#  I use the 'expr' command to...
expr 1 + 2

#  Addition
expr 1 + 2

#  Subtraction
expr 10 - 3

#  Multiplication
expr 2 \* 4

#  Division
expr 12 / 2

#  Remainder
expr 8 % 3

#  Logical Or
expr 1 \| 0

#  Logical And
expr 1 \& 0

#  Less Than
expr 99 \< 100

#  Less Than or equal to
expr 99 \<= 100

#  Equal to
expr 66 = 66

#  Not Equal to
expr 66 != 66

#  Greater Than
expr 231 \> 99

#  Greater Than or equal to
expr 231 \>= 99

#  Return length of string:
expr length 'Hello World.'

#  Return the substring at position 4 of length 9:
expr substr 'My Favourite String' 4 9

#  Return the index of the first character
#  in 'something' that is in the character set 'abcdef':
expr index something abcdef

#  Gives length of match:
expr aaaaaa : 'a\+'

#  Gives length of match:
expr match '0xF8' '0x[0-9a-fA-F][0-9a-fA-F]'

#  Extracts last 4 digits of phone number:
expr '555-123-1234' : '[0-9]\{3\}-[0-9]\{3\}-\([0-9]\{4\}\)'

#!/bin/bash
if [ $(expr 123 - 100) -gt 0 ]; then
        echo "Do something."
else
        echo "Do something else."
fi

expr 2 >= 1

expr: syntax error: unexpected argument ‘1’

man bash

...
   Redirecting Output
       Redirection of output causes the file whose name results from the expansion of word  to  be
       opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is
       not specified.  If the file does not exist it is created; if it does exist it is  truncated
       to zero size.

       The general format for redirecting output is:

              [n]>word
...

expr 2 \>= 1

1

expr 2 * 4

8

touch '+'

ls -l

total 0
-rw-rw-r-- 1 robert robert 0 Apr 19 11:51 +

expr 2 * 4

6

man glob

DESCRIPTION
       The  glob() function searches for all the pathnames matching pattern according to the rules
       used by the shell (see glob(7)).  No tilde expansion or parameter substitution is done;  if
       you want these, use wordexp(3).

expr 2 \* 4

8