# Bash / Bash Script
## Waht is Shell
A **Shell** is a command interpreter.
Commands can be executable files or built-ins.
## What is Shell Script
A **Shell Script** is a plain text file containing a set of various commands.
It can be used to automate repetitive tasks on Linux filesystem.

## What is Bash
Bash is a common and popular shell.

## shebang

#!/bin/bash

Specify the location of the shell interpreter in our operating system.
- on the first line
- no any space

## echo

echo [option] [string]

Display the standard output by passing the arguments.

## Comment
### Single Line Comment

# This is a single-line comment in Bash Script.  

### Multi Line Comment

<<COMMENTS
COMMENTS

: '
'

<<COMMENTS
This is a 
multi-line comment 
in Bash Script.  
COMMENTS

: '
This is also a 
multi-line comment 
in Bash Script.
'

## Quote
Used to handle texts and filenames with a space character.
- content within the quotes should be considered as a single item
- shell variable expansion will only work with double-quotes.

#!/bin/bash 
  
name="asdf"  
  
echo "$name"  
echo '$name' 

> asdf
> $name

## Variable

name=Joey
echo $name

The containers which store data.
- case sensitive
- untyped
- no need of using any quotes to define a variable with single character value
- no whitespace when doing assignment

### System-Defined Variables

echo $HOME
echo $PWD

- BASH - shell name
- BASH_VERSION - shell version
- HOME - home directory
- PWD - current working directory

## Command Line Arguments

./script.sh arg1 arg2 arg3

- $0 - the name of the script
- $1~$9 - the first 9 arguments
- $# - number of arguments
- $* - all arguments (as one word)
- $@ - all arguments (as array)

echo $1 $2

## Command Substitution

var=$(command_name)
echo $var

var=`command_name`
echo $var

It takes the output of the Bash command, stores in a variable, and display back with echo.

## Read User Input

read variable_name
read -p "name:" variable_name  # same prompt
read -sp "passward:" variable_name # silent mode
read -a array_name # read an array

## Arithmetic Operations

Sum=$((10 + 3))
((Sum=10 + 3))

### Operators
- \+
- \-
- \*
- \/
- \*\*
- \%
- \+=
- \-=
- \*=
- \/=
- \%=

### Let
Perform arithmetic operations

let "z=$((10 + 3))"

### Backticks
Similar to "let", but it does not save the result to a variable.

`expr $a + $b`

## If

test condition
[condition]
[[condition]]

if [ condition ]
then
statements
fi


if [ condition ];
then
statements
else
statements
fi

if [ condition ];  
then  
statements
elif [ condition ];  
then  
statements
else  
statements
fi  

### Operators
- !EXPRESSION - check if EXPRESSION is false
- \-n STRING - check if the length of STRING is greater than zero
- \-z STRING - check if the length of STRING is zero
- STRING1 == STRING2
- STRING1 != STRING2
- INTEGER1 -eq INTEGER2
- INTEGER1 -gt INTEGER2
- INTEGER1 -ge INTEGER2
- INTEGER1 -lt INTEGER2
- INTEGER1 -le INTEGER2
- \-d FILE - check if FILE exists and it is a directory
- \-e FILE - check if FILE exists

## Case

case expression in  
    pattern_1)  
        statements  
        ;;  
    pattern_2)  
        statements  
        ;;  
    pattern_3|pattern_4|pattern_5)  
        statements  
        ;;  
    pattern-n)  
        statements  
        ;;  
    *)  
        statements  
        ;;  
esac  

## Loop
### For

for variable in list  
do  
statements
done  

for (( expression1; expression2; expression3 ))  
do  
statements
done  

### While

while [ expressions ];  
do  
commands
done  

### Until

until [ expressions ];  
do  
commands
done  

## String
### Operator
- str1 = str2
- str1 != str2
- str1 < str2 (less than)
- str1 > str2 (greater than)
- -n str (length > 0)
- -z str (length == 0)
- ${#str} (return length)

## Pattern Matching
### Wildcard
- \* - match zero or more characters
- ? - match any single character
- [...] - match any of the characters in a set
- ?(patterns) - match zero or one occurrences of the patterns
- \*(patterns) - match zero or more occurrences of the patterns
- +(patterns) - match one or more occurrences of the patterns
- @(patterns) - match one occurrence of the patterns
- !(patterns) - match anything that doesn't match one of the patterns

### Curly braces

# cat log.txt log.csv
cat log.{txt,csv}

# cat all *.py and *.sh files
cat *{.py,.sh}

## Substitution
- ${foo%suffix} - remove suffix
- ${foo#prefix} - remove prefix
- ${foo%%suffix} - remove long suffix
- ${foo##prefix} - remove long prefix
- ${foo/from/to} - replace first match
- ${foo//from/to} - replace all
- ${foo/%from/to} - replace suffix
- ${foo/#from/to} - replace prefix

## Function

# Definition
function_name () 
{  
  statements
}  

# Calling
function_name

### Passing Arguments

# Calling with arguments
function_name () 
{  
  echo $1
  echo $2
  echo $3
}  

function_name arg1 arg2 arg3

### Variable Scope
- variables are defined as global variables by default

local name=Joey 

## Array
### Initialization

arr[0] = value_0
arr[1] = value_1
arr[2] = value_2

arr=(value_0 value_1 value_2)

### Array Length

${#arr[@]}

### Loop through the Array

for ele in ${arr[@]}  
do  
echo $ele
done  

for i in ${!arr[@]}  
do  
echo $i ":" ${arr[$i]}
done 

### Adding Element

arr+=(value)

### Remove Element

unset arr[index]

### Delete Array

unset arr

### Slice Array

arr_slice=(${arr[@]:m:n})  

### Check If Files Exist

FILE=/etc/resolv.conf
if [-f "$FILE"]; then
    echo "$FILE exists."
fi

- \-d FILE: True if the FILE exists and is a directory.
- \-e FILE: True if the FILE exists and is a file, regardless of type (node, directory, socket, etc.).
- \-f FILE: True if the FILE exists and is a regular file (not a directory or device).
- \-x FILE: True if the FILE exists and is executable.

### Stdio Redirection
- \>: redirect output (default 1>)
- \<: redirect input (default 0<)
- 0: stdin
- 1: stdout
- 2: stderr
- &>: 2>&1

# redirect the stderr to file
command 2> file

# '> file' redirect the stdout to file
# then '2>&1' redirect the stderr to stdout.
command > file 2>&1

# redirect both stdout/stderr to file
command &> file
command 2>&1 > file 

### Piping
redirect the stdout of the former program as the stdin of the latter program

# count the number of particular files
ls *.txt | grep test | wc -l

# sort the content of a file
$ cat file.txt | sort | cat > sortedFile.txt

# read the specific line of a file
cat file.txt | head -3 | tail -1

#### xargs
execute a command using STDIN as arguments.

# list all contents of files in the current dir
ls | xargs cat