sed 's/\(.*\) (nflg || \(.*\))\(.*\)/\1\2\3/' src/useradd.c > useradd.c.temp &&: This sed is used to fix a compilation bug which occurs due to a variable (nflg), being used but not defined. cp limits login.access and others: These files were not installed during the installation of the package so we copy them manually as those files are used to configure authentication details on the system. sed "s|/var/spool/mail|/var/mail|" login.defs.linux > /etc/login.defs: /var/spool/mail is the old location of the user mailboxes. The location that is used nowadays is /var/mail.