[sagar.shah@~]$ cat csh_script.csh
#!/bin/csh
echo $a;

[sagar.shah@~]$ csh csh_script.csh
a: Undefined variable.

[sagar.shah@~]$ set a=5

[sagar.shah@~]$ echo $a
5

[sagar.shah@~]$ csh csh_script.csh
a: Undefined variable.

[sagar.shah@~]$ setenv a 10

[sagar.shah@~]$ csh csh_script.csh
10

[sagar.shah@~]$ echo $a
5