lstrip() {
    # Usage: lstrip "string" "pattern"
    printf '%s\n' "${1##$2}"
}

$ lstrip "The Quick Brown Fox" "The "
Quick Brown Fox

rstrip() {
    # Usage: rstrip "string" "pattern"
    printf '%s\n' "${1%%$2}"
}

$ rstrip "The Quick Brown Fox" " Fox"
The Quick Brown

trim_string() {
    # Usage: trim_string "   example   string    "

    # Remove all leading white-space.
    # '${1%%[![:space:]]*}': Strip everything but leading white-space.
    # '${1#${XXX}}': Remove the white-space from the start of the string.
    trim=${1#${1%%[![:space:]]*}}

    # Remove all trailing white-space.
    # '${trim##*[![:space:]]}': Strip everything but trailing white-space.
    # '${trim%${XXX}}': Remove the white-space from the end of the string.
    trim=${trim%${trim##*[![:space:]]}}

    printf '%s\n' "$trim"
}

$ trim_string "    Hello,  World    "
Hello,  World

$ name="   John Black  "
$ trim_string "$name"
John Black

# shellcheck disable=SC2086,SC2048
trim_all() {
    # Usage: trim_all "   example   string    "

    # Disable globbing to make the word-splitting below safe.
    set -f

    # Set the argument list to the word-splitted string.
    # This removes all leading/trailing white-space and reduces
    # all instances of multiple spaces to a single ("  " -> " ").
    set -- $*

    # Print the argument list as a string.
    printf '%s\n' "$*"

    # Re-enable globbing.
    set +f
}

$ trim_all "    Hello,    World    "
Hello, World

$ name="   John   Black  is     my    name.    "
$ trim_all "$name"
John Black is my name.

case $var in
    *sub_string1*)
        # Do stuff
    ;;

    *sub_string2*)
        # Do other stuff
    ;;

    *)
        # Else
    ;;
esac

case $var in
    sub_string1*)
        # Do stuff
    ;;

    sub_string2*)
        # Do other stuff
    ;;

    *)
        # Else
    ;;
esac

startswith() {

    if [ "$(printf "%.${#1}s" "$2")" = "$1" ]; then
        return 0
    else
        return 1
    fi
}

$ startswith 'fo' 'foo'

case $var in
    *sub_string1)
        # Do stuff
    ;;

    *sub_string2)
        # Do other stuff
    ;;

    *)
        # Else
    ;;
esac

split() {
    # Disable globbing.
    # This ensures that the word-splitting is safe.
    set -f

    # Store the current value of 'IFS' so we
    # can restore it later.
    old_ifs=$IFS

    # Change the field separator to what we're
    # splitting on.
    IFS=$2

    # Create an argument list splitting at each
    # occurance of '$2'.
    #
    # This is safe to disable as it just warns against
    # word-splitting which is the behavior we expect.
    # shellcheck disable=2086
    set -- $1

    # Print each list value on its own line.
    printf '%s\n' "$@"

    # Restore the value of 'IFS'.
    IFS=$old_ifs

    # Re-enable globbing.
    set +f
}

$ split "apples,oranges,pears,grapes" ","
apples
oranges
pears
grapes

$ split "1, 2, 3, 4, 5" ", "
1
2
3
4
5

trim_quotes() {
    # Usage: trim_quotes "string"

    # Disable globbing.
    # This makes the word-splitting below safe.
    set -f

    # Store the current value of 'IFS' so we
    # can restore it later.
    old_ifs=$IFS

    # Set 'IFS' to ["'].
    IFS=\"\'

    # Create an argument list, splitting the
    # string at ["'].
    #
    # Disable this shellcheck error as it only
    # warns about word-splitting which we expect.
    # shellcheck disable=2086
    set -- $1

    # Set 'IFS' to blank to remove spaces left
    # by the removal of ["'].
    IFS=

    # Print the quote-less string.
    printf '%s\n' "$*"

    # Restore the value of 'IFS'.
    IFS=$old_ifs

    # Re-enable globbing.
    set +f
}

$ var="'Hello', \"World\""
$ trim_quotes "$var"
Hello, World

# Setting 'IFS' tells 'read' where to split the string.
while IFS='=' read -r key val; do
    # Skip over lines containing comments.
    # (Lines starting with '#').
    [ "${key##\#*}" ] || continue

    # '$key' stores the key.
    # '$val' stores the value.
    printf '%s: %s\n' "$key" "$val"

    # Alternatively replacing 'printf' with the following
    # populates variables called '$key' with the value of '$val'.
    #
    # NOTE: I would extend this with a check to ensure 'key' is
    #       a valid variable name.
    # export "$key=$val"
    #
    # Example with error handling:
    # export "$key=$val" 2>/dev/null ||
    #     printf 'warning %s is not a valid variable name\n' "$key"
done < "file"

head() {
    # Usage: head "n" "file"
    while IFS= read -r line; do
        printf '%s\n' "$line"
        i=$((i+1))
        [ "$i" = "$1" ] && return
    done < "$2"

    # 'read' used in a loop will skip over
    # the last line of a file if it does not contain
    # a newline and instead contains EOF.
    #
    # The final line iteration is skipped as 'read'
    # exits with '1' when it hits EOF. 'read' however,
    # still populates the variable.
    #
    # This ensures that the final line is always printed
    # if applicable.
    [ -n "$line" ] && printf %s "$line"
}

$ head 2 ~/.bashrc
# Prompt
PS1='➜ '

$ head 1 ~/.bashrc
# Prompt

lines() {
    # Usage: lines "file"

    # '|| [ -n "$line" ]': This ensures that lines
    # ending with EOL instead of a newline are still
    # operated on in the loop.
    #
    # 'read' exits with '1' when it sees EOL and
    # without the added test, the line isn't sent
    # to the loop.
    while IFS= read -r line || [ -n "$line" ]; do
        lines=$((lines+1))
    done < "$1"

    printf '%s\n' "$lines"
}

$ lines ~/.bashrc
48

count() {
    # Usage: count /path/to/dir/*
    #        count /path/to/dir/*/
    [ -e "$1" ] \
        && printf '%s\n' "$#" \
        || printf '%s\n' 0
}

# Count all files in dir.
$ count ~/Downloads/*
232

# Count all dirs in dir.
$ count ~/Downloads/*/
45

# Count all jpg files in dir.
$ count ~/Pictures/*.jpg
64

:>file

# OR (shellcheck warns for this)
>file

dirname() {
    # Usage: dirname "path"

    # If '$1' is empty set 'dir' to '.', else '$1'.
    dir=${1:-.}

    # Strip all trailing forward-slashes '/' from
    # the end of the string.
    #
    # "${dir##*[!/]}": Remove all non-forward-slashes
    # from the start of the string, leaving us with only
    # the trailing slashes.
    # "${dir%%"${}"}": Remove the result of the above
    # substitution (a string of forward slashes) from the
    # end of the original string.
    dir=${dir%%"${dir##*[!/]}"}

    # If the variable *does not* contain any forward slashes
    # set its value to '.'.
    [ "${dir##*/*}" ] && dir=.

    # Remove everything *after* the last forward-slash '/'.
    dir=${dir%/*}

    # Again, strip all trailing forward-slashes '/' from
    # the end of the string (see above).
    dir=${dir%%"${dir##*[!/]}"}

    # Print the resulting string and if it is empty,
    # print '/'.
    printf '%s\n' "${dir:-/}"
}

$ dirname ~/Pictures/Wallpapers/1.jpg
/home/black/Pictures/Wallpapers/

$ dirname ~/Pictures/Downloads/
/home/black/Pictures/

basename() {
    # Usage: basename "path" ["suffix"]

    # Strip all trailing forward-slashes '/' from
    # the end of the string.
    #
    # "${1##*[!/]}": Remove all non-forward-slashes
    # from the start of the string, leaving us with only
    # the trailing slashes.
    # "${1%%"${}"}:  Remove the result of the above
    # substitution (a string of forward slashes) from the
    # end of the original string.
    dir=${1%${1##*[!/]}}

    # Remove everything before the final forward-slash '/'.
    dir=${dir##*/}

    # If a suffix was passed to the function, remove it from
    # the end of the resulting string.
    dir=${dir%"$2"}

    # Print the resulting string and if it is empty,
    # print '/'.
    printf '%s\n' "${dir:-/}"
}

$ basename ~/Pictures/Wallpapers/1.jpg
1.jpg

$ basename ~/Pictures/Wallpapers/1.jpg .jpg
1

$ basename ~/Pictures/Downloads/
Downloads

# Loop from 0-10.
for i in 0 1 2 3 4 5 6 7 8 9 10; do
    printf '%s\n' "$i"
done

# Loop from var-var.
start=0
end=50

while [ "$start" -le "$end" ]; do
    printf '%s\n' "$start"
    start=$((start+1))
done

while IFS= read -r line || [ -n "$line" ]; do
    printf '%s\n' "$line"
done < "file"

# Greedy example.
for file in *; do
    [ -e "$file" ] || [ -L "$file" ] || continue
    printf '%s\n' "$file"
done

# PNG files in dir.
for file in ~/Pictures/*.png; do
    [ -f "$file" ] || continue
    printf '%s\n' "$file"
done

# Iterate over directories.
for dir in ~/Downloads/*/; do
    [ -d "$dir" ] || continue
    printf '%s\n' "$dir"
done

$ var="world"
$ eval "hello_$var=value"
$ eval printf '%s\n' "\$hello_$var"
value

# Set the value of var to var2 if var2 is greater than var.
# 'var2 > var': Condition to test.
# '? var2': If the test succeeds.
# ': var': If the test fails.
var=$((var2 > var ? var2 : var))

is_float() {
    # Usage: is_float "number"

    # The test checks to see that the input contains
    # a '.'. This filters out whole numbers.
    [ -z "${1##*.*}" ] &&
        printf %f "$1" >/dev/null 2>&1
}

$ is_float 1 && echo true
$

$ is_float 1.1 && echo true
$ true

is_int() {
    # usage: is_int "number"
    printf %d "$1" >/dev/null 2>&1
}

$ is_int 1 && echo true
$ true

$ is_int 1.1 && echo true
$

# Clear screen on script exit.
trap 'printf \\033[2J\\033[H\\033[m' EXIT

# Run a function on script exit.
# 'clean_up' is the name of a function.
trap clean_up EXIT

trap '' INT

# Right.
var="$(command)"

# Wrong.
var=`command`

# $() can easily be nested whereas `` cannot.
var="$(command "$(command)")"

"$EDITOR" "$file"

# NOTE: This variable may be empty, set a fallback value.
"${EDITOR:-vi}" "$file"

"$PWD"