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--- linux_wiki:use_shell_scripting_to_automate_system_maintenance_tasks [2016/09/21 22:58]
billdozor created
+++ linux_wiki:use_shell_scripting_to_automate_system_maintenance_tasks [2019/05/25 23:50] (current)
@@ Line 4: / Line 4: @@
 Review of common shell scripting syntax.
+----
+====== Lab Setup ======
+The following virtual machines will be used:
+  * server1.example.com (192.168.1.150) -> Create and test some scripts
+----
+====== Help ======
+Finding help in this section.
+  * Not many references. Be comfortable with basic bash scripting
+    * Variables
+    * Arguments
+    * Input
+    * Output
+    * Decisions (if statements, case)
+    * Loops
 ----
@@ Line 84: / Line 104: @@
 [ -f $1 ] || echo "This is not a file"
 </code>
+----
+====== 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>
+----
+====== 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
 ----