Bringing network interfaces up/down in Linux

Two methods can be used to bring interfaces up or down

1) Using "ip"

Usage:

# ip link set dev <interface> up # ip link set dev <interface> down

Example:

# ip link set dev eth0 up # ip link set dev eth0 down

2) Using "ifconfig"

Usage:

# /sbin/ifconfig <interface> up # /sbin/ifconfig <interface> down

Example:

# /sbin/ifconfig eth0 up # /sbin/ifconfig eth0 down

Fix GRUB2 Using Ubuntu LiveCD

Here's how:

1. Boot from the live CD or live USB, in "Try Ubuntu" mode.
2. Determine the partition number of your main partition. sudo fdisk -l, sudo blkid or GParted (which should already be installed, by default, on the live session) can help you here. I'm going to assume in this answer that it's /dev/sda2, but make sure you use the correct partition number for your system!
   If your main partition is in an LVM, the device will instead be located in /dev/mapper/, most likely, /dev/mapper/{volume}--{os}-root where {volume} is the LVM volume name and {os}is the operating system. Execute ls /dev/mapper for the exact name.
3. Mount your partition:

   sudo mount /dev/sda2 /mnt  #Replace sda2 with the partition from step 2
   

   If you have a separate /boot, /var or /usr partitions, repeat steps 2 and 3 to mount these partitions to /mnt/boot, /mnt/var and /mnt/usr respectively. For example,

   sudo mount /dev/sdXW /mnt/boot
   sudo mount /dev/sdXY /mnt/var
   sudo mount /dev/sdXZ /mnt/usr
   

   replacing sdXW, sdXY, and sdXZ with the respective partition numbers.
4. Bind mount some other necessary stuff:

   for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt$i"; done
   

5. If Ubuntu is installed in EFI mode (see this answer if you're unsure), use sudo fdisk -l | grep -i efi or GParted to find your EFI partition. It will have a label of EFI. Mount this partition, replacing sdXY with the actual partition number for your system:

   sudo mount /dev/sdXY /mnt/boot/efi
   

6. chroot into your Ubuntu install:

   sudo chroot /mnt
   

7. At this point, you're in your install, not the live session, and running as root. Update grub:

   update-grub
   

   If you get errors or if going up to step 7 didn't fix your problem, go to step 8. (Otherwise, it is optional.)
8. Depending on your situation, you might have to reinstall grub:

   grub-install /dev/sda
   update-grub # In order to find and add windows to grub menu.
   

9. If Ubuntu is installed in EFI mode, and EFI partition UUID has changed, you may need to update it in /etc/fstab. Compare it:

   blkid | grep -i efi
   grep -i efi /etc/fstab
   

   If current EFI partition UUID (from blkid) differs from the one in /etc/fstab, update /etc/fstab with current UUID.
10. If everything worked without errors, then you're all set:

    exit
    sudo reboot
    

11. At this point, you should be able to boot normally.

If you cannot boot normally, and didn't do step 8 because there were no error messages, try again with step 8.

• Sometimes giving GRUB2 the correct configuration for your partitions is not enough, and you must actually install it (or reinstall it) to the Master Boot Record, which step 8 does. Experience helping users in chat has shown that step 8 is sometimes necessary even when no error messages are shown.

Common administrative commands in Red Hat Enterprise Linux 5, 6, and 7

System basics

Task	RHEL5	RHEL6	RHEL7
View subscription information	/etc/sysconfig/rhn/systemid	/etc/sysconfig/rhn/systemid subscription-manager identity	subscription-manager identity
Configure subscription	rhn_register subscription-manager 1	rhn_register rhnreg_ks subscription-manager	subscription-manager2 rhn_register 3
View RHEL version information	/etc/redhat-release
View system profile	sosreport dmidecode hwbrowser	sosreport dmidecode lstopo lscpu

Basic configuration

Task	RHEL5	RHEL6	RHEL7
Graphical configuration tools	system-config-*		gnome-control-center
Text-based configuration tools	system-config-*-tui
Configure printer	system-config-printer
Configure network	system-config-network		nmcli nmtui nm-connection-editor
Configure system language	system-config-language		localectl
Configure time and date	system-config-date date		timedatectl date
Synchronize time and date	ntpdate /etc/ntp.conf		timedatectl /etc/chrony.conf ntpdate
Configure keyboard	system-config-keyboard		localectl
Configure SSH	/etc/ssh/ssh_config /etc/ssh/sshd_config ~/.ssh/config ssh-keygen

Jobs and services

Task	RHEL5	RHEL6	RHEL7
List all services	chkconfig --list ls /etc/init.d/		systemctl -at service ls /etc/systemd/system/*.service ls /usr/lib/systemd/system/*.service
List running services	service --status-all		systemctl -t service --state=active
Start/stop service	service name start service name stop		systemctl start name.service systemctl stop name.service
Enable/disable service	chkconfig name on chkconfig name off		systemctl enable name.service systemctl disable name.service
View service status	service name status		systemctl status name.service
Check if service is enabled	chkconfig name --list		systemctl is-enabled name
Create new service file or modify configuration	chkconfig --add		systemctl daemon-reload /etc/systemd/system/*.service
View run level/target	runlevel who -r		systemctl get-default who -r
Change run level/target	/etc/inittab init run_level		systemctl isolate name.target systemctl set-default
Configure logging	/etc/syslog.conf	/etc/rsyslog.conf	/etc/rsyslog.conf /etc/rsyslog.d/*.conf /var/log/journal systemd-journald.service
View logs	/var/log		/var/log journalctl
Configure system audit	add audit=1 to kernel cmdline auditctl /etc/audit/auditd.conf /etc/audit/audit.rules authconfig /etc/pam.d/system-auth pam_tty_audit kernel module
View audit output	aureport /var/log/faillog
Schedule/batch tasks	cron at batch
Find file by name	locate
Find file by characteristic	find
Create archive	tar cpio zip

Kernel, boot, and hardware

Task	RHEL5	RHEL6	RHEL7
Single user/rescue mode	append 1 or s or init=/bin/bash to kernel cmdline		append rd.break or init=/bin/bash to kernel cmdline
Shut down system	shutdown
Power off system	poweroff		poweroff systemctl poweroff
Halt system	halt		halt systemctl halt
Reboot system	reboot		reboot systemctl reboot
Configure default run level/target	/etc/inittab		systemctl set-default
Configure GRUB bootloader	/boot/grub/grub.conf		/etc/default/grub grub2-mkconfig grub-set-default
Configure kernel module	modprobe
View hardware configured	hwbrowser	lshw (in EPEL)
Configure hardware device	udev
View kernel parameters	sysctl -a cat /proc/cmdline
Load kernel module	modprobe
Remove kernel module	modprobe -r
View kernel version	rpm -q kernel uname -r

Software management

Task	RHEL5	RHEL6	RHEL7
Install software	yum install yum groupinstall		yum install yum group install
View software info	yum info yum groupinfo		yum info yum group info
Update software	yum update
Upgrade software	yum upgrade
Configure software repository	subscription-manager repos /etc/yum.repos.d/*.repo
Find package owning file	rpm -qf filename yum provides filename-glob
View software version	rpm -q packagename
View installed software	rpm -qa yum list installed

User management

Task	RHEL5	RHEL6	RHEL7
Graphical user management	system-config-users
Create user account	useradd
Delete user account	userdel
View/change user account details	usermod /etc/passwd vipw id
Create user group	groupadd
Delete user group	groupdel
Change group details	groupmod /etc/group
Change user password	passwd
Change user permissions	usermod visudo
Change group permissions	groupmod visudo
Change password policy	chage
View user sessions	w

File systems, volumes, and disks

Task	RHEL5	RHEL6	RHEL7
Default file system	ext3	ext4	xfs
Create/modify disk partitions	fdisk parted		fdisk gdisk parted ssm create
Format disk partition	mkfs.filesystem_type (ext4, xfs) mkswap		mkfs.filesystem_type (ext4, xfs) mkswap ssm create
Defragment disk space	copy data to new file system fsck (look for 'non-contiguous inodes')	copy data to new file system fsck (look for 'non-contiguous inodes') xfs_fsr
Mount storage	mount /etc/fstab		mount /etc/fstab ssm mount
Mount and activate swap	/etc/fstab swapon -a
Configure static mounts	/etc/fstab
View free disk space	df
View logical volume info	lvdisplay lvs vgdisplay vgs pvdisplay pvs
Create physical volume	pvcreate		pvcreate ssm create (if backend is lvm)
Create volume group	vgcreate		vgcreate ssm create (if backend is lvm)
Create logical volume	lvcreate		lvcreate ssm create (if backend is lvm)
Enlarge volumes formatted with default file system	vgextend lvextend resize2fs		vgextend lvextend xfs_growfs ssm resize
Shrink volumes formatted with default file system	resize2fs lvreduce vgreduce		XFS cannot currently be shrunk; copy desired data to a smaller file system.
Check/repair file system	fsck		fsck ssm check
View NFS share	showmount -e mount
Configure NFS share	/etc/exports service nfs reload		/etc/exports systemctl reload nfs.service
Configure on-demand auto-mounts	/etc/auto.master.d/*.autofs /etc/auto.*
Change file permissions	chmod chown chgrp umask (future file creation)
Change file attributes	chattr
Change access control list	setfacl

Networking

Task	RHEL5	RHEL6	RHEL7
Configure name resolution	/etc/hosts /etc/resolv.conf		/etc/hosts /etc/resolv.conf nmcli con mod
Configure hostname	/etc/sysconfig/network		hostnamectl /etc/hostname nmtui
View network interface info	ip addr ifconfig brctl		ip addr nmcli dev show teamdctl brctl bridge
Configure network interface	/etc/sysconfig/network-scripts/ifcfg-*		/etc/sysconfig/network-scripts/ifcfg-* nmcli con [add|mod|edit] nmtui nm-connection-editor
View routes	ip route
Configure routes	ip route add system-config-network /etc/sysconfig/route-iface		ip route add nmcli nmtui nm-connection-editor /etc/sysconfig/route-iface
Configure firewall	iptables and ip6tables /etc/sysconfig/ip*tables	iptables and ip6tables /etc/sysconfig/ip*tables system-config-firewall	firewall-cmd firewall-config
View ports/sockets	ss lsof netstat		ss lsof

Security and identity

Task	RHEL5	RHEL6	RHEL7
Configure system security	/etc/selinux/config chcon restorecon semanage setsebool system-config-selinux
Report on system security	sealert
LDAP, SSSD, Kerberos	authconfig authconfig-tui authconfig-gtk
Network users	getent

Resource management

Task	RHEL5	RHEL6	RHEL7
Trace system calls	strace
Trace library calls	ltrace
Change process priority	nice renice
Change process run location	taskset
Kill a process	kill pkill killall
View system usage	top ps sar iostat netstat vmstat mpstat numastat	top ps sar iostat netstat ss vmstat mpstat numastat tuna	top ps sar iostat ss vmstat mpstat numastat tuna
View disk usage	df	df iostat