Revision [543]
This is an old revision of HowToConditional made by DavidLee on 2009-05-21 19:09:51.
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 OperationsIf 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 |
