Comments in the code sometimes contain line references to bash's source code, e.g. # bash/parse.y L2626.

$ git clone git://git.sv.gnu.org/bash.git
$ cd bash
$ git checkout df2c55de9c87c2ee8904280d26e80f5c48dd6434 # commit used in
translating the code
$ ./configure
$ make CFLAGS=-g CFLAGS_FOR_BUILD=-g # debug info and don't optimize
$ gdb --args ./bash -c 'echo foo'

Useful things to look at when debugging bash:

• variables yylval, shell_input_line, shell_input_line_index
• breakpoint at yylex (token numbers to names is in file parser-built)
• breakpoint at read_token_word (corresponds to bashlex/tokenizer._readtokenword)
• xparse_dolparen, expand_word_internal (called when parsing $())

Motivation

I wrote this library for another project of mine, explainshell which needed a new parsing backend to support complex constructs such as process/command substitutions.

Releasing a new version

Suggestion for making a release environment:

python3 -m venv venv
source venv/bin/activate
pip install -r requirements.txt

• make tests
• bump version in setup.py
• git tag the new commit
• run python -m build
• run twine upload dist/*

License

The license for this is the same as that used by GNU bash, GNU GPL v3+.