Peng Yu
2018-02-07 21:15:58 UTC
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
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
Regards,
Peng
