Unix Shell: test command

1. Basic Usage

 #! /bin/bash  
 var_1=10  
 var_2=20  

 #First expression about "test" command  
 if test $var_1 -le $var_2  
 then  
   echo "var_1 is less than var_2"  
 else  
   echo "var_2 is less than var_1"  
 fi  

 #Second expression about "test" command, after the bracket,  
 #we must leave one space. Please keep in mind, [] is another   
 #expression of test command, all variables inside are taken as  
 #the argument of test command  
 if [ $var_1 -le $var_2 ]  
 then   
   echo "var_1 is less than var_2"  
 else  
   echo "var_2 is less than var_1"  
 fi  

2. test command expression

 #! /bin/bash  
 # -d decides whether the input is one directory  
 if [ -d "text" ]  
 then  
   echo "text is directory"  
 else  
   echo "text is not directory"  
 fi  
 #output: text is not directory  

 # -n decides whether the input is empty string  
 if [ -n "" ]  
 then  
   echo "this is not empty string"  
 else  
   echo "this is empty string"  
 fi  
 #output: this is empty string  

 # -z decides if input string is empty string or not  
 # this is "reversed way" of -n  
 if [ -z "" ]  
 then   
   echo "this is empty string"  
 else  
   echo "this is not empty string"  
 fi  
 # output: this is empty string  

 # -r decides if the input file is readable  
 if [ -r "text" ]  
 then  
   echo "text is readable"  
 else  
   echo "text is not readable"  
 fi  
 #text has read permission  
 #output: text is readable  

 # -s decides if input file is empty or not  
 if [ -s "text" ]  
 then  
   echo "text is not empty"  
 else  
   echo "text is empty"  
 fi  
 #text file is not empty  
 #output: text is not empty  

 # -w decides if input file is writable or not  
 if [ -w "text" ]  
 then   
   echo "text is writable"  
 else  
   echo "text is not writable"  
 fi  
 #text file is writable in system  
 #output: text is writable   

 # -x decides if input file is executable or not  
 if [ -x "text" ]  
 then   
   echo "text is executable"  
 else  
   echo "text is not executable"  
 fi  
 #text file is not executable in system  
 #output: text is not executable  

 # '=' could only be used to compare string, no way to   
 # compare number, following operation is just comparing  
 # "30+60" and "90", both are taken as string  
 if [ 30+60 = 90 ]  
 then   
   echo "equal string"  
 else  
   echo "unequal string"  
 fi  
 #output: unequal string  

 # '!=' could only be used compare string, same as above  
 if [ 30+60 != 90 ]  
 then  
   echo "unequal string"  
 else  
   echo "equal string"  
 fi  
 #output: unequal string  

 # -eq is used to compare numbers only, we use arithmatic expression below  
 # to compute "30+60"  
 if [ $((30+60)) -eq 90 ]  
 then  
   echo "equal number"  
 else  
   echo "unequal number"  
 fi  
 #output: equal number  

 # -ne is used to compare number only, decide whether 2 numbers are unequal  
 if [ $((30+60)) -ne 90 ]  
 then  
   echo "unequal number"  
 else  
   echo "equal number"  
 fi  
 #output: equal number  

 # -lt is used to compare numbers only, decide whether first number is less  
 # than 2nd number  
 if [ 50 -lt 90 ]  
 then   
   echo "50 is less than 90"  
 else  
   echo "50 is larger than 90"  
 fi  
 #output: 50 is less than 90  

 # -gt is used to compare numbers only, decide whether first number is larger  
 # than 2nd number  
 if [ 90 -gt 50 ]  
 then  
   echo "90 is greater than 50"  
 else  
   echo "90 is less than 50"  
 fi  
 #output: 90 is greater than 50  

 # -le is used to compare numbers only, decide whether first number is less  
 # than or equal to 2nd number  
 if [ 50 -le 50 ]  
 then  
   echo "50 is less than or equal to 50"  
 else  
   echo "50 is larger than 50"  
 fi  
 #output: 50 is less than or equal to 50  

 # -ge is used to compare numbers only, decide whether first number is greater  
 # than or equal to 2nd number  
 if [ 50 -ge 50 ]  
 then  
   echo "50 is greater than or equal to 50"  
 else  
   echo "50 is less than 50"  
 fi  
 #output: 50 is greater than or equal to 50  

3. Require arguements
script:

 #! /bin/bash  

 file=""  

 #normally we use double quote to circle the $file, if $file is empty  
 #sometimes in some system, it will cause error, since "test" command need arguments!   
 #With double quotes, even if $file is empty, we just take one empty string as arguement.  
 if [ -r "$file" ]  
 then  
   echo "file is readable"  
 else  
   echo "file is not readable"  
 fi  

4. String Comparisons
script:

 #! /bin/bash  

 var=""  

 #Normally we add one characterer before the string used to be compared  
 #Since if one of string is empty, in some system, it may make "test"  
 #command confuse  
 if [ "W" = "W$var" ]  
 then  
   echo "two strings are equal"  
 else  
   echo "two strings are not equal"  
 fi  

5. Numeric test is only for integer
script_1

 #! /bin/bash  

 var_1=4.5  
 var_2=4  

 if [ $var_1 -ge $var_2 ]  
 then  
   echo "var_1 is larger"  
 else  
   echo "var_2 is larger"  
 fi  

terminal:

 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_1  
 ./script_1: line 6: [: 4.5: integer expression expected  
 var_2 is larger