test: vyhodnotit výraz

Originální popis anglicky: test - evaluate expression

Návod, kniha: POSIX Programmer's Manual

STRUČNĚ

test [expression]

[ [expression] ]

The test utility shall evaluate the expression and indicate the result of the evaluation by its exit status. An exit status of zero indicates that the expression evaluated as true and an exit status of 1 indicates that the expression evaluated as false.

In the second form of the utility, which uses "[]" rather than test, the application shall ensure that the square brackets are separate arguments.

OPTIONS

The test utility shall not recognize the "--" argument in the manner specified by guideline 10 in the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.

No options shall be supported.

OPERANDS

The application shall ensure that all operators and elements of primaries are presented as separate arguments to the test utility.

The following primaries can be used to construct expression:

-b file: True if file exists and is a block special file.

-c file: True if file exists and is a character special file.

-d file: True if file exists and is a directory.

-e file: True if file exists.

-f file: True if file exists and is a regular file.

-g file: True if file exists and its set-group-ID flag is set.

-h file: True if file exists and is a symbolic link.

-L file: True if file exists and is a symbolic link.

-n string: True if the length of string is non-zero.

-p file: True if file is a FIFO.

-r file: True if file exists and is readable. True shall indicate that permission to read from file will be granted, as defined in File Read, Write, and Creation .

-S file: True if file exists and is a socket.

-s file: True if file exists and has a size greater than zero.

-t file_descriptor: True if the file whose file descriptor number is file_descriptor is open and is associated with a terminal.

-u file: True if file exists and its set-user-ID flag is set.

-w file: True if file exists and is writable. True shall indicate that permission to write from file will be granted, as defined in File Read, Write, and Creation .

-x file: True if file exists and is executable. True shall indicate that permission to execute file will be granted, as defined in File Read, Write, and Creation . If file is a directory, true shall indicate that permission to search file will be granted.

-z string: True if the length of string string is zero.

string: True if the string string is not the null string.

s1 = s2: True if the strings s1 and s2 are identical.

s1 != s2: True if the strings s1 and s2 are not identical.

n1 -eq n2: True if the integers n1 and n2 are algebraically equal.

n1 -ne n2: True if the integers n1 and n2 are not algebraically equal.

n1 -gt n2: True if the integer n1 is algebraically greater than the integer n2.

n1 -ge n2: True if the integer n1 is algebraically greater than or equal to the integer n2.

n1 -lt n2: True if the integer n1 is algebraically less than the integer n2.

n1 -le n2: True if the integer n1 is algebraically less than or equal to the integer n2.

expression1 -a expression2: True if both expression1 and expression2 are true. The -a binary primary is left associative. It has a higher precedence than -o.

expression1 -o expression2: True if either expression1 or expression2 is true. The -o binary primary is left associative.

With the exception of the -h file and -L file primaries, if a file argument is a symbolic link, test shall evaluate the expression by resolving the symbolic link and using the file referenced by the link.

These primaries can be combined with the following operators:

! expression: True if expression is false.

( expression ): True if expression is true. The parentheses can be used to alter the normal precedence and associativity.

The primaries with two elements of the form:


-primary_operator primary_operand

are known as unary primaries. The primaries with three elements in either of the two forms:


primary_operand -primary_operator primary_operand
 

primary_operand primary_operator primary_operand

are known as binary primaries. Additional implementation-defined operators and primary_operators may be provided by implementations. They shall be of the form - operator where the first character of operator is not a digit.

The algorithm for determining the precedence of the operators and the return value that shall be generated is based on the number of arguments presented to test. (However, when using the "[...]" form, the right-bracket final argument shall not be counted in this algorithm.)

In the following list, $1, $2, $3, and $4 represent the arguments presented to test:

0 arguments:: Exit false (1).

1 argument:: Exit true (0) if $1 is not null; otherwise, exit false.

2 arguments:

*: If $1 is '!' , exit true if $2 is null, false if $2 is not null.

*: If $1 is a unary primary, exit true if the unary test is true, false if the unary test is false.

*: Otherwise, produce unspecified results.

3 arguments:

*: If $2 is a binary primary, perform the binary test of $1 and $3.

*: If $1 is '!' , negate the two-argument test of $2 and $3.

*: If $1 is '(' and $3 is ')' , perform the unary test of $2.

*: Otherwise, produce unspecified results.

4 arguments:

*: If $1 is '!' , negate the three-argument test of $2, $3, and $4.

*: If $1 is '(' and $4 is ')' , perform the two-argument test of $2 and $3.

*: Otherwise, the results are unspecified.

>4 arguments:: The results are unspecified.

On XSI-conformant systems, combinations of primaries and operators shall be evaluated using the precedence and associativity rules described previously. In addition, the string comparison binary primaries '=' and "!=" shall have a higher precedence than any unary primary.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of test:

LANG: Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)

LC_ALL: If set to a non-empty string value, override the values of all the other internationalization variables.

LC_CTYPE: Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments).

LC_MESSAGES: Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.

NLSPATH: Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS

Default.

STDOUT

Not used.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

None.

EXIT STATUS

The following exit values shall be returned:

0: expression evaluated to true.

1: expression evaluated to false or expression was missing.

>1: An error occurred.

CONSEQUENCES OF ERRORS

Default.

The following sections are informative.

APPLICATION USAGE

Scripts should be careful when dealing with user-supplied input that could be confused with primaries and operators. Unless the application writer knows all the cases that produce input to the script, invocations like:


test "$1" -a "$2"

should be written as:


test "$1" && test "$2"

to avoid problems if a user supplied values such as $1 set to '!' and $2 set to the null string. That is, in cases where maximal portability is of concern, replace:


test expr1 -a expr2

with:


test expr1 && test expr2

and replace:


test expr1 -o expr2

with:


test expr1 || test expr2

but note that, in test, -a has higher precedence than -o while "&&" and "||" have equal precedence in the shell.

Parentheses or braces can be used in the shell command language to effect grouping.

Parentheses must be escaped when using sh; for example:


test \( expr1 -a expr2 \) -o expr3

This command is not always portable outside XSI-conformant systems. The following form can be used instead:


( test expr1 && test expr2 ) || test expr3

The two commands:


test "$1"
test ! "$1"

could not be used reliably on some historical systems. Unexpected results would occur if such a string expression were used and $1 expanded to '!' , '(' , or a known unary primary. Better constructs are:


test -n "$1"
test -z "$1"

respectively.

Historical systems have also been unreliable given the common construct:


test "$response" = "expected string"

One of the following is a more reliable form:


test "X$response" = "Xexpected string"
test "expected string" = "$response"

Note that the second form assumes that expected string could not be confused with any unary primary. If expected string starts with '-' , '(' , '!' , or even '=' , the first form should be used instead. Using the preceding rules without the XSI marked extensions, any of the three comparison forms is reliable, given any input. (However, note that the strings are quoted in all cases.)

Because the string comparison binary primaries, '=' and "!=" , have a higher precedence than any unary primary in the greater than 4 argument case, unexpected results can occur if arguments are not properly prepared. For example, in:


test -d $1 -o -d $2

If $1 evaluates to a possible directory name of '=' , the first three arguments are considered a string comparison, which shall cause a syntax error when the second -d is encountered. One of the following forms prevents this; the second is preferred:


test \( -d "$1" \) -o \( -d "$2" \)
test -d "$1" || test -d "$2"

Also in the greater than 4 argument case:


test "$1" = "bat" -a "$2" = "ball"

syntax errors occur if $1 evaluates to '(' or '!' . One of the following forms prevents this; the third is preferred:


test "X$1" = "Xbat" -a "X$2" = "Xball"
test "$1" = "bat" && test "$2" = "ball"
test "X$1" = "Xbat" && test "X$2" = "Xball"

PŘÍKLADY POUŽITÍ

1.: Exit if there are not two or three arguments (two variations):


if [ $# -ne 2 -a $# -ne 3 ]; then exit 1; fi
if [ $# -lt 2 -o $# -gt 3 ]; then exit 1; fi

2.: Perform a mkdir if a directory does not exist:


test ! -d tempdir && mkdir tempdir

3.: Wait for a file to become non-readable:


while test -r thefile
do
    sleep 30
done
echo '"thefile" is no longer readable'

4.: Perform a command if the argument is one of three strings (two variations):


if [ "$1" = "pear" ] || [ "$1" = "grape" ] || [ "$1" = "apple" ]
then
    command
fi
 

case "$1" in
    pear|grape|apple)  command ;;
esac

RATIONALE

The KornShell-derived conditional command (double bracket [[]]) was removed from the shell command language description in an early proposal. Objections were raised that the real problem is misuse of the test command ( [), and putting it into the shell is the wrong way to fix the problem. Instead, proper documentation and a new shell reserved word ( !) are sufficient.

Tests that require multiple test operations can be done at the shell level using individual invocations of the test command and shell logicals, rather than using the error-prone -o flag of test.

XSI-conformant systems support more than four arguments.

XSI-conformant systems support the combining of primaries with the following constructs:

expression1 -a expression2: True if both expression1 and expression2 are true.

expression1 -o expression2: True if at least one of expression1 and expression2 are true.

( expression ): True if expression is true.

In evaluating these more complex combined expressions, the following precedence rules are used:

*: The unary primaries have higher precedence than the algebraic binary primaries.

*: The unary primaries have lower precedence than the string binary primaries.

*: The unary and binary primaries have higher precedence than the unary string primary.

*: The ! operator has higher precedence than the -a operator, and the -a operator has higher precedence than the -o operator.

*: The -a and -o operators are left associative.

*: The parentheses can be used to alter the normal precedence and associativity.

The BSD and System V versions of -f are not the same. The BSD definition was:

-f file: True if file exists and is not a directory.

The SVID version (true if the file exists and is a regular file) was chosen for this volume of IEEE Std 1003.1-2001 because its use is consistent with the -b, -c, -d, and -p operands ( file exists and is a specific file type).

The -e primary, possessing similar functionality to that provided by the C shell, was added because it provides the only way for a shell script to find out if a file exists without trying to open the file. Since implementations are allowed to add additional file types, a portable script cannot use:


test -b foo -o -c foo -o -d foo -o -f foo -o -p foo

to find out if foo is an existing file. On historical BSD systems, the existence of a file could be determined by:


test -f foo -o -d foo

but there was no easy way to determine that an existing file was a regular file. An early proposal used the KornShell -a primary (with the same meaning), but this was changed to -e because there were concerns about the high probability of humans confusing the -a primary with the -a binary operator.

The following options were not included in this volume of IEEE Std 1003.1-2001, although they are provided by some implementations. These operands should not be used by new implementations for other purposes:

-k file: True if file exists and its sticky bit is set.

-C file: True if file is a contiguous file.

-V file: True if file is a version file.

The following option was not included because it was undocumented in most implementations, has been removed from some implementations (including System V), and the functionality is provided by the shell (see Parameter Expansion .

-l string: The length of the string string.

The -b, -c, -g, -p, -u, and -x operands are derived from the SVID; historical BSD does not provide them. The -k operand is derived from System V; historical BSD does not provide it.

On historical BSD systems, test -w directory always returned false because test tried to open the directory for writing, which always fails.

Some additional primaries newly invented or from the KornShell appeared in an early proposal as part of the conditional command ( [[]]): s1 > s2, s1 < s2, str = pattern, str != pattern, f1 -nt f2, f1 -ot f2, and f1 -ef f2. They were not carried forward into the test utility when the conditional command was removed from the shell because they have not been included in the test utility built into historical implementations of the sh utility.

The -t file_descriptor primary is shown with a mandatory argument because the grammar is ambiguous if it can be omitted. Historical implementations have allowed it to be omitted, providing a default of 1.

FUTURE DIRECTIONS

None.

SOUVISEJÍCÍ

File Read, Write, and Creation , find

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

2003	IEEE/The Open Group