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Will I be able to login into the system if the root filesystem is 100% full?

Configuration: home is not a partition but is also placed on the root (i.e. /home is a directory on the partition holding root /), var and tmp are separate partitions and in good state.

  • No, I guess. You will be reverted back to login prompt – SHW Dec 23 '13 at 10:17
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You should be able to log in as root, because usually a percentage of the partition's size is reserved in order to always enable root login for rescue operations and such. See this U&L Q&A:

Reserved space for root on a filesystem - why?

What you won't be able to do, however, is log in as a regular user from your display manager then switch to root or use sudo from a shell in a terminal. You have two alternatives instead:

  1. Switch to a VT (press Ctrl + Alt + F2, for example), log in as root from there and free some space.
  2. At boot time opt for single user mode to get a root shell that would also help you free some space for your regular login.

This assumes that the reason your partition was filled up is due to regular user activity and not activity by root processes. In such cases, you might need to resort to mounting the partition on a Live system and freeing up the space from there. Thanks to Alexios' comment for bringing this up.

  • That is, provided it wasn't root or a daemon running as the super user that filled up the filesystem in the first place. In that case, a single-user boot (or in extreme circumstances, rescue boot) may be necessary. – Alexios Dec 23 '13 at 13:44
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    @Alexios You're right. Thank you for pointing this out. Will add to answer. – Joseph R. Dec 23 '13 at 15:32
  • Once bitten, twice shy. I've had this exact situation on old Solaris boxes. And rescuing them was very annoying. ;) – Alexios Dec 23 '13 at 15:49
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    @Alexios System administration: come for the fun, stay for the war stories :) – Joseph R. Dec 23 '13 at 15:54

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