Today I Learned

A common construct an bash scripts is an if block like so:

if [ -z "$STRING" ]; then
  echo "Length of string is zero"
fi

The syntax seems similar to what you find in many other languages. For example

if (string.length) {
  console.log('Length of string is zero');
}

But if you've tried using a bash if block, you may have noticed that you'll get an error if you omit the space between the brackets and their content:

if [-z "$STRING" ]; then
  echo "Length of string is zero"
fi
[-z: command not found

This seemed like Bash was just being a bit picky about the formatting here. But today I learned this isn't so.

In fact, [ is essentially just an alias for the test command: it's not just for grouping things. So, when you run

[-z "$STRING" ]

you are actually running

test-z "$STRING"

And of course, test-z is not a command, so it produces an error.

You can resize images at the command line with Image Magick. First, install it:

sudo apt install imagemagick

Then convert:

convert original.png -resize 100x100 new.png

Works great! For more basic use cases, see this article at Digital Ocean.

Bash aliases only work at the beginning of the command. But this doesn't mean they only work at the beginning of the command line. For example, given the alias

alias clip="xclip -sel clipboard"

It works fine when piping.

$ someCommand | clip

Now the output of someCommand is loaded to your clipboard.

The xargs command can be used to feed the output of one program as variadic arguments to another program.

For example, here's how you can delete all Git branches that match the glob feat/*:

git branch --list 'feat/*' | xargs git branch -D

To kill a process by name, rather than job number or PID, you can use pkill. For example, this will kill all open VSCode instances.

pkill code

Today I accidentally turned on the Redshift program, and it was flickering annoyingly. I started to look for a solution online, but then remembered:

pkill redshift

Problem solved. Use this power wisely.

Instead of using up and down to cycle through terminal history, you can use Ctrl + p and Ctrl+n, for slightly better typing ergonomics.

Instead of creating new files with touch, you can use the redirection operator:

# this works
$ touch foo.txt

# but this is easier
$ > foo.txt

The redirection operator is also more generally capable, because you can add text to the file that you are creating.

echo node_modules > .gitignore

You can use Alt + * to insert all completions at the shell. For instance, if you type rm foo followed by tab, and there are multiple files starting with foo in the CWD, then you'll see a list of these files. If you want to remove all of them, type Alt + * to insert them all as arguments to rm.

When killing a process, it's usually better to not use the -9 flag. The command kill -9 immediately kills a process, without giving it time to clean up after itself. Instead, try using kill -15 first. This allows a process to terminate itself gracefully. kill -9 is better reserved as a command of last resort.