Why do the system accounts shutdown, halt and sync have login shells?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux All versions

Issue

  • Why do the following system accounts (UID < 500) have login shells?

    # awk -F: '($1!="root" && $3<500 && $7!="/sbin/nologin") {print}' /etc/passwd
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/hal

Resolution

  • Instead of logging on as a regular user and then issuing the 'sudo  shutdown' command, simply log in as user "shutdown", and the login takes  care of running the shutdown command immediately.

    # grep shutdown /etc/passwd
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
# grep shutdown /etc/shadow
shutdown:*:14784:0:99999:7:::

  • If login with this shutdown user was possible (the * in the second field prevents that here), then the system would run shutdown.

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