====How To test a condition====

Conditions can be tested with either the boolean operations ""||"" and "&&" or with 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 ; }
%% 

See [[CommandTest test]] for information on the test ("[") command

====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
%%

XML Expressions can also be used as conditons [[HowToXMLCondition]]

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[[CommandTest]]
[[HowTo]]
[[CategoryHowTo]]