Several methods of self-increment (plus 1) of shell integer variables

#!/bin/sh

A=1

a=$ (($a + 1))

a=$[$a +1]

A= ' expr $a + 1 '

Let a++

Let A+=1

((a++))

Echo $a

1. Classical methods
Length= ' expr length + 1 '
2. Bash only
$ ((length++))

1. i= ' expr $i + 1 ';
2. Let i+=1;
3. ((i++));
4. i=$[$i +1];
5. i=$ (($i + 1))

How to set the self-increment variable in the shell

Loop=1
For i in ' ls '
Do
# loop= ' expr $loop + 1 '
# Let Loop+=1
# ((loop++))
# loop=$[$loop +1]
loop=$ (($loop + 1))
echo "$i"
echo "Num: $loop"
Done

Note: loop= ' expr $loop + 1 ', the loop variable cannot have spaces around it, and is not a single quotation mark;

In addition, for a fixed number of cycles, can be achieved by the SEQ command, there is no need for the increment of the variable, the example is as follows:
#!/bin/bash
For j in $ (SEQ 1 5)
Do
Echo $j

Done