For imp and exp you can supply parameters in a file. You can change permissions on this file in the file system to make it readable only by the people you choose. Check the documentation for the PARFILE command line option. The option you will want to include in the PARFILE is the USERID option.
For SQLPLUS you can specify /nolog option and supply connection command as the first line of the sql script you are executing.
sqlplus /nolog @myscript.sql
And then in myscript.sql:
For SQLPlus I usually recommend creating a file or using a "here" document so you can protect the username/password from being viewed with "ps -ef" command in Unix/Linux. If the username/password is contained in a script file or sql file you can protect using appropriate user/group read permissions. Then you can keep the user/pass inside the file like this in a shell script:
sqlplus -s /nolog <<EOF
select blah from blah;