When I’m working on a little bash script, I’ll often make a backup, each time I save and test. Then I can revert back, if I need to. The syntax I’ll use is to cp and then curly-bracket the output into .bak files (that’s a 90s era file extension I use for such nonsense): cp filename.sh{,.bak} So if I’m writing a script called MYSCRIPT.sh: cp MYSCRIPT.sh{,.bak} The resultant backup of the script is MYSCRIPT.sh.bak.

Pretty much every script I’m working on these days must be run as root. Checking what user is running something is pretty straight forward, as there’s a built-in shell variable for $USER that contains the user running a script. To see this real quick, simply run the following: echo $USER You can then put this into your scripts. I’ve been using the same block of code for decades, which can be run in a script by itself if you’d like to paste this into one. if [[ $USER != "root" ]]; then echo "This script must be run as root" else echo "You are root" exit 1 fi Note: Keep in mind that the built-in $USER variable is case sensitive. Obviously, most people won’t keep the lines that contain the else and you are root echo statements. You can just remove these or replace them with the meat of your script that requires elevated privileges to run. Enjoy.

We tend to use a lot of commands in the Terminal app. That is, after all, what it’s there fore. And there’s a nice history of what we do. There are also a number of ways to view and manage the bash history. The simplest of which is the history command, which will show the previous commands run. Here, we’ll simply run it: history Keep in mind that this shows the history based on context, so if you sudo bash, you’ll potentially see a different history. You can also use the bash built-in fc command, which has the additional awesomeness of being able to edit and re-run commands from the history. To start, we’ll simply look at showing the last 16 commands using the -l option: fc -l You can also constraint entries in the output by specific line numbers. For example, to see lines 12 through 18, simply use them as the first two positions of the command after fc: fc 12 18 You can load the history into an editor and remove or add entries using fc without any options: fc To exit the editor, hit control-z. I’ve written in the past about using substitution. For example, sudo !! to run the last command. fc can do some basic substitution as well. For example, use the -s to start substation and then enter a string, which will append whatever you like before a command. So the following would put sudo in front and re-run the previous command: fc -s sudo And let’s say that you were doing a find for a string of krypted. To then swap that string with charles: fc -s krypted=charles Overall, the bash history can be incredibly useful. I frequently pipe the output of a series of lines into a new file with a .sh at the end as a starting point for scripts and use these substitution options to save myself a bunch of time not retyping longer commands. Enjoy.

The cd command has lots of fun little shortcuts. One I use frequently is the -. The ~ always takes you to your home directory, but using cd – will take you to the last directory you were in. For example, if you do the following on a Mac: cd ~ Then you do .. (which is a shortcut for the directory above the one you’re in): cd .. Then pwd will show that you’re in /Users. But, if you cd to – again: cd - Now you’re back in your home folder. The – expands to OLDPWD. Quick tip. Nothing more to see here.

The options for Open Directory continue to get more refined, aligning with opendirectoryd. The odutil command is becoming more and more useful with each version of OS X. Let’s inspect the directory service cache, using odutil with the show verb and the cache option: odutil show cache You can also view statistics for opendirectoryd using that show verb but with the statistics option: odutil show statistics And to see everything, use odutil with the show verb and the all option to get plenty of data to grep through: odutil show all The final show option we’ll look at is configuration. Here, you will also need to feed a directory nodename into the command: odutil show configuration /Search Now, /Search is a node but there are a lot. You can use show with nodes to see a listing of all the nodes: odutil show nodes You can then see which pids have references to opendirectoryd as well as the nodenames, reference IDs, and session IDs. All of this can be very helpful when troubleshooting Open Directory issues. One thing I find I do pretty frequently is resetting statistics then repeating a process that is causing a problem so I can view only the updated statistics. To do so: odutil reset statistics You can also disable statistics (I’ve seen them create performance concerns: odutil set statistics off Or to turn them back on: odutil set statistics on Once upon a time you could killall DirectoryService with a -usr level to set various logging levels. With opendirectoryd, we can still do that, but it’s less cludgy with odutil. Here, we’ll set the logging level as detailed as we can get: odutil set log debug Other levels, in ascending order of verbosity, include alert, critical, error, warning, notice, and info.

When you run a kill command to stop a process from bash or the javax.realtime.POSIXSignalHandler class, you’re sending what’s known as a POSIX signal to the process. These signals can be called via their numeric representations or the signal (e.g. with the -s option of the kill command). Signals include the following:
  • 1: SIGHUP – Close the controlling terminal when the controlling process dies
  • 2: SIGINT – Send a keyboard interrupt
  • 3: SIGQUIT – Quit from a keyboard/terminal
  • 4: SIGILL – Terminate illegal instruction with a core dump and don’t restart
  • 5: SIGTRAP – Send a trace/break trap (with core dump)
  • 6: SIGABRT – Process an abort signal
  • 7: SIGEMT – Send the signal when emulator traps happen
  • 8:SIGFPE – Terminate floating point errors (erroneous arithmetic operations) with a core dump
  • 9: SIGKILL – Kill a signal outright (kill cannot be ignored)
  • 10: SIGBUS – Terminate access (some portion of a memory object) with a core dump
  • 11: SIGSEGV – Terminate with a core dump – Invalid memory reference
  • 12: SIGSYS – Bad system call
  • 13: SIGPIPE – Terminate and write on the pipe
  • 14: SIGALRM – Timed kill of a signal
  • 15: SIGTERM – Software termination of a signal
  • 16: SIGUSR1 – User defined signal 1, with SIGUSR2 as user defined signal 2
  • 17: SIGTERM – Software termination of a signal
  • 18: SIGCHLD – Child status change
  • 19: SIGPWR – Send the signal when the system encounters a power failure
  • 20: SIGWINCH – Send the signal to a process when the window changes
  • 21: SIGURG – Ignore signal, high bandwidth data
  • 22: SIGIO – Terminate pollable event
  • 23: SIGSTOP – Stop executing (cannot be ignored or caught by an exception)
  • 24: SIGTSTP – Terminate a stop signal.
  • 25: SIGCONT – If stopped, continue executing a process
  • 26: SIGTTIN – Background process is attempting to read
  • 27: SIGTTOU – Background process is attempting to write
  • 28: SIGVTALTM – Expired virtual timer
  • 29: SIGPROF – Terminate Profiling timer
  • 30: SIGXCPU – Past the CPU time limit, terminate with a core dump
  • 31: SIGXFSZ – Past the file size limit, terminate with a core dump
  • 32: SIGWAITING – Suspend execution of the process until a defined signal is sent
  • 33: SIGLWP – Send when the implementing threading requires a signal
  • 34: SIGFREEZE – Deprecated
  • 35: SIGTHAW – Deprecated
  • 36: SIGCANCEL – Deprecated
  • 37: SIGLOST – Send the signal when encountering a lost file lock
To put these in practice, let’s use the kill command from bash, with the -s option followed by SIGTERM and then the pid number: sudo kill -s SIGTERM 20341

In bash, you can run multiple commands in a single line of a script. You do so by separating them with a semi-colon (;). The great thing about this is that if you end up using a variable, you can pass it on to subsequent commands. Here, we’re going to string three commands together and then echo the output: a=1;b=2;c=$a+$b;echo $c because we told c to be $a + $b, the $a expands to 1 and the $b expands to 2, we throw them together and then echo out the contents of c$ which appears as follows: 1+2 Now, we could have this thing do math as well, by wrapping the mathematical operation in double-parenthesis, which bash treats as an arithmetic expansion: a=1;b=2;c=(($a+$b));echo $c The output this one is simply 3.

Merry Christmas ya’ll!
On the first day of Christmas my true love gave to me one 32 gig iPad On the second day of Christmas my true love gave to me two bash one-liners On the third day of Christmas my true love gave to me three Red Hat servers On the fourth day of Christmas my true love gave to me four email blasts On the fifth day of Christmas my true love gave to me five retweets On the sixth day of Christmas my true love gave to me six regular expressions On the seventh day of Christmas my true love gave to me seven lines of perl On the eighth day of Christmas my true love gave to me eight app store apps On the ninth day of Christmas my true love gave to me nine AWS instances On the tenth day of Christmas my true love gave to me ten Active Directory forests On the eleventh day of Christmas my true love gave to me 11 crappy python scripts On the twelfth day of Christmas my true love gave to me 12 craft brews
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