bash script

1) shell scripting

2) how to create and use variables

3) Testing and decision making

4) command line argument processing

5) input and output, include user input

6) if statements

7) exit statuses

8) functions

9)

10)

11)

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13)


#!/bin/bash

#comment

echo "Hello World!"

myName="name" #variable

#all variable are trited as a string
# no space on either side of =

#declar a constant
declare -r NUM1=5

num2=4

num3=$((NUM1+num2))
num3=$((NUM1-num2))
num3=$((NUM1*num2))
num3=$((NUM1/num2))

echo "5 + 4 = $num3"

echo $((5**2))
echo $(( 5%4 ))


i += 2
i + i =2


rand=5
let rand+=4
echo "$rand"

echo"rand++ = $((  rand ++ ))"
echo"++rand = $(( ++ rand ))"
echo"rand-- = $((  rand -- ))"
echo"--rand = $((  --rand ))"


num7=1.2
num8=3.4
num9=$(pythin - c "print $num7+$num8")
echo $num9


cat<< END
This text
prints
on many lines
END

#Functions

getDate(){
    date

    return
}

getDate


name="name" #global var

demLocal(){
   local name="Paul" #local var
   return
}

demLocal

echo "$name"

getSum(){
  local num=$1
  local num=$2

  local sum=$((num3+num4))

  echo $sum
}

num1=5
num2=6

sum=$(getSum num1 num2)
echo "the sum is $sum"


#conditionals

read -p "What is your name? " name

echo "Hello $name"

read -p "How old are you? " age

if [$age -ge 16]   #another options eq ne le lt ge gt
then
  echo "You can drive"
elif [$age -eq 15]
then
   echo "You can drive next year"
else
   echo "You can't drive"
fi



read -p "Enter a number : " num

if (( num == 10)); then
  echo "Your number equals 10"
fi

if ((num >= 10)); then
  echo "It is greater then 10"
else
  echo "It is less then ten"
fi


if (( (( num % 2)) == 0)); then
  echo "It is even"
fi


#Logical operators

if (( (( num > 0)) && ((num <11)) ));
then
  echo "$num is between 1 and 10"
fi

touch samp_file  && vim samp_file

[ -d samp_dir] || mkdir samp_dir


str1=""
str2="Sad"
str3="Happy"

if [ "$str1" ]; then
  echo "$str1 is not null"
fi

if [ -z "$str1" ]; then
  echo "str1 has not value"
fi

if ["$str2" === "$str3" ]; then
  echo "$str2 equals $str3"
elfi [ "$str2" != "$str3" ] ; then
  echo "$str2 is not equal to @str3"
fi


if ["$str2" > "$str3" ];  then
  echo "$str2 is greater then $str3"
elif [ "$str2" < "$str3"]; then
  echo "$str2 is less then $str3"
fi

# files

file1=".test_file1"
file2="./test_file2"

if [ - e "$file1"]; then
  echo "$file1 exists"

  if [-f "$file1"]; then
    echo "$file1 is a normal file"
  fi

  if [ -r "$file1" ]; then
    echo "$file1" is readable"
  fi

  if [ -w "$file1" ]; then
    echo "$file1 is writable"
  fi

  if [ -e "$file1" ]; then
    echo "$file1 is executable"
  fi

  if [ -d "$file1" ]; then
    echo "$file1 is a directory"
  fi

  if [ -L "$file1" ]; then
    echo "$file1 is a symbolic link"
  fi

  if [ -p "$file1" ]; then
    echo "$file1 is named pipe"
  fi

  if [ -S "$file1" ]; then
    echo "$file1 is a network sockeet"
  fi

  if [ -G "$file1" ]; then
    echo "$file1 is owned by the group"
  fi

  if [ -O "$file1" ]; then
    echo "$file1 is owned by the userid"
  fi

fi

# Reg exp

#Extendet test


read -p "Validate Date: " date

pat="^[0-9]{8}$"

if [[ $date =~ $pat ]] ; then
  echo "$date is valid"
else
  echo "$date not valid"
fi


read -p "Enter 2 Numbers to Sum : " num1 num2

sum=$((num1+num2))

echo "$num1 + $num2 = $sum"

read -sp "enter the secret code" secret

if [ $secret" == "password" ]; then
  echo"Enter"
else
  echo "Wrong Password"
fi

#Parametr expation


OIFS="$IFS"

IFS=","

read -p "Enter 2 numbers to add separated by a comma" num1 num2

num1=${num1//[[:blank:]]/}
num2=${num1//[[:blank:]]/}

sum=$((num1+num2))

sum "$num1 + $num2 = $sum"

IFS="$OIFS"


name="name"

echo "${name}'s toy"

samp_string ="Thr dog climed the tree"
echo "${samp_string//dog/cat}"

echo "I am ${name:=name}"


#case

read -p "How old are you: " age

case $age in
[0-4])
  echo "To young for school"
  ;;
5)
  echo "Go to Kindergarten"
  ;;
[6-9]|1[0-8])
  grade=$((age-5))
  echo "Go to grade $grade"
  ;;
*)
  echo "You are to old for school"
  ;;
esac

can_vote= 0
age=18

((age >=18?(can_vote=1):(can_vote=0)))
echo "Can Vote : $can_vote"


rand_str="A randomstrging"

echo "String Length : ${#rand_str}"
echo "${rand_str:2}"
echo "${rand_str:2:7}"


2)Indirect expansion

If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion.

However, reading on from there:

The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below.

${!prefix*} Names matching prefix. Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable.

In other words, your particular example ${!N*} is an exception to the rule you quoted. It does, however, work as advertised in the expected cases, such as:

$ export xyzzy=plugh ; export plugh=cave

$ echo ${xyzzy}  # normal, xyzzy to plugh
plugh

$ echo ${!xyzzy} # indirection, xyzzy to plugh to cave
cave

3)

It refers to the positional parameters $1 ... $n.

${1:-default} means "if parameter 1 is unset or empty, then use default instead".

Caveat: do not confuse ${1:-2} with ${1: -2}. With bash, the latter is substituted with the last two characters of $1.

Example:

$ set --
$ echo "${1:-2}"
2

$ set 345 678
$ echo "${1:-2}"
345

$ echo "${1: -2}"
45

4)

This technique allows for a variable to be assigned a value if another variable is either empty or is undefined. NOTE: This "other variable" can be the same or another variable.

excerpt

${parameter:-word}
    If parameter is unset or null, the expansion of word is substituted. 
    Otherwise, the value of parameter is substituted.

NOTE: This form also works, ${parameter-word}. If you'd like to see a full list of all forms of parameter expansion available within Bash then I highly suggest you take a look at this topic in the Bash Hacker's wiki titled: "Parameter expansion".

Examples

variable doesn't exist

$ echo "$VAR1"

$ VAR1="${VAR1:-default value}"
$ echo "$VAR1"
default value

variable exists

$ VAR1="has value"
$ echo "$VAR1"
has value

$ VAR1="${VAR1:-default value}"
$ echo "$VAR1"
has value

The same thing can be done by evaluating other variables, or running commands within the default value portion of the notation.

$ VAR2="has another value"
$ echo "$VAR2"
has another value
$ echo "$VAR1"

$

$ VAR1="${VAR1:-$VAR2}"
$ echo "$VAR1"
has another value

More Examples

You can also use a slightly different notation where it's just VARX=${VARX-<def. value>}.

$ echo "${VAR1-0}"
has another value
$ echo "${VAR2-0}"
has another value
$ echo "${VAR3-0}"
0

In the above $VAR1 & $VAR2 were already defined with the string "has another value" but $VAR3 was undefined, so the default value was used instead, 0.

Another Example

$ VARX="${VAR3-0}"
$ echo "$VARX"
0

Checking and assigning using := notation

Lastly I'll mention the handy operator, :=. This will do a check and assign a value if the variable under test is empty or undefined.

Example

Notice that $VAR1 is now set. The operator := did the test and the assignment in a single operation.

$ unset VAR1
$ echo "$VAR1"

$ echo "${VAR1:=default}"
default
$ echo "$VAR1"
default

However if the value is set prior, then it's left alone.

$ VAR1="some value"
$ echo "${VAR1:=default}"
some value
$ echo "$VAR1"
some value

Handy Dandy Reference Table

    

References

• Parameter Expansions - Bash Hackers Wiki
• 10.2. Parameter Substitution
• Bash Parameter Expansions

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20) what to use in the script insted of "if" - Best Practices

a) exit codes

; - to chain commands together

echo  "sth" ; echo "sth2"

b) logical operators

&& - "and and" only runs second command if first succeed

cat file && echo "sth2"

|| - "or or" it runs second command only if first one not succeed

cat file || echo "sth2"

& - |"and" it runs both of command one after another without wait for first command to end running

sleep 5 & echo "sth2"

21)

100)
a)https://www.youtube.com/watch?v=hwrnmQumtPw