====== Use Shell Scripting To Automate System Maintenance Tasks ======

**General Information**

Review of common shell scripting syntax. 

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====== Lab Setup ======

The following virtual machines will be used:
  * server1.example.com (192.168.1.150) -> Create and test some scripts

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====== Help ======

Finding help in this section.
  * Not many references. Be comfortable with basic bash scripting
    * Variables
    * Arguments
    * Input
    * Output
    * Decisions (if statements, case)
    * Loops

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====== General Layout ======

General bash script layout
<code bash>
#!/bin/bash

echo "Hello world"
exit 0
</code>

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====== Arguments ======

Sending and parsing arguments to scripts.
<code bash>
#!/bin/bash

echo "The first argument is: $1"
echo "The second argument is: $2"

echo "There are $# total arguments."
echo "All of the arguments are: $@"

# Loop through arguments one at a time
for myarg in $@; do
  echo "Argument is: $myarg"
done
</code>

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====== Getting User Input ======

User input with the read command
<code bash>
#!/bin/bash

# If there is no argument passed, prompt for input
if [ -z $1 ]; then
  echo "Enter something: "
  read myname
else
  # There is an argument, use that
  myname=$1
fi

echo "You entered: $myname"
exit 0
</code>

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====== Conditionals ======

If/else conditional testing
<code bash>
if [ -f $1 ]; then
  echo "Argument is a file"
elif [ -d $1 ]; then
  echo "Argument is a directory"
else
  echo "Argument is something else..."
fi

exit 0
</code>

\\
Logical operators
<code bash>
# Logical AND: If first command is true, execute second command
[ -f $1 ] && echo "This is a file"

# Logical OR: If first command is true, do not execute second command
[ -f $1 ] || echo "This is not a file"
</code>

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====== Loops ======

Iterating through data.

===== For Loops =====

Useful for processing ranges of information.

Specific amount of iterations
<code bash>
for (( count=1; count <= 100; count++ )); do
  echo "The count is: $count"
done

exit 0
</code>

\\
Unknown iterations through a range of data
<code bash>
for node in $(cat system_list.txt); do
  ssh $node "uptime"
done
</code>

\\
Range of numbers
<code bash>
for number in {100..200}; do
  ping -c 1 192.168.1.$number > /dev/null && echo "192.168.1.$number is up"
done
</code>

===== While Loops =====

While loops are useful for monitoring something or to repeat something forever and break under certain conditions.

\\
Loop infinitely
<code bash>
# Keep checking to see if a specific host is up (the first argument to the script)
while true; do
  ping -c 1 $1
  
  # if the last command (ping) returns successful (0 exit code)
  if [[ $? -eq 0 ]]; then
    echo "$1 is up"
    exit 0
  else
    sleep 5
  fi

done
</code>

\\
Monitor a process
<code bash>
# While output results are true, go back to sleep
while ps aux | grep $1 | grep -v grep; do
  sleep 5
done

# Mail when no results from ps (process stopped)
echo "The process($1) is no longer running." | mail -s "Process $1 stopped" root
</code>

===== Case Evaluation =====

Evaluate data and provide matches for expected values.

<code bash>
case $1 in
  start)
    start_program
  ;;
  stop)
    stop_program
  ;;
  status)
    status_of_program
  ;;
  *)
    echo "Usage: $0 (start|stop|status)"
  ;;
esac
</code>

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====== Debugging ======

To debug a script, execute it with a special argument:
<code bash>
bash -x myscript.sh
</code>
  * -x -> Shows line by line what the script is doing, which allows easier debugging when it breaks

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