References:

https://ryanstutorials.net/bash-scripting-tutorial/ this tutorial

http://tldp.org/LDP/abs/html/string-manipulation.html string manipulation in bash

#shebang

#!/bin/bash

#built-in variables

$0 - The name of the Bash script.

$1 - $9 - The first 9 arguments to the Bash script. (As mentioned above.)

$# - How many arguments were passed to the Bash script.

$@ - All the arguments supplied to the Bash script.

$? - The exit status of the most recently run process.

$$ - The process ID of the current script.

$USER - The username of the user running the script.

$HOSTNAME - The hostname of the machine the script is running on.

$SECONDS - The number of seconds since the script was started.

$RANDOM - Returns a different random number each time is it referred to.

$LINENO - Returns the current line number in the Bash script.

#display current environment variables

env

#execute a command and assign output to a variable

myvar=$( ls /etc | wc -l )

export myvar

inside "" (double quotes) the variable substitution takes place, inside '' (single quotes) no.

#read from stdin

read var1

read -p "your name" name

read -ps "silent password" pw

STDIN - /proc/<processID>/fd/0 or /proc/self/fd/0

STDOUT - /proc/<processID>/fd/1 or /proc/self/fd/1

STDERR - /proc/<processID>/fd/2 or /proc/self/fd/2

#reading from stdin

cat /dev/stdin

#arithmetics

let a=5+4

expr 5+4

a=$(( 5+4 ))

#print length of a variable

myvariable="hello"

echo ${#myvariable}

#if statement

if [ $1 -gt 100 ]

then

echo Hey that\'s a large number.

pwd

fi

this help about expressions can be displayed with "man test":

! EXPRESSION The EXPRESSION is false.

-n STRING The length of STRING is greater than zero.

-z STRING The lengh of STRING is zero (ie it is empty).

STRING1 = STRING2 STRING1 is equal to STRING2

STRING1 != STRING2 STRING1 is not equal to STRING2

INTEGER1 -eq INTEGER2 INTEGER1 is numerically equal to INTEGER2

INTEGER1 -gt INTEGER2 INTEGER1 is numerically greater than INTEGER2

INTEGER1 -lt INTEGER2 INTEGER1 is numerically less than INTEGER2

-d FILE FILE exists and is a directory.

-e FILE FILE exists.

-r FILE FILE exists and the read permission is granted.

-s FILE FILE exists and it's size is greater than zero (ie. it is not empty).

-w FILE FILE exists and the write permission is granted.

-x FILE FILE exists and the execute permission is granted.

test 001 = 1

echo $? (result is 0)

test 001 -eq 1

echo $? (result is 1)