# pure sh bible

A collection of pure POSIX `sh` alternatives to external processes


**NOTE**: If anything seen here is **not** POSIX `sh`, open an issue. This is a living document that is open to change and improvement.


## Table of Contents

<!-- vim-markdown-toc GFM -->

* [STRINGS](#strings)
    * [Strip pattern from start of string](#strip-pattern-from-start-of-string)
    * [Strip pattern from end of string](#strip-pattern-from-end-of-string)
    * [Trim all white-space from string and truncate spaces](#trim-all-white-space-from-string-and-truncate-spaces)
    * [Check if string contains a sub-string](#check-if-string-contains-a-sub-string)
    * [Check if string starts with sub-string](#check-if-string-starts-with-sub-string)
    * [Check if string ends with sub-string](#check-if-string-ends-with-sub-string)
* [FILES](#files)
    * [Get the first N lines of a file](#get-the-first-n-lines-of-a-file)
    * [Get the number of lines in a file](#get-the-number-of-lines-in-a-file)
    * [Count files or directories in directory](#count-files-or-directories-in-directory)
    * [Create an empty file](#create-an-empty-file)
* [FILE PATHS](#file-paths)
    * [Get the directory name of a file path](#get-the-directory-name-of-a-file-path)
    * [Get the base-name of a file path](#get-the-base-name-of-a-file-path)
* [ESCAPE SEQUENCES](#escape-sequences)
    * [Text Colors](#text-colors)
    * [Text Attributes](#text-attributes)
    * [Cursor Movement](#cursor-movement)
    * [Erasing Text](#erasing-text)
* [PARAMETER EXPANSION](#parameter-expansion)
    * [Replacement](#replacement)
    * [Length](#length)
    * [Default Value](#default-value)
* [CONDITIONAL EXPRESSIONS](#conditional-expressions)
    * [File Conditionals](#file-conditionals)
    * [Variable Conditionals](#variable-conditionals)
    * [Variable Comparisons](#variable-comparisons)
* [ARITHMETIC OPERATORS](#arithmetic-operators)
    * [Assignment](#assignment)
    * [Arithmetic](#arithmetic)
    * [Bitwise](#bitwise)
    * [Logical](#logical)
    * [Miscellaneous](#miscellaneous)

<!-- vim-markdown-toc -->


# STRINGS

## Strip pattern from start of string

**Example Function:**

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

**Example Usage:**

```shell
$ lstrip "The Quick Brown Fox" "The "
Quick Brown Fox
```

## Strip pattern from end of string

**Example Function:**

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

**Example Usage:**

```shell
$ rstrip "The Quick Brown Fox" " Fox"
The Quick Brown
```

## Trim all white-space from string and truncate spaces

This is an alternative to `sed`, `awk`, `perl` and other tools. The
function below works by abusing word splitting to create a new string
without leading/trailing white-space and with truncated spaces.

**Example Function:**

```sh
# shellcheck disable=SC2086,SC2048
trim_all() {
    # Usage: trim_all "   example   string    "
    set -f
    set -- $*
    printf '%s\n' "$*"
    set +f
}
```

**Example Usage:**

```shell
$ trim_all "    Hello,    World    "
Hello, World

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

## Check if string contains a sub-string

**Using a case statement:**

```shell
case $var in
    *sub_string*)
        # Do stuff
    ;;

    *sub_string2*)
        # Do more stuff
    ;;

    *)
        # Else
    ;;
esac
```

## Check if string starts with sub-string

**Using a case statement:**

```shell
case $var in
    sub_string*)
        # Do stuff
    ;;

    sub_string2*)
        # Do more stuff
    ;;

    *)
        # Else
    ;;
esac
```

## Check if string ends with sub-string

**Using a case statement:**

```shell
case $var in
    *sub_string)
        # Do stuff
    ;;

    *sub_string2)
        # Do more stuff
    ;;

    *)
        # Else
    ;;
esac
```

# FILES

## Get the first N lines of a file

Alternative to the `head` command.

**Example Function:**

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

**Example Usage:**

```shell
$ head 2 ~/.bashrc
# Prompt
PS1='➜ '

$ head 1 ~/.bashrc
# Prompt
```

## Get the number of lines in a file

Alternative to `wc -l`.

**Example Function:**

```sh
lines() {
    # Usage: lines "file"
    while read -r _; do
        lines=$((lines+1))
    done < "$1"

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

**Example Usage:**

```shell
$ lines ~/.bashrc
48
```

## Count files or directories in directory

This works by passing the output of the glob to the function and then counting the number of arguments.

**Example Function:**

```sh
count() {
    # Usage: count /path/to/dir/*
    #        count /path/to/dir/*/
    printf '%s\n' "$#"
}
```

**Example Usage:**

```shell
# 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
```

## Create an empty file

Alternative to `touch`.

```shell
:>file

# OR (shellcheck warns for this)
>file
```

# FILE PATHS

## Get the directory name of a file path

Alternative to the `dirname` command.

**Example Function:**

```sh
dirname() {
    # Usage: dirname "path"
    printf '%s\n' "${1%/*}/"
}
```

**Example Usage:**

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

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

## Get the base-name of a file path

Alternative to the `basename` command.

**Example Function:**

```sh
basename() {
    # Usage: basename "path"
    path=${1%/}
    printf '%s\n' "${path##*/}"
}
```

**Example Usage:**

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

$ basename ~/Pictures/Downloads/
Downloads
```

# ESCAPE SEQUENCES

Contrary to popular belief, there is no issue in utilizing raw escape sequences. Using `tput` abstracts the same ANSI sequences as if printed manually. Worse still, `tput` is not actually portable. There are a number of `tput` variants each with different commands and syntaxes (*try `tput setaf 3` on a FreeBSD system*). Raw sequences are fine.

## Text Colors

**NOTE:** Sequences requiring RGB values only work in True-Color Terminal Emulators.

| Sequence | What does it do? | Value |
| -------- | ---------------- | ----- |
| `\e[38;5;<NUM>m` | Set text foreground color. | `0-255`
| `\e[48;5;<NUM>m` | Set text background color. | `0-255`
| `\e[38;2;<R>;<G>;<B>m` | Set text foreground color to RGB color. | `R`, `G`, `B`
| `\e[48;2;<R>;<G>;<B>m` | Set text background color to RGB color. | `R`, `G`, `B`

## Text Attributes

| Sequence | What does it do? |
| -------- | ---------------- |
| `\e[m`  | Reset text formatting and colors.
| `\e[1m` | Bold text. |
| `\e[2m` | Faint text. |
| `\e[3m` | Italic text. |
| `\e[4m` | Underline text. |
| `\e[5m` | Slow blink. |
| `\e[7m` | Swap foreground and background colors. |


## Cursor Movement

| Sequence | What does it do? | Value |
| -------- | ---------------- | ----- |
| `\e[<LINE>;<COLUMN>H` | Move cursor to absolute position. | `line`, `column`
| `\e[H` | Move cursor to home position (`0,0`). |
| `\e[<NUM>A` | Move cursor up N lines. | `num`
| `\e[<NUM>B` | Move cursor down N lines. | `num`
| `\e[<NUM>C` | Move cursor right N columns. | `num`
| `\e[<NUM>D` | Move cursor left N columns. | `num`
| `\e[s` | Save cursor position. |
| `\e[u` | Restore cursor position. |


## Erasing Text

| Sequence | What does it do? |
| -------- | ---------------- |
| `\e[K` | Erase from cursor position to end of line.
| `\e[1K` | Erase from cursor position to start of line.
| `\e[2K` | Erase the entire current line.
| `\e[J` | Erase from the current line to the bottom of the screen.
| `\e[1J` | Erase from the current line to the top of the screen.
| `\e[2J` | Clear the screen.
| `\e[2J\e[H` | Clear the screen and move cursor to `0,0`.


# PARAMETER EXPANSION

## Replacement

| Parameter | What does it do? |
| --------- | ---------------- |
| `${VAR#PATTERN}` | Remove shortest match of pattern from start of string. |
| `${VAR##PATTERN}` | Remove longest match of pattern from start of string. |
| `${VAR%PATTERN}` | Remove shortest match of pattern from end of string. |
| `${VAR%%PATTERN}` | Remove longest match of pattern from end of string. |

## Length

| Parameter | What does it do? |
| --------- | ---------------- |
| `${#VAR}` | Length of var in characters.

## Default Value

| Parameter | What does it do? |
| --------- | ---------------- |
| `${VAR:-STRING}` | If `VAR` is empty or unset, use `STRING` as its value.
| `${VAR-STRING}` | If `VAR` is unset, use `STRING` as its value.
| `${VAR:=STRING}` | If `VAR` is empty or unset, set the value of `VAR` to `STRING`.
| `${VAR=STRING}` | If `VAR` is unset, set the value of `VAR` to `STRING`.
| `${VAR:+STRING}` | If `VAR` is not empty, use `STRING` as its value.
| `${VAR+STRING}` | If `VAR` is set, use `STRING` as its value.
| `${VAR:?STRING}` | Display an error if empty or unset.
| `${VAR?STRING}` | Display an error if unset.


# CONDITIONAL EXPRESSIONS

For use in `[ ]` `if [ ]; then` and `test`.

## File Conditionals

| Expression | Value  | What does it do? |
| ---------- | ------ | ---------------- |
| `-b`       | `file` | If file exists and is a block special file.
| `-c`       | `file` | If file exists and is a character special file.
| `-d`       | `file` | If file exists and is a directory.
| `-e`       | `file` | If file exists.
| `-f`       | `file` | If file exists and is a regular file.
| `-g`       | `file` | If file exists and its set-group-id bit is set.
| `-h`       | `file` | If file exists and is a symbolic link.
| `-p`       | `file` | If file exists and is a named pipe (*FIFO*).
| `-r`       | `file` | If file exists and is readable.
| `-s`       | `file` | If file exists and its size is greater than zero.
| `-t`       | `fd`   | If file descriptor is open and refers to a terminal.
| `-u`       | `file` | If file exists and its set-user-id bit is set.
| `-w`       | `file` | If file exists and is writable.
| `-x`       | `file` | If file exists and is executable.
| `-L`       | `file` | If file exists and is a symbolic link.
| `-S`       | `file` | If file exists and is a socket.

## Variable Conditionals

| Expression | Value | What does it do? |
| ---------- | ----- | ---------------- |
| `-z`       | `var` | If the length of string is zero.
| `-n`       | `var` | If the length of string is non-zero.

## Variable Comparisons

| Expression | What does it do? |
| ---------- | ---------------- |
| `var = var2` | Equal to.
| `var != var2` | Not equal to.
| `var -eq var2` | Equal to (*algebraically*).
| `var -ne var2` | Not equal to (*algebraically*).
| `var -gt var2` | Greater than (*algebraically*).
| `var -ge var2` | Greater than or equal to (*algebraically*).
| `var -lt var2` | Less than (*algebraically*).
| `var -le var2` | Less than or equal to (*algebraically*).


# ARITHMETIC OPERATORS

## Assignment

| Operators | What does it do? |
| --------- | ---------------- |
| `=`       | Initialize or change the value of a variable.

## Arithmetic

| Operators | What does it do? |
| --------- | ---------------- |
| `+` | Addition
| `-` | Subtraction
| `*` | Multiplication
| `/` | Division
| `**` | Exponentiation
| `%` | Modulo
| `+=` | Plus-Equal (*Increment a variable.*)
| `-=` | Minus-Equal (*Decrement a variable.*)
| `*=` | Times-Equal (*Multiply a variable.*)
| `/=` | Slash-Equal (*Divide a variable.*)
| `%=` | Mod-Equal (*Remainder of dividing a variable.*)

## Bitwise

| Operators | What does it do? |
| --------- | ---------------- |
| `<<` | Bitwise Left Shift
| `<<=` | Left-Shift-Equal
| `>>` | Bitwise Right Shift
| `>>=` | Right-Shift-Equal
| `&` | Bitwise AND
| `&=` | Bitwise AND-Equal
| `\|` | Bitwise OR
| `\|=` | Bitwise OR-Equal
| `~` | Bitwise NOT
| `^` | Bitwise XOR
| `^=` | Bitwise XOR-Equal

## Logical

| Operators | What does it do? |
| --------- | ---------------- |
| `!` | NOT
| `&&` | AND
| `\|\|` | OR

## Miscellaneous

| Operators | What does it do? | Example |
| --------- | ---------------- | ------- |
| `,` | Comma Separator | `((a=1,b=2,c=3))`