[Help-bash] bash way of variable substitution

[Peng Yu]

Hi,

I'd like to do variable substitution using the bash syntax (only allow
things like ${x} or $x without allowing anything else in bash) instead
of using other tools like m4. In order to not to have a conflict with
environmental variables, I did not use bash for this implementation.
Instead, I come up with the following implement in python. But I am
not how variable substitution is implemented internally in bash and
whether there are any other better and cleverer implementations.

Does anybody have any other suggestions about a better implementation?

$ cat varbashsub0.py
#!/usr/bin/python
# vim: set noexpandtab tabstop=2 shiftwidth=2 softtabstop=-1 fileencoding=utf-8:

import sys
import re

rpl_dict = {}

for s, d in zip(sys.argv[1::2], sys.argv[2::2]):
    rpl_dict[s] = d

regex=r'(\\\\)|(\\\$)' +
'|\$(([A-Za-z_][A-Za-z0-9_]*)|{([A-Za-z_][A-Za-z0-9_]*)})'
r = re.compile(regex)

def rpl_str(r, s, rpl_dict):
    result = []
    prev_end = 0
    for m in r.finditer(s):
        if m.group(1):
            varval = '\\'
        elif m.group(2):
            varval = '$'
        else:
            varname = m.group(4)
            if varname is None:
                varname = m.group(5)

            if varname in rpl_dict:
                varval = rpl_dict[varname]
            else:
                varval = ''

        result.append(s[prev_end:m.start()])
        result.append(varval)
        prev_end = m.end()

    result.append(s[prev_end:])

    return result

for line in sys.stdin:
    line = line.rstrip('\n')
    print ''.join(rpl_str(r, line, rpl_dict))

$ ./varbashsub0.py <<< '\\\$a'
\$a
$ ./varbashsub0.py <<< '\$'
$
$ ./varbashsub0.py <<< 'x${ab}y'
xy
$ ./varbashsub0.py <<< 'x$ab y'
x y
$ ./varbashsub0.py a 1 b 2 <<< '${a}$b'
12
$ ./varbashsub0.py a 1 b 2 <<< 'x${a}${b}y'
x12y

-- 
Regards,
Peng