Dev Journal 1/24/2020

Today I wrote a bunch of scripts to bootstrap a computer and setup dotfiles! Here are some things I learned.

Making cron jobs

You can edit your crontab file with the command crontab -e. This brings up a file with your cronjobs, which follow the pattern:

# min hou dom mon dow command
0 5 * * * echo "Hi!" >> ~/hi.log
23 12 4 6 * echo "SUPER HI" >> ~/hi.log

The first line is a comment to show the pattern. Comments are ignored. The second line runs every day at 5:00 AM. Since it is run from your home directory as you, a file should show up with all of the "hi" logs tomorrow morning! The third line schedules a SUPER HI at 12:53 PM (just after noon) on June 4.

Your crontab can be found in /var/spool/cron/crontabs/$USER (although only root can actually see into the crontabs directory).

crontab -l outputs the entire current crontab to STDOUT (or an error message to STDERR if the user doesn't have one yet).

You can overwrite the crontab by using a single - to specify the crontab reads from stdin: echo "0 0 * * 1 ls" | crontab -

Checking for unstaged changes

You can use

git diff --quiet

and the exit code will be 1 (failure) if there are unstaged changes and 0 (pass) if the working directory is clean.

Checking for upstream changes

After running a git fetch, you can use

git diff origin/master --quiet

The same way as above. The only difference is that it also checks unstaged changes, so if you want to only check against your current master branch, you have to git stash your changes first. You can git stash pop them back when you're done.

Sending variables to awk

You can input variables to awk! This is really good for passing shell variables into awk, which should really be single quoted.

$ awk -F: -v user=$USER '$1 ~ user {print $7}' /etc/passwd
/usr/bin/zsh

Looks like you can't use them in regular expressions for patterns.

Checking a user's default shell

Granted, I ended up finding a much easier check to see if my default shell was zsh or not:

$ grep '^ryan.*zsh$' /etc/passwd
ryan:x:1000:1000:,,,:/home/ryan:/usr/bin/zsh

Apparently, after some research, there is a command getent that lets you query the various system databases for specific info. So this works:

$ getent passwd ryan
ryan:x:1000:1000:,,,:/home/ryan:/usr/bin/zsh

# And thus:
$ getent passwd ryan | awk -F: '{print $7}'
/usr/bin/zsh

Also neat is that if there is no key present that matches, it will do exit code of 2, so:

if getent passwd florpnorp > /dev/null; then
    echo "Found that user"
else
    echo "Not present"
fi
# => Not present