Add bash parameter substitution to config's variable

[iMacadan]

Bash param substitution would be a nice to have feature.
Not all of them would be worth it, but, for example, it can certainly help to reduce the need of creating python renamer plugins.

# Bash Parameter Substitution 
# Variable                      Description
${#var}                         # Find the length of the string
${var%pattern}                  # Remove from shortest rear (end) pattern
${var%%pattern}                 # Remove from longest rear (end) pattern
${var:num1:num2}                # Substring
${var#pattern}                  # Remove from shortest front pattern
${var##pattern}                 # Remove from longest front pattern
${var/pattern/string}           # Find and replace (only replace first occurrence)
${var//pattern/string}          # Find and replace all occurrences
${!prefix*}                     # Expands to the names of variables whose names begin with prefix.
${var,}
${var,pattern}                  # Convert first character to lowercase.
${var,,}
${var,,pattern}                 # Convert all characters to lowercase.
${var^}
${var^pattern}                  # Convert first character to uppercase.
${var^^}
${var^^pattern}                 # Convert all character to uppercase.

[petersilva]

• please provide references for where you got this list.
• var^^pattern ... I don't know what the pattern is in upper casing... does it mean only upper case what matches the pattern?

Is the pattern a regex? I notice that shell/bash regexes are different from python ones, and that sarra must use the python ones?

Having a trailing pattern is making it pretty complex to figure out what to do.

[petersilva]

If you have something like ${var^^^h} if after the ^^ you have a regex ^h, which means only upper-case if it starts with h. ... and braces are a thing in python regexes, so now have to deal with nested braces... getting hairy.

UPDATE: above pattern is wrong... see patterns link a few posts down.

[petersilva]

need an actual parser at this point... which is probably not a bad thing, just work.

[reidsunderland]

There's details about all the substitutions supported by bash online:

https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

https://tldp.org/LDP/abs/html/parameter-substitution.html

[petersilva]

the tests/sarracenia/config_test.py file could contain test cases for all these, and then it would just be a matter of running:

pytest -o log_cli=true tests/sarracenia/config_test.py

so there is a good framework to build unit tests for an implementation (that would be in sarracenia/config.py .. ( variableExpansion routine.)

[petersilva]

reading up on the references, the patterns are not regexes, but a different language akin to globbing but different again. sigh... how many pattern matching dialects to implement in a single app.

https://www.gnu.org/software/bash/manual/html_node/Pattern-Matching.html

[petersilva]

I'm tempted to put in something totally idiosyncractic like... a means of embedding python in the config file. The shell stuff looks complicated to parse, not that poweful, and not that obvious wondering if having a means of embedding python in the config result will be cleaner/easier to understand in future.

so... embedding python to do this stuff is basically exactly what a DESTFNSCRIPT is for, I think. so if you don't have these substitutions, you need to implement one of those, or obviously an after_accept in a normal callback... so one can make the substitutions by other means. The request is about making that doable in a one liner, without having to write any code.

[petersilva]

It would be good to see some sample use cases for this feature.

• When have people seen the need for this?
• Why not just supply the values in the correct form?

[iMacadan]

My original ref. was https://www.cyberciti.biz/tips/bash-shell-parameter-substitution-2.html An example use for it would be when a product's file name that contains a lower case station id which needs to be put into an uppercase subdir at destination.
But, my bad, this example falls short...