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HOWTO: Move or migrate user accounts from old Linux server to a new Linux server


Linux Tux logoQuestion: How do I Move or migrate user accounts to from old Linux server a new Cent OS Linux server including mails? This new system a fresh installation.

Answer: You can migrate users from old Linux server to new Linux sever with standard commands such as tar, awk, scp and others. This is also useful if you are using old Linux distribution like Redhat 9 or Debian 2.x.


Following files/directories are required for traditional Linux user management:

  • /etc/passwd – contains various pieces of information for each user account
  • /etc/shadow – contains the encrypted password information for user’s accounts and optional the password aging information.
  • /etc/group – defines the groups to which users belong
  • /etc/gshadow – group shadow file (contains the encrypted password for group)
  • /home – All User data is stored here.
  • /var/spool/mail – Generally user emails are stored here.

You need to backup all of the above files and directories from old server to new Linux server.

Commands to type on the “old” Linux system

First create a tar ball of old users (old Linux system). Create a working directory:
# mkdir /root/move/
Setup UID filter limit variable:
# export UGIDLIMIT=500
Now copy /etc/passwd accounts to /root/move/passwd.mig using awk to filter out system account (i.e. only copy user accounts)
# awk -v LIMIT=$UGIDLIMIT -F: '($3>=LIMIT) && ($3!=65534)' /etc/passwd > /root/move/passwd.mig
Copy /etc/group file:
# awk -v LIMIT=$UGIDLIMIT -F: '($3>=LIMIT) && ($3!=65534)' /etc/group > /root/move/group.mig
Copy /etc/shadow file:
# awk -v LIMIT=$UGIDLIMIT -F: '($3>=LIMIT) && ($3!=65534) {print $1}' /etc/passwd | tee - |egrep -f - /etc/shadow > /root/move/shadow.mig
Copy /etc/gshadow (rarely used):
# cp /etc/gshadow /root/move/gshadow.mig
Make a backup of /home and /var/spool/mail dirs:
# tar -zcvpf /root/move/home.tar.gz /home
# tar -zcvpf /root/move/mail.tar.gz /var/spool/mail

Notes:

  • Users that are added to the Linux system always start with UID and GID values of as specified by Linux distribution or set by admin. Limits according to different Linux distro:
    • RHEL/CentOS/Fedora : Default is 500 and upper limit is 65534 (/etc/libuser.conf)
    • Debian and Ubuntu Linux : Default is 1000 and upper limit is 29999 (/etc/adduser.conf)
  • You should never ever create any new system user accounts on the newly installed Cent OS Linux. So above awk command filter out UID according to Linux distro.
  • export UGIDLIMIT=500 – setup UID start limit for normal user account. Set this value as per your Linux distro.
  • awk -v LIMIT=$UGIDLIMIT -F: ‘($3>=LIMIT) && ($3!=65534)’ /etc/passwd > /root/move/passwd.mig – You need to pass UGIDLIMIT variable to awk using -v option (it assigns value of shell variable UGIDLIMIT to awk program variable LIMIT). Option -F: sets the field separator to : . Finally awk read each line from /etc/passwd, filter out system accounts and generates new file /root/move/passwd.mig. Same logic is applies to rest of awk command.
  • tar -zcvpf /root/move/home.tar.gz /home – Make a backup of users /home dir
  • tar -zcvpf /root/move/mail.tar.gz /var/spool/mail – Make a backup of users mail dir

Use scp or usb pen or tape to copy /root/move to a new Linux system.
# scp -r /root/move/* user@newlinuxserver.com:/path/to/location

Commands to type on the “new” Linux system

First, make a backup of current users and passwords:
# mkdir /root/newsusers.bak
# cp /etc/passwd /etc/shadow /etc/group /etc/gshadow /root/newsusers.bak

Now restore passwd and other files in /etc/
# cd /path/to/location
# cat passwd.mig >> /etc/passwd
# cat group.mig >> /etc/group
# cat shadow.mig >> /etc/shadow
# /bin/cp gshadow.mig /etc/gshadow

Please note that you must use >> (append) and not > (create) shell redirection.

Now copy and extract home.tar.gz to new server /home
# cd /
# tar -zxvf /path/to/location/home.tar.gz

Now copy and extract mail.tar.gz (Mails) to new server /var/spool/mail
# cd /
# tar -zxvf /path/to/location/mail.tar.gz

Finally reboot the system; when the Linux comes back up, your user accounts will work as they did before on old system:
# reboot

Note: It is also worth running pwck and grpck after doing the copy. These commands can correct mismatches.

Please note that if you are new to Linux perform above commands in a sandbox environment. Above technique can be used to UNIX to UNIX OR UNIX to Linux account migration. You need to make couple of changes but overall the concept remains the same.

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3 Responses to “HOWTO: Move or migrate user accounts from old Linux server to a new Linux server”

  1. xPAT says:

    UP UP , what to do if you have already added system account on the new server?

  2. xPAT says:

    or if i may rephrase, several users have been added to the new server prior to migrating old users (to the new server). Would the migration process still work? What would be the effect to the previously/initially added users? What would be the next best thing to do if you have several users added prior to migration? Thanks.

  3. Amalia Garrison says:

    Hi mate. I am still really new in blogging and all things around this field. There are various jargons I still don’t understand. I’m not pretty sure I can write half decent to yours. I will browse the entire blog maybe I will be able to grasp your writing style a little.

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