Bash

You can assign a varaible using the = equals operator. However, its important to note in bash scripting that if you add spaces between the assignment then the variable will NOT be assigned.

# variable is assigned correctly
MY_VAR="foo"

# variable is not assigned correctly at all
MY_VAR = "foo"

You can check if a variable is null using the -z operator. This returns true if the argument IS null.

test -z $MY_VAR # -> true

Scripts

You can create a bash script by giving a file the .sh extension.

my_script.sh

To start writing a script you must open it with a shebang that points to the bash interpreter so that the script can run. The first line of the script will always be a path to the interpreter for bash to use. You can then use any bash command you would normally use in the console inside your script and the commands will be executed procedural with bash waiting for each command to finish before executing another.

#!/bin/bash

You can run a bash script by simply typing the name of the script file (or path to the file prepended by a ./).

$ ./my_script.sh

You can also execute bash scripts using the sh or bash commands.

$ sh my_script.sh
$ bash my_script.sh

You can get positional arguments passed when calling a bash script by using $<ARGUMENT_NUMBER> to access the arguments. For example for the script

# my_script.sh
#!/bin/bash
echo $1
echo $2

Then passing in two arguments Hello and world! as space separated arguments will automatically be put into the $1 and $2 variables. This can go up arbitrarily high depdending on how mant arguments you pass in.

my_script.sh Hello world!
# Output:
# Hello
# world!

Results

You can check the outcome result of the last command run using the $? variable. This will be either a 0 or 1 depending on whether the command succeeded.

echo $1

Ternary

You can create a “ternary-like” statement by surrounding the condition of the statement with [[]] double square brackets and then using the && and || symbols to separate the conditions.

[[ <CONDITION> ]] && <IF_TRUE> || <IF_FALSE>

For example

[[ 1 != 2 ]] && echo "Not Equal!" || echo "Equal!"
# => Not Equal!

Operating System

You can test whether a script is running in a Linux or OSX (or other?) environment by using the uname property against the name of system environment.

if [ "$(uname)" == "Darwin" ]
then
  echo "We're on Mac!"
elif [ "$(uname)" == "Linux" ]
then
  echo "We're a cool kid!"
fi

You can also get these values directly in the terminal by simply typing uname. The default argument for this command is -s.

$ uname
Linux