Linux provides us with some expansions for command, arithmetic, addition and subtraction, comparison, ternary, assignment, and process substitution operations.

Command

We can execute the output as a command by using the $() or `` symbols.

Let’s use the $() symbols to do this first:

                    
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                          echo "ls -l" > ls_l_command_file
cat ls_l_command_file
$(cat ls_l_command_file)    # This will run the file content "ls -l" as a command.

Alternatively, we can use backtick symbols to achieve the same effect:

                    
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                          `cat ls_l_command_file`    # This will run the file content "ls -l" as a command.

Arithmetic

We can use arithmetic expansion to quickly calculate numerical values.

Some of the operators we can use are:

Operator	Meaning
VARIABLE++ VARIABLE‑‑	self increasing or deceasing after the variable is obtained
++VARIABLE ‑‑VARIABLE	self increasing or deceasing before the variable is obtained
- +	negative or positive number
! ~	logical or bitwise negation
**	exponentiation
* / %	multiplication, division, remainder
+ -	addition, subtraction
<= >= < >	less than equals, greater than equals, less than, greater than
== !=	equal or not equal
expr ? expr : expr	ternary operator
= *= /= %= += -= <<= >>= &= ^=	assign the operation result to a variable

Addition and Subtraction

                    
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                          echo $((1+1))    # 2
echo $((1-1))    # 0

Comparison

                    
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                          echo $((1<=1))    # true
echo $((1>=1))    # true
echo $((1<1))    # false
echo $((1>1))    # false
echo $((1==1))    # true
echo $((1!=1))    # false

Ternary Operator

Positive and negative numbers are true, only zero is false

                    
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                          echo $((1?20:-20))    # 20
echo $((0?20:-20))    # -20
echo $((-1?20:-20))    # 20

Prints true or false of the ternary operation result

Assignment Operator

                    
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                          NUM=2
echo $((NUM*=NUM))    # 4
echo $((NUM*=2))    # 8
echo $((NUM*=4))    # 32

Process Substitution

We can use the <() symbols to use the result of the command output as a file:

                    
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                          cat <(ls)    # sequentially prints the listed files
tac <(ls)    # sequentially prints the listed files in reversed order