Mac OS X Leopard - ARCAism

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CHAPTER 19 EXTENDING THE POWER OF DARWIN
One thing to keep in mind is that once the condition is met, the script will execute that block
and then exit the if block entirely, so if condition1 is met, the script will not check for condition2
or run the code in that block.
There are a number of ways to create conditional statements; one of the most common is to
use a mathematical test condition as listed in Table 19-1 (the Bash alternate expressions will
work only with Bash and not a traditional Bourne shell).
Table 19-1. Mathematic Test Expressions for sh and Bash
Expression (sh) Bash Alternate Result
$x -eq $y $x == $y True if $x is equal to $y
$x -ne $y $x != $y True if $x is not equal to $y
$x -lt $y $x < $y True if $x is less than $y
$x -gt $y $x > $y True if $x is greater than $y
$x -le $y $x = $y True if $x is greater than or equal to $y
Another common conditional statement is to check on the existence of a file or directory.
This is especially handy if you are creating workflow or other scripts that interact with the file
system. An if statement that checks for the existence of a particular file would look like this:
if [ -e /path/to/file ]
then
If file exists do this
else
If file doesn't exist do this
fi
Here the -e option checks for the existence of the file. Table 19-2 lists some possible options.
Table 19-2. Common Options for Testing File Attributes in Shell Scripts
Option Result
-e file True if the file exists at all
-f file True if file exists and it is a regular file
-d file True if file exists and it is a directory
-r file True if file exists and it is readable
-w file True if file exists and it is writeable
-x file True if file exists and it is executable
It is also possible to test multiple conditions using logical and/or statements. This allows you
to check either whether multiple statements are all true or whether one of many statements is
true. This is done using either && or || (that’s the long bar over the \ key) between the conditions.
[ condition1 ] && [ condition2 ] will return true if both conditions are true, while [ condition1 ]
|| [ condition2 ] will return true if either condition is true.