2020-07-22 21:17:48 +00:00
|
|
|
|
# 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:
|
|
|
|
|
|
|
2020-07-31 13:43:25 +00:00
|
|
|
|
```console
|
|
|
|
|
|
$ 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!
|
|
|
|
|
|
```
|
2020-07-22 21:17:48 +00:00
|
|
|
|
|
2020-07-24 13:46:16 +00:00
|
|
|
|
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`).
|
2020-07-22 21:17:48 +00:00
|
|
|
|
|
|
|
|
|
|
`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
|
2020-07-23 08:44:54 +00:00
|
|
|
|
by `out` is now “valid”. If you try to build the derivation again, Nix
|
2020-07-22 21:17:48 +00:00
|
|
|
|
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:
|
|
|
|
|
|
|
2020-07-31 13:43:25 +00:00
|
|
|
|
```console
|
|
|
|
|
|
$ nix-build -A hello
|
|
|
|
|
|
waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'
|
|
|
|
|
|
```
|
2020-07-22 21:17:48 +00:00
|
|
|
|
|
|
|
|
|
|
So it is always safe to run multiple instances of Nix in parallel (which
|
|
|
|
|
|
isn’t the case with, say, `make`).
|
