How to check if a shell script is being run by root

BASH stores a user's ID in $UID variable. Your effective user ID is stored in $EUID variable.

#Old way: just add a simple check at the start of the script:

#!/bin/bash
# Init
FILE="/tmp/out.$$"
GREP="/bin/grep"
#....
# Make sure only root can run our script
if [ "$(id -u)" != "0" ]; then
  echo "This script must be run as root" 1>&2
  exit 1
fi

#A new way by using EUID

#!/bin/bash
# Init
FILE="/tmp/out.$$"
GREP="/bin/grep"
#....
# Make sure only root can run our script
if [[ $EUID -ne 0 ]]; then
  echo "This script must be run as root" 1>&2
  exit 1
fi
# ...

#only root can mount /dev/sdb1
#!/bin/bash
if [[ $EUID -ne 0 ]]; then
 echo "You must be a root user" 2>&1
 exit 1
else
 mount /dev/sdb1 /mnt/disk2
fi