1. Bash

Quick reference for Bash scripting.

1.1. Data Types

By default, bash treats all variables as character strings. Depending on the context in which the variables are used affects whether bash interprets the variables as integers, strings, booleans, or arrays.

$ A="1"; B="2"
$ X="Hello"; Y="World"
$ echo "${A} ${B}"
1 2
$ echo "${X} ${Y}"
Hello World
$ echo $((${A} + ${B}))
3
$ echo $((${X} + ${Y}))
0

Expressions written inside (()) are evaluated as mathematical expressions. Note that adding the strings "Hello" and World returns 0.

1.2. Square Brackets (The Test Command)

Square brackets are referred to as the test command and are used to test if the statement within the brackets are true are true of false.

if [ "a" = "a" ]; then echo "True"; else echo "False"; fi    # True
if [ "a" = "b" ]; then echo "True"; else echo "False"; fi    # False
if [ "aa" = "aa" ]; then echo "True"; else echo "False"; fi  # False

X=3
Y=5

# Less than or equal to
if [ $X -le $Y ]; then echo "True"; else echo "False"; fi    # True

# Greater than or equal to
if [ $X -ge $Y ]; then echo "True"; else echo "False"; fi    # False

# Less than
if [ $X -lt $Y ]; then echo "True"; else echo "False"; fi    # True

# Greater than
if [ $X -gt $Y ]; then echo "True"; else echo "False"; fi    # False

1.3. For Loops

For loops can be defined over a list of items.

# Integer items
for i in 1 2 3
do
    echo "This is element $i"
done

# String items
for item in "item 1" "item 2" "item 3"
do
    echo "$item"
done

Braces can be used to define ranges, {x..y}. Note that ranges are inclusive so x and y are included in the range.

# 0, 1, 2, 3, 4, 4
for i in {0..5}
do
    echo "Item $i"
done

# 0, 2, 4, 6, 8
for i in {0..10..2}
do
    echo "Item $i"
done

1.4. Parameter Expansion

In shell scripts, the value of a parameter can be substituted into into a string using the ${variable_name} syntax.

$ X="Hello this is some text."
$ echo "${X}"
Hello this is some text.

Parameter expansions allows us to add characters inside the curly braces to change value that is substituted.

${parameter:-word}

If parameter is unset or null, substitute word instead.

$ X="Hello"
$ echo "${X:-"Word"}"
Hello
$ echo "${Y:-"Word"}"
Word
$ X=
$ echo "${X:-"Word"}"
Word

${parameter:offset:length}

Expands the substring of the value of parameter that starts at offset and has length length:

$ X="Hello"
$ echo "${X:1:3}"
ell
$ echo "${X:1}"
ello

1.4.1. Single vs Double Quotes

Single quotes can be used to avoid treat characters literally.

$ x="a"
$ echo "x: ${x}"
x: a
$ echo 'x: ${x}'
x: ${x}

1.4.2. Single vs Double Quotes in PS1

The PS1 variable can be changed to customise the command prompt. Unfortunately, single and double quotes behaviour slightly differently here.

$ PS1="$(pwd) $ "
/home/alex $ cd documents
/home/alex $ pwd
/home/alex/documents

Above the PS1 is changed to show the working directory at the start of the prompt followed by a $. Unfortunately, changing into the documents directory does not update the prompt even pwd shows that the directory has been changed.

If single quotes are used instead of double quotes, we get the expected behaviour.

$ PS1='$(pwd) $ '
/home/alex $ cd documents
/home/alex/documents $ pwd
/home/alex/documents
/home/alex/documents $ ..
/home/alex $

According to this stackoverflow thread https://stackoverflow.com/questions/23879159/how-can-this-ps1-variable-be-defined-in-single-quotes, this is special behaviour for the PS1 variable. Every time a new prompt is generated (for example by changing directory) using single quotes will expand the substitution. However, when double quotes are used, the substitution is expanded when PS1 is defined and thus will not update.

Not also even though single quotes are used, the $, (, and ) are not treated literally.