"expr" Command

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expr - evaluate expressions

root:~# man expr

In Examples,
Note-1 Beware that many operators need to be escaped or quoted for shells. Comparisons are arithmetic if both ARGs are numbers, else lexicographical. Pattern matches return the string matched between \( and \) or null; if \( and \) are not used, they return the number of characters matched or 0.
Note-2 Exit status is 0 if EXPRESSION is neither null nor 0, 1 if EXPRESSION is null or 0, 2 if EXPRESSION is syntactically invalid, and 3 if an error occurred.
Note-3 Operator has been separated by space otherwise treated as one expression as shown in Example-1.1

Example-1

1.1 ARG1 | ARG2
ARG1 if it is neither null nor 0, otherwise ARG2

root:~# expr 2 | 3
-bash: 3: command not found
root:~# expr 2\|3
2|3
root:~# expr 2 \| 3
2
root:~# expr '' \| 4
4

1.2 ARG1 & ARG2
ARG1 if neither argument is null or 0, otherwise 0

root:~# expr 1 \& 2
1
root:~# expr 1 \& 0
0

1.3 ARG1 < ARG2
Compare ARG1 is less than ARG2

root:~# expr 3 \< 2
0

1.4 ARG1 <= ARG2
ARG1 is less than or equal to ARG2

root:~# expr 3 \<= 2
0
root:~# expr 3 \<= 3
1

1.5 ARG1 > ARG2
ARG1 is greater than ARG2

root:~# expr 4 \> 5
0
root:~# expr 5 \> 4
1

1.6 ARG1 >= ARG2
ARG1 is greater than or equal to ARG2

root:~# expr 5 \>= 4
1
root:~# expr 5 \>= 5
1

Example-2

2.1 ARG1 = ARG2
ARG1 is equal to ARG2

root:~# expr 4 = 5
0
root:~# expr 4 = 4
1

2.2 ARG1 != ARG2
ARG1 is unequal to ARG2

root:~# expr 4 != 5
1
root:~# expr 4 != 4
0

2.3 ARG1 + ARG2
arithmetic sum of ARG1 and ARG2

root:~# expr 4 + 4
8
root:~# expr -3 + 5
2
root:~# expr 4.4 + 5.412
expr: non-integer argument

2.4 ARG1 - ARG2
arithmetic difference of ARG1 and ARG2

root:~# expr 4 - 4
0
root:~# expr 4 - 6
-2
root:~# expr 4.4 - 5.412
expr: non-integer argument

2.5 ARG1 * ARG2
arithmetic product of ARG1 and ARG2

root:~# expr 4 * 5
expr: syntax error
root:~# expr 4 \* 5
20

2.6 ARG1 / ARG2
arithmetic quotient of ARG1 divided by ARG2

root:~# expr 4 / 2
2
root:~# expr 4 / 0
expr: division by zero

2.7 ARG1 % ARG2
arithmetic remainder of ARG1 divided by ARG2

root:~# expr 4 % 3
1

Example-3

3.1 length STRING
length of STRING

root:~# expr length 'Hello World, How are you?'
25

3.2 STRING : REGEXP or match STRING REGEXP
anchored pattern match of REGEXP in STRING

root:~# expr 'Hello World, How are you?' : 'Hello'
5
root:~# expr 'Hello World, How are you?' : 'How'
0
root:~# expr 'Hello World, How are you?' : '.*'
25
root:~# expr match 'Hello World, How are you?' '.*'
25

As shown, Partial match is allowed but it always start search within start of the string so anything in between pattern search wont be successful. Upon successful, returns the number of characters matched.

3.3 substr STRING POS LENGTH
To Extract sub-string from string following is syntax:

root:~# expr substr candidinformation 7 11
information

3.4 index STRING CHAR
index in STRING where any CHAR is found, or 0

root:~# expr index candidinformation o
10
root:~# expr index candidinformation a
2

Example-4

4.1 Increment Shell Variable

root:~# count=0
root:~# echo $count
0
root:~# count=`expr $count + 1`
root:~# echo $count
1