Bash Programming - 02 Arithmetics

This post is Part Two of Bash Programming. In this post, we will focus on doing arithmetics with Bash.

Operators

Basic

• +
• -
• * (Use \ to escape)
• / (Result is integer, not float)
• %

expr: Only among integers. e.g.

expr 3 \* -2 / 5
>>>  -1

Logic

• ! (Not used in shell’s arithmetics)
• &
• |

e.g.

expr 1 \& 2
>>>  1

expr 2 \& 0
>>>  0

expr 2 \| 0
>>> 2

In commands:

• 1 && 2
• 1 || 2 (exec. 2 only if 1 fails)

Relational

• =
• >
• <
• <=
• >=

Commands

expr

• No power or floats

(( )) （是 [ ] 的进化版）

• e.g. echo $((2**3))

• e.g.

    
  a=0
  ((a=a+3))
  echo $a
  
  >>>  3
  

let: assignment & simple arithmetics

• (( ))‘s command version
• e.g. let 1+1

declare: calculations use -i

a=1+1
declare -i a
echo $a
    
>>>  2

bc: Linux calculator (ints & floats)

• e.g. echo "1.1*3" | bc

Conditions

[[ ]] equals to test command

test 3 -gt 2  	->   [[ 3 > 2 ]]
echo $?

>>> 0   	# True

File type:

• -d: Is it a dir.
• -e: Does it contain this file
• -r: Is it read-only
• e.g.

  [ -d /home/test ]
  echo $?
  
  [[ -r /home/test && -e /test/hello ]]
  echo $?

Special

Convert decimal to binary:

echo "obase=2; ibase=10; 11" | bc -l

>>>  1011

Convert binary to decimal:

echo $(( 2#1011 ))	

>>>  11

Get random numbers (Default: 0 - 32767):

echo $RANDOM

Get random numbers (0 - 255):

expr $RANDOM / 128	

More flexible way:

shuf -i 100-300 -n 4

• Range (-i)
• # of outputs (-n)