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 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
   otherwise commands

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
  echo File myfile.xml does not exist

XML Expressions can also be used as conditons HowToXMLCondition


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