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   test.1p    ( 1 )

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Использование в приложениях (Application usage)

The XSI extensions specifying the -a and -o binary primaries and the '(' and ')' operators have been marked obsolescent. (Many expressions using them are ambiguously defined by the grammar depending on the specific expressions being evaluated.) Scripts using these expressions should be converted to the forms given below. Even though many implementations will continue to support these obsolescent forms, scripts should be extremely careful when dealing with user-supplied input that could be confused with these and other primaries and operators. Unless the application developer 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 even on XSI-conformant systems depending on the expressions specified by expr1, expr2, and expr3. 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"

Note that none of the following examples are permitted by the syntax described:

[-f file] [-f file ] [ -f file] [ -f file test -f file ]

In the first two cases, if a utility named [‐f exists, that utility would be invoked, and not test. In the remaining cases, the brackets are mismatched, and the behavior is unspecified. However:

test ! ]

does have a defined meaning, and must exit with status 1. Similarly:

test ]

must exit with status 0.