sftp from mainframe to unix server without password

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http://www.unix.com – Hi, Let me just give a background of what the current process is before I proceed with my doubts. Currently, I have a job in mainframes that is scheduled to run on a particular date and time every month(it is fully automated and no manual intervention is there in running that job). What that job does is that it will connect to a unix server using ftp command and get a file and then FTP some files from Mainframes back to that unix server. In Mainframes itself I am giving this FTP command to get and put files in unix server. Now, I am in a need to use sftp command instead of ftp command to (HowTos)