2.6 KiB
Building and Testing
You can now try to build Hello. Of course, you could do nix-env -i hello, but you may not want to install a possibly broken package just
yet. The best way to test the package is by using the command
nix-build, which builds a Nix expression and creates a symlink named
result in the current directory:
$ nix-build -A hello
building path `/nix/store/632d2b22514d...-hello-2.1.1'
hello-2.1.1/
hello-2.1.1/intl/
hello-2.1.1/intl/ChangeLog
...
$ ls -l result
lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1
$ ./result/bin/hello
Hello, world!
The -A option selects the hello attribute. This is faster than
using the symbolic package name specified by the name attribute
(which also happens to be hello) and is unambiguous (there can be
multiple packages with the symbolic name hello, but there can be
only one attribute in a set named hello).
nix-build registers the ./result symlink as a garbage collection
root, so unless and until you delete the ./result symlink, the output
of the build will be safely kept on your system. You can use
nix-build’s -o switch to give the symlink another name.
Nix has transactional semantics. Once a build finishes successfully, Nix
makes a note of this in its database: it registers that the path denoted
by out is now “valid”. If you try to build the derivation again, Nix
will see that the path is already valid and finish immediately. If a
build fails, either because it returns a non-zero exit code, because Nix
or the builder are killed, or because the machine crashes, then the
output paths will not be registered as valid. If you try to build the
derivation again, Nix will remove the output paths if they exist (e.g.,
because the builder died half-way through make install) and try again. Note that there is no “negative caching”: Nix
doesn't remember that a build failed, and so a failed build can always
be repeated. This is because Nix cannot distinguish between permanent
failures (e.g., a compiler error due to a syntax error in the source)
and transient failures (e.g., a disk full condition).
Nix also performs locking. If you run multiple Nix builds simultaneously, and they try to build the same derivation, the first Nix instance that gets there will perform the build, while the others block (or perform other derivations if available) until the build finishes:
$ nix-build -A hello
waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'
So it is always safe to run multiple instances of Nix in parallel (which
isn’t the case with, say, make).