redhat

Understanding The Linux /etc/passwd File

The file’s name originates from one of its initial functions as it contained the data used to verify passwords of user accounts. However, on modern Unix systems the security-sensitive password information is instead often stored in a different file using shadow passwords, or other database implementations.

To view an example of a user in the passwd file, run the following command specifying what user you’d like to view. In my example, I’ve grepped the mysql user.

# cat /etc/passwd | grep mysql

To see all users, run it without the grep as I’ve done here:

# cat /etc/passwd

passwd-file

The fields, in order from left to right, are:

  1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
  2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
  3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
  4. Group ID (GID): The primary group ID (stored in /etc/group file)
  5. User ID Info: The comment field. It allow you to add extra information about the users such as user’s full name, phone number etc. This field use by finger command.
  6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
  7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell.
redhat

Rename Root Volume Group (VG) on Linux Server

This walk-through worked on a virtual server running RHEL 6.7 in a VMware environment.

As I rebuilt an existing VMware guest that was being scheduled to go into production, I realized I did not rename the default volume group from vg_hostname to rootvg.

You must be root user and it is extremely important to backup the files below. It’s also important to know how to boot up in rescue mode with other boot media in the event there was a typo or other unforeseen issue.

Backup fstab file

cp /etc/fstab /etc/fstab.orig

Backup grub.conf file

cp /boot/grub/grub.conf /boot/grub/grub.conf.orig

Rename volume group

vgrename /dev/vg_OLDname /dev/rootvg

Change all instances of the old volume group in the following files:
Edit /etc/grub.conf (which is a symbolic link to /boot/grub/grub.conf)

vim /etc/grub.conf

Search and replace

:%s/vg_OLDname/rootvg/g

Edit fstab file

vim /etc/fstab

Search and replace

:%s/vg_OLDname/rootvg/g

Move boot image

mv /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r).img.backup
dracut -v /boot/initramfs-$(uname -r).img $(uname -r)

Verify your work
Reboot

If your system comes back up, you are golden. If it does not…