Red Hat Linux 7 Unleashed

By William Ball

• Table of Contents
  • Copyright
  • About the Lead Authors
  • About the Contributing Authors
  • Acknowledgments
  • Tell Us What You Think!
  • Introduction
  • I. Red Hat Linux Installation and User Services
  • Chapter 1. Introduction to Red Hat Linux
  • Chapter 2. Installation of Your Red Hat System
  • Chapter 3. LILO and Other Boot Managers
  • Chapter 4. Configuring the X Window System, Version 11
  • Chapter 5. Window Managers
  • Chapter 6. Connecting to the Internet
  • Chapter 7. IRC, ICQ, and Chat Clients
  • Chapter 8. Using Multimedia and Graphics Clients
  • II. Configuring Services
  • Chapter 9. System Startup and Shutdown
  • Chapter 10. SMTP and Protocols
  • Chapter 11. FTP
  • Chapter 12. Apache Server
  • Chapter 13. Internet News
  • Chapter 14. Domain Name Service and Dynamic Host Configuration Protocol
  • Chapter 15. NIS: Network Information Service
  • Chapter 16. NFS: Network Filesystem
  • Chapter 17. Samba
  • III. System Administration and Management
  • Chapter 18. Linux Filesystems, Disks, and Other Devices
  • Chapter 19. Printing with Linux
  • Chapter 20. TCP/IP Network Management
  • Chapter 21. Linux System Administration
  • Chapter 22. Backup and Restore
  • Chapter 23. System Security
  • IV. Red Hat Development and Productivity
  • Chapter 24. Linux C/C++ Programming Tools
  • Chapter 25. Shell Scripting
    • Creating and Executing a Shell Program
    • Variables
    • Positional Parameters
    • Built-In Variables
    • Special Characters
    • Comparison of Expressions
    • Iteration Statements
    • Conditional Statements
    • Miscellaneous Statements
    • Functions
    • Summary
  • Chapter 26. Automating Tasks
  • Chapter 27. Configuring and Building Kernels
  • Chapter 28. Emulators, Tools, and Window Clients
  • V. Appendixes
  • A. The Linux Documentation Project
  • B. Top Linux Commands and Utilities
  • C. The GNU General Public License
  • D. Red Hat Linux RPM Package Listings

Comparison of Expressions

The way the logical comparison of two operators (numeric or string) is done varies slightly in different shells. In pdksh and bash, a command called test can be used to achieve comparisons of expressions. In tcsh, you can write an expression to accomplish the same thing.

pdksh and bash

This section covers comparisons using the pdksh or bash shells. Later in the chapter, the section tcsh contains a similar discussion for the tcsh shell.

The syntax of the test command is as follows:

test expression

or

[ expression ]

Both forms of test commands are processed the same way by pdksh and bash. The test commands support the following types of comparisons:

• String comparison
• Numeric comparison
• File operators
• Logical operators

String Comparison

The following operators can be used to compare two string expressions:

Next are some examples comparing two strings, string1 and string2, in a shell program called compare1:

#!/bin/sh
string1="abc"
string2="abd"
if [ $string1 = $string2 ]; then
   echo "string1 equal to string2"
else
   echo "string1 not equal to string2"
fi

if [ $string2 != string1 ]; then
   echo "string2 not equal to string1"
else
   echo "string2 equal to string2"
fi

if [ $string1 ]; then
   echo "string1 is not empty"
else
   echo "string1 is empty"
fi

if [ -n $string2 ]; then
   echo "string2 has a length greater than zero"
else
   echo "string2 has length equal to zero"
fi

if [ -z $string1 ]; then
   echo "string1 has a length equal to zero"
else
  echo "string1 has a length greater than zero"
fi

If you execute compare1, you get the following result:

string1 not equal to string2
string2 not equal to string1
string1 is not empty
string2 has a length greater than zero
string1 has a length greater than zero

If two strings are not equal in size, the system pads out the shorter string with trailing spaces for comparison. That is, if the value of string1 is abc and that of string2 is ab, string2 will be padded with a trailing space for comparison purposes--it will have a value of ab .

Number Comparison

The following operators can be used to compare two numbers:

The following examples compare two numbers, number1 and number2, in a shell program called compare2:

#!/bin/sh
number1=5
number2=10
number3=5

if [ $number1 -eq $number3 ]; then
   echo "number1 is equal to number3"
else
   echo "number1 is not equal to number3"
fi

if [ $number1 -ne $number2 ]; then
   echo "number1 is not equal to number2"
else
   echo "number1 is equal to number2"
fi

if [ $number1 -gt $number2 ]; then
   echo "number1 is greater than number2"
else
   echo "number1 is not greater than number2"
fi

if [ $number1 -ge $number3 ]; then
   echo "number1 is greater than or equal to number3"
else
   echo "number1 is not greater than or equal to number3"
fi

if [ $number1 -lt $number2 ]; then
   echo "number1 is less than number2"
else
   echo "number1 is not less than number2"
fi

if [ $number1 -le $number3 ]; then
   echo "number1 is less than or equal to number3"
else
   echo "number1 is not less than or equal to number3"
fi

When you execute the shell program compare2, you get the following results:

number1 is equal to number3
number1 is not equal to number2
number1 is not greater than number2
number1 is greater than or equal to number3
number1 is less than number2
number1 is less than or equal to number3

File Operators

The following operators can be used as file comparison operators:

Assume that a shell program called compare3 is in a directory with a file called file1 and a subdirectory dir1 under the current directory. Assume file1 has a permission of r-x (read and execute permission) and dir1 has a permission of rwx (read, write, and execute permission). The code for compare3 would look like this:

#!/bin/sh
if [ -d $dir1 ]; then
   echo "dir1 is a directory"
else
   echo "dir1 is not a directory"
fi

if [ -f $dir1 ]; then
   echo "file1 is a regular file"
else
   echo "file1 is not a regular file"
fi

if [ -r $file1 ]; then
   echo "file1 has read permission"
else
   echo "file1 does not have read permission"
fi

if [ -w $file1 ]; then
   echo "file1 has write permission"
else
   echo "file1 does not have write permission"
fi

if [ -x $dir1 ]; then
   echo "dir1 has execute permission"
else
   echo "dir1 does not have execute permission"
fi

If you execute the file compare3, you get the following results:

dir1 is a directory
file1 is a regular file
file1 has read permission
file1 does not have write permission
dir1 has execute permission

Logical Operators

Logical operators are used to compare expressions using the rules of logic. The characters represent NOT, AND, and OR.

This example named logic uses the file and directory mentioned in the previous compare3 example.

#!/bin/sh
if [ -x file1 -a -x dir1 ]; then
   echo file1 and dir1 are executable
else
   echo at least one of file1 or dir1 are not executable
fi

if [ -w file1 -o -w dir1 ]; then
   echo file1 or dir1 are writable
else
   echo neither file1 or dir1 are executable
fi

if [ ! -w file1 ]; then
   echo file1 is not writable
else
   echo file1 is writable
fi

If you execute logic, it will yield the following result:

file1 and dir1 are executable
file1 or dir1 are writable
file1 is not writable

tcsh

As stated earlier, the comparisons are different under tcsh from what they are under pdksh and bash. This section explains the same concepts as the section "pdksh and bash," but it uses the syntax necessary for the tcsh shell environment.

String Comparison

The following operators can be used to compare two string expressions:

The following examples compare two strings, string1 and string2, in the shell program compare1:

#!/bin/tcsh
set string1 = "abc"
set string2 = "abd"

if  (string1 == string2)  then
   echo "string1 equal to string2"
else
   echo "string1 not equal to string2"
endif

if  (string2 != string1)  then
   echo "string2 not equal to string1"
else
   echo "string2 equal to string1"
endif

If you execute compare1, you get the following results:

string1 not equal to string2
string2 not equal to string1

Number Comparison

These operators can be used to compare two numbers:

The next examples compare two numbers, number1 and number2, in a shell program called compare2:

#!/bin/tcsh
set number1 = 5
set number2 = 10
set number3 = 5

if  ($number1 > $number2)  then
   echo "number1 is greater than number2"
else
   echo "number1 is not greater than number2"
endif

if  ($number1 >= $number3) then
   echo "number1 is greater than or equal to number3"
else
   echo "number1 is not greater than or equal to number3"
endif

if  ($number1 < $number2)  then
   echo "number1 is less than number2"
else
   echo "number1 is not less than number2"
endif

if  ($number1 <= $number3) then
   echo "number1 is less than or equal to number3"
else
   echo "number1 is not less than or equal to number3"
endif

Executing the shell program compare2, you get the following results:

number1 is not greater than number2
number1 is greater than or equal to number3
number1 is less than number2
number1 is less than or equal to number3

File Operators

These operators can be used as file comparison operators:

The following examples are based on a shell program called compare3, which is in a directory with a file called file1 and a subdirectory dir1 under the current directory. Assume that file1 has a permission of r-x (read and execute permission) and dir1 has a permission of rwx (read, write, and execute permission).

The following is the code for the compare3 shell program:

#!/bin/tcsh
if  (-d dir1) then
   echo "dir1 is a directory"
else
   echo "dir1 is not a directory"
endif

if (-f dir1)  then
   echo "file1 is a regular file"
else
   echo "file1 is not a regular file"
endif

if (-r file1) then
   echo "file1 has read permission"
else
   echo "file1 does not have read permission"
endif

if (-w file1) then
   echo "file1 has write permission"
else
   echo "file1 does not have write permission"
endif

if (-x dir1) then
   echo "dir1 has execute permission"
else
   echo "dir1 does not have execute permission"
endif

if (-z file1) then
   echo "file1 has zero length"
else
   echo "file1 has greater than zero length"
endif

If you execute the file compare3, you get the following results:

dir1 is a directory
file1 is a regular file
file1 has read permission
file1 does not have write permission
dir1 has execute permission
file1 has greater than zero length

Logical Operators

Logical operators are used with conditional statements. These operators are used to negate a logical expression or to perform logical ANDs and ORs.

This example named logic uses the file and directory mentioned in the previous compare3 example.

#!/bin/tcsh
if ( -x file1 && -x dir1 ) then
   echo file1 and dir1 are executable
else
   echo at least one of file1 or dir1 are not executable
endif

if ( -w file1 || -w dir1 ) then
   echo file1 or dir1 are writable
else
   echo neither file1 or dir1 are executable
endif

if ( ! -w file1 ) then
   echo file1 is not writable
else
   echo file1 is writable
endif

If you execute logic, it will yield the following result:

file1 and dir1 are executable
file1 or dir1 are writable
file1 is not writable

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