How to know if the running platform is Ubuntu or CentOS with help of a Bash script?

Unfortunately, there is no surefire, simple way of getting the distribution name. Most major distros are moving towards a system where they use /etc/os-release to store this information. Most modern distributions also include the lsb_release tools but these are not always installed by default. So, here are some approaches you can use:

1. Use /etc/os-release

   awk -F= '/^NAME/{print $2}' /etc/os-release
   

2. Use the lsb_release tools if available

   lsb_release -d | awk -F"\t" '{print $2}'
   

3. Use a more complex script that should work for the great majority of distros:

   # Determine OS platform
   UNAME=$(uname | tr "[:upper:]" "[:lower:]")
   # If Linux, try to determine specific distribution
   if [ "$UNAME" == "linux" ]; then
       # If available, use LSB to identify distribution
       if [ -f /etc/lsb-release -o -d /etc/lsb-release.d ]; then
           export DISTRO=$(lsb_release -i | cut -d: -f2 | sed s/'^\t'//)
       # Otherwise, use release info file
       else
           export DISTRO=$(ls -d /etc/[A-Za-z]*[_-][rv]e[lr]* | grep -v "lsb" | cut -d'/' -f3 | cut -d'-' -f1 | cut -d'_' -f1)
       fi
   fi
   # For everything else (or if above failed), just use generic identifier
   [ "$DISTRO" == "" ] && export DISTRO=$UNAME
   unset UNAME
   

4. Parse the version info of gcc if installed:

   CentOS 5.x

   $ gcc --version
   gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-54)
   Copyright (C) 2006 Free Software Foundation, Inc.
   

   CentOS 6.x

   $ gcc --version
   gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)
   Copyright (C) 2010 Free Software Foundation, Inc.
   

   Ubuntu 12.04

   $ gcc --version
   gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
   Copyright (C) 2011 Free Software Foundation, Inc.
   

   Ubuntu 14.04

   $ gcc --version
   gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2
   Copyright (C) 2013 Free Software Foundation, Inc.
   This is free software; see the source for copying conditions.  There is NO
   warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
   

This has basically been directly copied from @slm's great answer to my question here.

You don't need bash to do such task, and I'd suggest using a high-level approach to avoid dealing with files like /etc/version and /etc/issue (I don't have /etc/version on 13.10).

So my recommendation is to use this command instead:

python -mplatform | grep -qi Ubuntu && sudo apt-get update || sudo yum update

python platform module will work on both systems, the rest of the command will check if Ubuntu is returned by python and run apt-get else yum.

Here's a simple answer that I find works across all versions of Ubuntu / CentOS / RHEL by the mere presence of the files (not failsafe of course if someone is randomly dropping /etc/redhat-release on your Ubuntu boxes, etc):

if [ -f /etc/redhat-release ]; then
  yum update
fi

if [ -f /etc/lsb-release ]; then
  apt-get update
fi