Lecture07.pptx - LECTURE 7: INFO 1211 – OPERATING SYSTEM’S...

LECTURE 7:
INFO 1211 – OPERATING SYSTEM’S PRINCIPLES AND APPLICATIONS
DAVID WANG
COMPUTING SCIENCE AND INFORMATION TECHNOLOGY

OUTLINE
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Shell Programming – Control Constructs
•
Shell Programming – loops
•
Shell Programming – Shell functions etc.

CONTROL CONSTRUCTS
•
The Bourne Shell control constructs can alter the flow of
control within the script
•
Flow control
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script logic branches based on conditional testing or
pattern matching
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if, then, else statements
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case statements
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Iteration
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execute repeatedly a series of commands
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for loop, until loop, while loop

CONTROL CONSTRUCTS
boolean-
expression
statements
false
true
boolean-
expressions
statements
true
false

TESTING CONDITIONS
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Test is a simple but powerful comparison utility
•
It is used by virtually every shell script written
•
The test utility
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evaluates expressions and returns a boolean value
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true (equal to zero) or false (not equal to zero)
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Format : test expression
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Do not create a program called test and try to run it!
•
as this shell builtin will be called instead of your program!
constants, variables, and operators

TESTING CONDITIONS
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Test is not often called directly
•
more frequently called as [
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Format : [ condition ]
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A space character must appear after the left bracket and
before the right bracket
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[ is a symbolic link to test
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$ type [
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[ is a shell builtin
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$ which [
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/usr/bin/[
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$ ls -l /usr/bin/[
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lrwxr-xr-x 1 root root 4 Apr 28 13:41 /usr/bin/[ -> test

TESTING FILE STATUS
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The test utility can be used to test file types information
•
-d filename
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true if the filename exists and is a directory.
• -f filename
•
true if filename exists and is regular file.
•
-L filename
• true if filename exist and is a symbolic link.
•
-r filename
•
true if filename is readable
•
-w filename
•
true if filename is writable
•
-x filename
•
true if filename is executable

TESTING FILE STATUS
(CONT.)
•
File characteristics tests options
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-e FILE
•
True if FILE exists.
•
-s FILE
•
True if FILE exists and has a size greater than zero.
•
FILE1 -nt FILE2
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True if FILE1 is newer (according to modification date) than
FILE2.
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FILE1 -ot FILE2
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True if FILE1 is older (according to modification date) than
FILE2.
•
FILE1 -ef FILE2
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True if FILE1 and FILE2 have the same device and inode
numbers, i.e., if they are hard links to each other.

USING LOGICAL OPERATORS
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Compare one file's status against another or itself
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-a Logical AND
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-o Logical OR
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! Logical NOT
•
Example
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$ [ -w foo1 -a -w foo2 ]
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$ [ -w foo1 -o -x foo1 ]

TESTING STRINGS
•
The general form of evaluating criterion for strings is:
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string1 op string2
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op string
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string
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op can be
•
= The two strings are equal
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!= The two strings are not equal
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-z The length of string is zero
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-n The length of string is nonzero
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Example
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$ [ -z $EDITOR ]
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$ [ $EDITOR = "vi" ]
Is this env variable unset?