shell-scripts that only executes itself if it changed since the previous run.

am-i-changed.sh

#!/bin/sh -                                   
#===============================================================================
#                                               
#          FILE: am-i-changed.sh                   
#                                                  
#         USAGE: ./am-i-changed.sh                 
#                                                  
#   DESCRIPTION: simple snippet to test if the current file has changed, thus needs to be re-run
#                                                  
#       OPTIONS: ---                               
#  REQUIREMENTS: ---                               
#          BUGS: ---                               
#         NOTES: ---                               
#        AUTHOR: Elmar Athmer (elmar@athmer.org),                                                        
#  ORGANIZATION:                                   
#       CREATED: 12.12.2014 10:08                  
#      REVISION:  ---                              
#===============================================================================
                                                   
set -o nounset                                     
set -o errexit                                     
                                                   
FILE=/tmp/am-i-changed-status.md5                  
touch $FILE                                        
                                                   
LAST_HASH=$(cat $FILE)                             
CURRENT_HASH=$(cat $0 | md5sum | xargs)            
                                                   
if [ "${LAST_HASH}" = "${CURRENT_HASH}" ]; then 
    echo "nothing to do"                           
    exit 0                                         
else                                               
    echo "i did change, so continue running this script"
    echo $CURRENT_HASH > $FILE                     
fi                                                 
                                                   
                                                   
echo "Hello World!"

$ ./am-i-changed.sh # first execution
i did change, so continue running this script
Hello World!
$ ./am-i-changed.sh # second execution
nothing to do