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Testing Values and Files

configure scripts need to test properties of many files and strings. Here are some portability problems to watch out for when doing those tests.

The test program is the way to perform many file and string tests. It is often invoked by the alternate name `[', but using that name in Autoconf code is asking for trouble since it is an m4 quote character.

If you need to make multiple checks using test, combine them with the shell operators `&&' and `||' instead of using the test operators `-a' and `-o'. On System V, the precedence of `-a' and `-o' is wrong relative to the unary operators; consequently, POSIX does not specify them, so using them is nonportable. If you combine `&&' and `||' in the same statement, keep in mind that they have equal precedence.

To enable configure scripts to support cross-compilation, they shouldn't do anything that tests features of the host system instead of the target system. But occasionally you may find it necessary to check whether some arbitrary file exists. To do so, use `test -f' or `test -r'. Do not use `test -x', because 4.3BSD does not have it.

Another nonportable shell programming construction is


The intent is to set var to value only if it is not already set, but if var has any value, even the empty string, to leave it alone. Old BSD shells, including the Ultrix sh, don't accept the colon, and complain and die. A portable equivalent is

: ${var=value}

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