Skip to content

Latest commit

 

History

History
64 lines (44 loc) · 3.31 KB

Cool_Bash_Shell_Scripts.adoc

File metadata and controls

64 lines (44 loc) · 3.31 KB

Instructor Sander van Vugt

Generic notes

  • Gather CPU usage, PID, and process name, sort on highest CPU usage: ps -eo pcpu,pid -o comm= | sort -k1 -n -r

  • To run indefinitely: while true or while :

  • USAGE=${USAGE%.*} removes any decimal value from a number (makes it a whole number but without rounding)

    • Seems like % removes what follows it: USAGE=${USAGE%??} removes the last two characters

      • Note sure why but USAGE=${USAGE%*} seems to increment the final digit

  • NOTE: When checking status of something, then processing the data, check once then process against that saved data

    • Otherwise the status might change between status checks

  • [ $USAGE2 -gt 80 ] && [ $PID1 = $PID2 ] && mail -s "CPU load of $PNAME is above 80%"

    • If $USAGE2 is greater than 80 then (and) if $PID1 is equal to $PID2 then send an email

  • Can use bash -x (debug mode) to see what bash is executing

  • Need to research source in a bash script. Something to do with preventing sub-shells from being spawned

    • exec causes the subsequent command to run in the current shell

  • $# represents the number of arguments given when executing the script

  • This is known as a "here document"

cat << EOF >> /tmp/file
...
EOF

  • Better than ps aux | grep ssh | grep -v grep can just put [] around the first letter in the expression, i.e. ps aux | grep [s]sh. This makes grep match all processes that start with that first letter (the exact same pattern matching bahavior as without []'s), but since the grep process has [ as the first character, i.e. [s]sh it won’t match.

  • If statements are documented in the bash man page, tests are documented in the test man page ==== Notes on scripts in GH

  • 1_cpu-hog does sorting and comparing two values gathered over time

  • 2_cv: Using bash tests. Check manpage for test

([ -z $1 ] && exit 52) || ([ -f $1 ] && vi $1) ||  cd $1

  • IMPORTANT: This doesn’t work, because the ()'s cause the tests to run in sub-shells. Works when putting each test on different lines and removing the ()'s and ||'s

  • If the first argument to the command does not exist then exit with a status of 1, else if the argument is a file then vi the file, else cd to the directory

  • Need to research the difference between -z and -e

  • Take note that ()'s contain the different tests and what should be done if the test is true. That allows the ||'s to execute the next test

  • Can use any exit code and then be able to analyze or react to what caused the exit

    • 4_reboot-test.sh runs some processes, creates an after-reboot script, reboots, then executes the script from within /etc/profile when the user logs in

    • 3_choosedir uses select with if/then/else to give the user a menu of options (very cool!)

  • Uses if [ -n "dir"] to test if the value of dir is a number

    • 9_processmonitor uses a while statement that seems to act as long as the command exits 0

    • 7_traps shows how to have the script react based on SIGNALS are sent to it (otherwise the trap goes to the parent bash shell)

  • trap "commands to run" EXIT (EXIT is trap signal 5, ctrl+c, man 7 trap to see all)