Bash scripting - basics · Truely random!

Bash version tested

BruceWayne:[~] $ bash --version
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin18)
Copyright (C) 2007 Free Software Foundation, Inc

Conditional statements

`if` condition

if EXPRESSION
  then DO SOMETHING
elif EXPRESSION
  then DO WHATEVER NEEDS TO BE DONE
else DO THE OTHERTHING
fi

#one liner
if CONDITION; then DO SOMETHING; elif EXPRESSION; then DO WHATEVER NEEDS TO BE DONE; else DO THE OTHERTHING; fi;

use [[ ]] when using variables in conditional statements. The whitespaces between [[]] is necessary.

#echoes if the file exists
if [[ -e $filename ]]
  then echo "file exists"
fi

#use ! for negation
#echoes if the directory does not exist
if ! [[ -e $dirname ]]
  then echo "directory does not exist"
fi

`for` loop

for ITEM in ITEMS;
  do echo $ITEM;
done

#example
BruceWayne:[~] $ for x in marvel dc; do echo $x; done
marvel
dc
BruceWayne:[~] $ 

Traditional C type for loop

for ((initialize; condition; expression))
do
  SOMETHING
done
#also supports break and continue.

‘while` loop

#syntax structue - while; do; done;
i="0"

while [ $i -lt 4 ]
do
  echo $i
  i=$[$i+1]
done

‘until’ loop

#similar to while loop
#syntax structure - until; do; done;

Command line arguments

Use $1 to get the first argument ,$2 for 2nd argument and so on..
Use *$ to get all arguments.
Use $# to get count of arguments.
$@ prints all arguments. When called with double quotes ex: “$@”, All arguments will be printed with each having double quotes.
```
#example
"arg1" "arg2" "arg3"
```
$* similar to $@ but when double quoted (ex: “$*”), All the arguments will be enclosed with in a single double quote.
```
#example
"arg1 arg2 arg3"
```
If the no of arugements go beyond 9 curly braces should be used.
```
#example
echo $9
echo ${10}
```
$0 - is special variable which prints the name of the file.

Using regular expressions

if [[ "Han Solo" =~ ^Han ]]; then echo "star wars"; fi

The above code outputs:

BruceWayne:[~] $ if [[ "Han Solo" =~ ^Han ]]; then echo "star wars"; fi
star wars
BruceWayne:[~] $ 
# =~ succeeds if the string on the left has a match for regex on the right.

Variables

It’s a good habbit to use curly braces around variables. For instance, While $var works but its better to have ${var}.

Attributes can be set to variables using declare.

  #example
    declare -i test_var

    #Now $test_var will hold only numeric values. If a string is assigned to it, A zero will be set instead.

We can use declare +i test_var to remove the attribute
Also use -r (ex: declare -ir test_var) to make test_var read only
Once test_var has been set with integer attribute, arithmetic operations can be performed.
```
#example
test_var="1+3"
echo $test_var #will display 4
```

Exporting variables

By default variable’s scope is within the script that is being executed.
We cannot pass variables to another script. Even when a script is being execute inside its parent. To resolve this issue we can export variables
```
#syntax
export somevar=<value>
declare -x somevar=<value>
```
Note:
1. Although exported variables can be accessed by all scripts in that environment, attributes cannot be exported along with variables. for instance: declare -i x. now x is an integer variable. but exporting it will make it loose integer property.
2. Subscript cannot pass variables to parent script even through exporting.

Return codes

A bash program always ends with a return code.
If we use other programs in a bash script, It’s good to know about the return codes of those programs.
0 - success & 1 to 255 - error.
use exit 1 to end program execution.

Operators on strings

Operator	Meaning
-eq	equality
-ne	not equal
-lt	less than
-gt	greater than
-z	returns true if a variable is empty

Logical AND & OR

AND && - It executes 2nd statement only when the first statement succeeds.
```
#example
mkdir newdir && cd newdir
```
OR || - It executes the 2nd statement only when the previous one fails.
```
#example
tk || echo "command not recognized"
```

Switch case

#sample
case x in 
	pattern1)
		do something;;
	pattern2)
		do something;;
	pattern3)
		do something;;
	*)
		do somethiing;;
esac

Command grouping

#syntax
{ command1; command2; command3; }

Use cases:

Can use i/o redirection for the whole group.

Arithmetic expressions

#syntax
(( expression here ))

#example
(( ++var )) #increments integer value in var by 1

#arithmetic variable assignment
#example
(( line_count = (cat file.txt | wc -l) ))
#line_count will hold integer which is the count of lines in file.txt

#command substitution for arithmetic expressions:
line_count=$((cat file.txt | wc -l))

let variable #holds integer
let x=10*10 #will hold 100

(( arithmetic expression )) can also be used in if, While unlike other operations 0 is false and values greater than 0 is true.
```
#example
(( 0 )) || echo 'false' #this will output false
```
Numbers leading with 0 will be interpreted as octal.
```
#example
#010 will be interpreted as 8
```

Arrays

Saving values in a array(no declaration required)

arr[0]="Chandler"
arr[1]="Joey"
arr[2]="Ross"

Declaration
```
declare -a x
```
Initializing
```
arr=(5,4,3,2,1,a,b,c)
```

Retrieving a data from an array.

echo ${arr[2]} #would out put "ross"

#Retrieving all values
echo ${x[@]}
echo ${x[*]}

#Get number of elements in the array
echo ${#arr[@]}

#Display indices instead of value
echo ${!arr[@]}

#Display indices and values
echo declare -p arr

Note

Arrays cannot be exported.
We can also skip indices and directly put a value at any index.
```
#example
x[0]="Joey"
x[10]="Dr. Drake ramoray"
```

Bash version tested

Conditional statements

if condition

for loop