Tech Reflect

Rather than write a script for something you may only need once, there’s an easy way to repeat a series of command-line commands. This works in bash and may in other shells:

while [ 1 ]; do <some command>; <some other command>; sleep <some delay in seconds>; done;

So, for example:

while [ 1 ]; do date; uptime; sleep 10; done;

This will generate:
Mon Mar 19 13:20:30 PDT 2018
13:20 up 1:23, 3 users, load averages: 3.25 3.87 3.51
Mon Mar 19 13:20:40 PDT 2018
13:20 up 1:24, 3 users, load averages: 3.31 3.86 3.51
Mon Mar 19 13:20:50 PDT 2018
13:20 up 1:24, 3 users, load averages: 5.44 4.30 3.67

Just use Ctrl-C to end.

Extra credit

Add a counter so that it runs 10 times only:

for n in $(seq 1 10); do date; uptime; sleep 10; done

Shell aliases are woefully underused. Painfully, I frequently see people do the following:

• Type a shell command with the same 4 arguments cd into the same folders
• ssh into the same machines or devices
• Run the same sequence of commands

Aliases are super easy. Just open your ~/.bashrc file (or equivalent) and start adding them. I’ll now give a bunch of examples I use.

“ls” command

I have many variations I use on the “ls” command. I don’t even remember what the arguments mean anymore because I’ve been using these shortcuts for so long.

# default arguments I like
alias l='ls -AG' 

# Long listing
alias ll='ls -aAlGh'

# Do a grep afterwards (My most often used)
alias lg='ls -AlGOh@ | grep -i '

# Newest files at bottom (My second most often used)
alias lt='ls -AltrGhu' 

# Largest files at bottom
alias lz='ls -AlGrhS' 

# Extended attributes

If you’ve ever had a bizarre problem in Xcode and you’ve given up hope, this post is for you. Some frustrating things you might see:

• Breakpoints stop working
• A framework or library can’t be found that the compiler found moments ago
• Symbols in linked frameworks are no longer found
• This is not a comprehensive list.

I wrote this script a decade ago and in this frustrating moments, I run it, and go get a refreshing beverage while my computer reboots. It’s kind of unbelievable how often this has saved me. Maybe it will help you at least once in your life.

Note: Read the script first as it deletes your caches and DerivedData folder. It shouldn’t be a problem but you should be aware what it’s doing.

#!/bin/zsh

rm -rf ~/Library/Developer/Xcode/DerivedData 

#edit this to point to your DerivedData folder
rm -rf ~/Library/Caches/*Xcode*

sudo find /var/folders -iname xcode -exec mv \{\} 

It’s fairly straightforward to leverage launchd to run a script either at specified intervals or at specific times of the day. I find that most people only want to do fairly simple things, so I’ve kept this simple.

The way launchd works is that you create a plist, then run a command to either load or unload the plist. Here is an example plist that will run the script called “rsync-backup-work” every 3 hours. The first 3 lines are just boilerplate for a plist.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
     "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
     <key>Label</key>
          <string>org.cricket.rsync-backup-work</string
     <key>ProgramArguments</key>
          <array>
               <string>/Users/admin/bin/rsync-backup-work</string
          </array>
     <key>StartInterval</key>
          <integer>10800</integer>
</dict>
</plist>

This has only three keys:

• Label → Unique identifier (this can just be the name of the plist without the plist suffix to make it simple)
• ProgramArguments → First item in the array is the path to an executable, the rest would be