CODEWITHSUNDEEP
bashcsh
Ryan C. Thompson
Ryan C. Thompson

Reputation: 42080

Code that is a no-op in bash but stops with an error message in csh?

I am working with someone on a data analysis project and we frequently document the steps we perform by putting them into small shell scripts. The problem is that I use bash and the other person uses csh. The other person has a habit of using source to run these scripts instead of executing them directly (this habit probably dates back to times when spawning an extra shell was an extravagent waste of resources, so it's probably too entrenched to change) , and I want to have my scripts (which are, of course, bash scripts) simply stop with a message reminding the user to run them with bash instead of csh when this person sources them from within csh. At the same time, I would like them to continue to function as bash scripts.

So is there some code I can put at the beginning of my scripts that is a no-op in bash but will signal an error and cancel the execution of the rest of the file (but not kill the shell itself) when sourced from cshell?

Upvotes: 3

Views: 374

Answers (2)

fork2execve
fork2execve

Reputation: 1650

This is not easy, as you cannot assign variables the same way or run if statements the same way.

You can use csh's meagre string parsing skills against itself. The following executes cleanly in all shells, include KSH, BASH, ZSH, CSH and SH on all platforms that I tested it on (Linux, AIX, HP-UX, Solaris):

test '\\' = "\\" && echo "CSH detected"

The idea that is used is here is that backslashes are not special in double quoted strings on CSH, whereas all other shells do see them as different.

However, that is only half an answer as what do you want to do if you don't want to simply exit the script if the 'wrong' shell is detected? Well, you may want to have a Bournish sh part to your script and a csh part.

If you can keep the csh code limited to code that does not use single quotes, the following will work everywhere:

test '\\' = "\\" && goto csh
# Just skip the block containing the csh code. Again we use csh's meagre string parsing capabilities against it.
false || csh_code_block='
csh:
  ... csh code goes here ...
  exit 0
'
... sh code goes here ...

If you are not worried about HP-UX's csh (which seems a little better than others in parsing) you could replace the multi-line single quoted command with a 'HERE' document (<<CSH_BLOCK ... CSH_BLOCK). You can't just reverse the order either, as the 'goto' statement doesn't like all syntax that it skips over.

Upvotes: 1

thiton
thiton

Reputation: 36059

This is harder than I thought due to csh's ancient variable substitution flavor. However, $?BASH_VERSION expands to 0 (not set) in csh and to 0BASH_VERSION (or whatever the last commands' RV was) in bash. So,

test "$?BASH_VERSION" = 0 && exit 1

should do the trick.

Upvotes: 6

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