10.6 The test Command

   

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Branching decisions are made depending on the result of a test command. The test command can perform tests on numeric and string data as well as on files. The test command returns a true or false value. The true value is always zero, while false is a number other than zero. Usually this number is one. You can check the result code of the test command to make a branching decision. The test command can be used in explicit or implicit modes. In the explicit mode, the test command is used as follows .

 $  test "ABC" = "abc"  

In the implicit mode, the word " test " is not there; square brackets are used instead.

 $  [ "ABC" = "abc" ]  

The command does not print anything on the terminal screen. The result code can be checked using the $? variable as explained earlier in the chapter.

Testing Numeric Values

Tests can be performed to compare two or more integers. The relations that can be used with numeric data are shown in Table 10-3.

Table 10-3. Numeric Tests

Relation	Description
-eq	Equality check
-ne	Not equal
-lt	Less than
-gt	Greater than
-le	Less than or equal to
-ge	Greater than or equal to

Numeric testing will be used in shell programs later in this chapter.

Testing String Values

The string values can be checked for equality and nonequality. Other than that, a single string can be tested if it has a zero length or not. The string operations are shown in Table 10-4.

Table 10-4. String Tests

Operation	Description
string1 = string2	True if string1 and string2 are equal
string1 != string2	True if string1 is not equal to string2
-z string	True if string length is zero
-n string	True if string length is nonzero
string	True if string length is nonzero

Testing Files

Testing on files can be performed in many ways. Some of these are shown in Table 10-5. A list of other supported file tests can be found using the man sh-posix command.

Table 10-5. File Tests

Operation	Description
-d file	True if the file is a directory
-f file	True if the file exists and is a normal file (not a directory)
-s file	True if the file is more than zero bytes in length
-r file	True if the file is readable
-w file	True if the file is writable
-e file	True if the file exists
-L file	True if the file is a symbolic link
file1 -nt file2	True if file1 is newer than file2
file1 -ot file2	True if file1 is older than file2
-x file	True if the file is executable

For example, if file file1 exists, and you use the following command to check its existence, you will get an exit code of zero (true).

 $  [ -f file1 ]  $ $  echo $?  0 $ 

Testing with Logical Operators

Logical operations can be performed on two expressions with one of the logical operators shown in Table 10-6.

Table 10-6. Logical Operators

Operation	Description
expr1 -o expr2	Logical OR, true if either expr1 or expr2 is true
expr1 -a expr2	Logical AND, true if both expr1 and expr2 are true
! expr	Logical NOT, true if expr is false

The following code segment tests files represented by the first two command line arguments and prints a message if both files exist.

 #!/usr/bin/sh if [ -f  -a -f  ] then    echo "Test successful" fi 

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