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This is an old revision of HowToConditional made by DavidLee on 2009-05-21 19:04:57.

 

How To test a condition


Conditions can be tested with either the boolean operations "
" and "&&" or with the the if/elif/else/then clauses.
The conditional test the return status of commands. If the command returns 0 then that is considered "true", otherwise the condition is "false". A common command used for conditionals is the "test" command, usually using the "[ test ]" syntax.


Boolean Operations


If you only need execute a single command or a small block then boolean operations are the easiest way

for example

if "command" returns successfully then print "is success"

$ command && echo is success


If the file "myfile.txt" exists then read the first line from the file and print it.

$ [ -e myfile.txt ] && { read VAR < myfile.txt ;  echo $VAR ; }



Structured Conditionals (if / elif / else / fi )


If the condition or conditional operations are more complicated then the structured conditionals are more useful.
This is the if / elif / else / fi construct.

The general form is
if  condition1 ; then 
   condition1 commands
elif  condition2 ; then
   condition2 commands
else 
   otherwise commands
fi


The "elif" and "else" parts are optional.

For example, if the file "myfile.xml" exists then read it into a variable otherwise print an error message

if [ -f myfile.xml ] ; then 
   xread var < myfile.xml
else
  echo File myfile.xml does not exist
fi







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