Scope of "setenv" in csh Versus "export" in bash
From what I've read, setenv in csh and export in bash are equivalent. However, I found some strange issues while working with the two.
When I open putty and start typing:
setenv TEMP yes
echo $TEMP //this give me "yes"
then I go to bash and type
echo $TEMP //this give me "yes" as well
However, if I do it in the opposite order, it wouldn't give the same results. Specifically, when I go to bash first and type
export TEMP=no
echo $TEMP //this give me "no"
then I go back to csh and type
echo $TEMP // this give me "Undefined Variable"
Shouldn't it give me "no" as well? Am I missing something?
Thank you!
Answer
Exporting a variable means that a copy of that variable is placed into the environment of any newly created child processes. It is a copy of the variable; if the child process modifies the variable, the parent does not see the modification. Moreover, if a child exports a variable, it does not become visible in the parent.
Hence, your two cases are asymmetrical. When you start in csh, export a variable, and then start bash, bash sees the exported variable. When you then export a new variable in bash and exit from bash to go back to csh, all of the variables created in the bash session disappear.
If you were to export a variable in bash and then start up a child csh (by typing csh), you would almost certainly see the exported variable.