Checking condition in shell scipt by test or [expr]

With test command or expression within bracket (for example, [expression here]) you can check whether condition is true inside shell script. If condition is true then it returns zero (0) otherwise it returns non-zero value for false.
Checking condition by test
# cat check_gt10.sh
if test $1 -gt 10
then
echo "$1 is greater than 10"
fi

# chmod +x check_gt10.sh

# ./check_gt10.sh 19
19 is greater than 10

# ./check_gt10.sh 5

Checking condition by [expression]
# vi check_gt10_2.sh
if [ $1 -gt 10 ]
then
echo "$1 is greater than 10"
fi

# chmod +x check_gt10_2.sh

# ./check_gt10_2.sh 10

# ./check_gt10_2.sh 11
11 is greater than 10

With test or [expression] you can check condition of
-integer numbers.
-file types.
-character strings

Integer Number Comparison:

Usage in shell script	Meaning	Mathematical Statements	Inside Shell Script
			Check by test statement with if condition	Check by [ expr ] statement with if condition
-eq	is equal to	3 == 4	if test 3 -eq 4	if [ 3 -eq 4 ]
-ne	is not equal to	3 != 4	if test 3 -ne 4	if [ 3 -ne 4 ]
-lt	is less than	3 <>	if test 3 -lt 4	if [ 3 -lt 4 ]
-le	is less than or equal to	3 <= 4	if test 3 -le 4	if [ 3 -le 4 ]
-gt	is greater than	3 > 4	if test 3 -gt 4	if [ 3 -gt 4 ]
-ge	is greater than or equal to	3 >= 4	if test 3 -ge 4	if [ 3 -ge 4 ]

Strings Comparison:

Operator	Meaning of the using
string1 = string2	string1 is equal to string2
string1 != string2	string1 is NOT equal to string2
string1	string1 is NOT NULL or not defined
-n string1	string1 is NOT NULL and does exist
-z string1	string1 is NULL and does exist

Files Comparison:

Test	Meaning of the text
-s file	Non empty file
-f file	Is File exist or normal file and not a directory
-d dir	Is Directory exist and not a file
-w file	Is writeable file
-r file	Is read-only file
-x file	Is file is executable

Comparison with logical operators:

Operator	Meaning of using
! expression	Logical NOT
expression1 -a expression2	Logical AND
expression1 -o expression2	Logical OR

Related Documents

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http://arjudba.blogspot.com/2009/04/what-is-sharp-bang-appear-at-first.html

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Control Characters in Linux and shell script

List of comparison made by test operator in shell script

http://arjudba.blogspot.com/2009/10/list-of-operators-in-linux-shell-script.html