.\" -*- mode: troff; coding: utf-8 -*-
.TH "nix3-eval" "1" ""
.RS
.PP
\fBWarning\fR 
.br
This program is
\fB\fBexperimental\fR\fR
and its interface is subject to change.
.RE
.SH Name
.LP
\f(CRnix eval\fR - evaluate a Nix expression
.SH Synopsis
.LP
\f(CRnix eval\fR [\fIoption\fR\[u2026]] \fIinstallable\fR
.SH Examples
.IP "\(bu" 3
Evaluate a Nix expression given on the command line:
.LP
.EX
# nix eval --expr '1 + 2'
.EE
.IP "\(bu" 3
Evaluate a Nix expression to JSON:
.LP
.EX
# nix eval --json --expr '{ x = 1; }'
{\(dqx\(dq:1}
.EE
.IP "\(bu" 3
Evaluate a Nix expression from a file:
.LP
.EX
# nix eval --file ./my-nixpkgs hello.name
.EE
.IP "\(bu" 3
Get the current version of the \f(CRnixpkgs\fR flake:
.LP
.EX
# nix eval --raw nixpkgs#lib.version
.EE
.IP "\(bu" 3
Print the store path of the Hello package:
.LP
.EX
# nix eval --raw nixpkgs#hello
.EE
.IP "\(bu" 3
Get a list of checks in the \f(CRnix\fR flake:
.LP
.EX
# nix eval nix#checks.x86_64-linux --apply builtins.attrNames
.EE
.IP "\(bu" 3
Generate a directory with the specified contents:
.LP
.EX
# nix eval --write-to ./out --expr '{ foo = \(dqbar\(dq; subdir.bla = \(dq123\(dq; }'
# cat ./out/foo
bar
# cat ./out/subdir/bla
123
.EE
.SH Description
.LP
This command evaluates the given Nix expression and prints the
result on standard output.
.SH Output format
.LP
\f(CRnix eval\fR can produce output in several formats:
.IP "\(bu" 3
By default, the evaluation result is printed as a Nix expression.
.IP "\(bu" 3
With \f(CR--json\fR, the evaluation result is printed in JSON format. Note
that this fails if the result contains values that are not
representable as JSON, such as functions.
.IP "\(bu" 3
With \f(CR--raw\fR, the evaluation result must be a string, which is
printed verbatim, without any quoting.
.IP "\(bu" 3
With \f(CR--write-to\fR \fIpath\fR, the evaluation result must be a string or
a nested attribute set whose leaf values are strings. These strings
are written to files named \fIpath\fR/\fIattrpath\fR. \fIpath\fR must not
already exist.
.SH Options
.IP "\(bu" 3
\fB\f(CR--apply\fR\fR \fIexpr\fR
.IP
Apply the function \fIexpr\fR to each argument.
.IP "\(bu" 3
\fB\f(CR--json\fR\fR
.IP
Produce output in JSON format, suitable for consumption by another program.
.IP "\(bu" 3
\fB\f(CR--raw\fR\fR
.IP
Print strings without quotes or escaping.
.IP "\(bu" 3
\fB\f(CR--read-only\fR\fR
.IP
Do not instantiate each evaluated derivation. This improves performance, but can cause errors when accessing store paths of derivations during evaluation.
.IP "\(bu" 3
\fB\f(CR--write-to\fR\fR \fIpath\fR
.IP
Write a string or attrset of strings to \fIpath\fR.
.SS
Common evaluation options
.IP "\(bu" 3
\fB\f(CR--arg\fR\fR \fIname\fR \fIexpr\fR
.IP
Pass the value \fIexpr\fR as the argument \fIname\fR to Nix functions.
.IP "\(bu" 3
\fB\f(CR--arg-from-file\fR\fR \fIname\fR \fIpath\fR
.IP
Pass the contents of file \fIpath\fR as the argument \fIname\fR to Nix functions.
.IP "\(bu" 3
\fB\f(CR--arg-from-stdin\fR\fR \fIname\fR
.IP
Pass the contents of stdin as the argument \fIname\fR to Nix functions.
.IP "\(bu" 3
\fB\f(CR--argstr\fR\fR \fIname\fR \fIstring\fR
.IP
Pass the string \fIstring\fR as the argument \fIname\fR to Nix functions.
.IP "\(bu" 3
\fB\f(CR--debugger\fR\fR
.IP
Start an interactive environment if evaluation fails.
.IP "\(bu" 3
\fB\f(CR--eval-store\fR\fR \fIstore-url\fR
.IP
The \fBURL of the Nix store\fR
to use for evaluation, i.e. to store derivations (\f(CR.drv\fR files) and inputs referenced by them.
.IP "\(bu" 3
\fB\f(CR--impure\fR\fR
.IP
Allow access to mutable paths and repositories.
.IP "\(bu" 3
\fB\f(CR--include\fR\fR / \f(CR-I\fR \fIpath\fR
.IP
Add \fIpath\fR to search path entries used to resolve \fBlookup paths\fR
.IP
This option may be given multiple times.
.IP
Paths added through \f(CR-I\fR take precedence over the \fB\f(CRnix-path\fR configuration setting\fR and the \fB\f(CRNIX_PATH\fR environment variable\fR.
.IP "\(bu" 3
\fB\f(CR--override-flake\fR\fR \fIoriginal-ref\fR \fIresolved-ref\fR
.IP
Override the flake registries, redirecting \fIoriginal-ref\fR to \fIresolved-ref\fR.
.SS
Common flake-related options
.IP "\(bu" 3
\fB\f(CR--commit-lock-file\fR\fR
.IP
Commit changes to the flake\(cqs lock file.
.IP "\(bu" 3
\fB\f(CR--inputs-from\fR\fR \fIflake-url\fR
.IP
Use the inputs of the specified flake as registry entries.
.IP "\(bu" 3
\fB\f(CR--no-registries\fR\fR
.IP
Don\(cqt allow lookups in the flake registries.
.RS
.IP
\fBDEPRECATED\fR
.IP
Use \fB\f(CR--no-use-registries\fR\fR instead.
.RE
.IP "\(bu" 3
\fB\f(CR--no-update-lock-file\fR\fR
.IP
Do not allow any updates to the flake\(cqs lock file.
.IP "\(bu" 3
\fB\f(CR--no-write-lock-file\fR\fR
.IP
Do not write the flake\(cqs newly generated lock file.
.IP "\(bu" 3
\fB\f(CR--output-lock-file\fR\fR \fIflake-lock-path\fR
.IP
Write the given lock file instead of \f(CRflake.lock\fR within the top-level flake.
.IP "\(bu" 3
\fB\f(CR--override-input\fR\fR \fIinput-path\fR \fIflake-url\fR
.IP
Override a specific flake input (e.g. \f(CRdwarffs/nixpkgs\fR). This implies \f(CR--no-write-lock-file\fR.
.IP "\(bu" 3
\fB\f(CR--recreate-lock-file\fR\fR
.IP
Recreate the flake\(cqs lock file from scratch.
.RS
.IP
\fBDEPRECATED\fR
.IP
Use \fB\f(CRnix flake update\fR\fR instead.
.RE
.IP "\(bu" 3
\fB\f(CR--reference-lock-file\fR\fR \fIflake-lock-path\fR
.IP
Read the given lock file instead of \f(CRflake.lock\fR within the top-level flake.
.IP "\(bu" 3
\fB\f(CR--update-input\fR\fR \fIinput-path\fR
.IP
Update a specific flake input (ignoring its previous entry in the lock file).
.RS
.IP
\fBDEPRECATED\fR
.IP
Use \fB\f(CRnix flake update\fR\fR instead.
.RE
.SS
Logging-related options
.IP "\(bu" 3
\fB\f(CR--debug\fR\fR
.IP
Set the logging verbosity level to \(oqdebug\(cq.
.IP "\(bu" 3
\fB\f(CR--log-format\fR\fR \fIformat\fR
.IP
Set the format of log output; one of \f(CRraw\fR, \f(CRinternal-json\fR, \f(CRbar\fR or \f(CRbar-with-logs\fR.
.IP "\(bu" 3
\fB\f(CR--print-build-logs\fR\fR / \f(CR-L\fR
.IP
Print full build logs on standard error.
.IP "\(bu" 3
\fB\f(CR--quiet\fR\fR
.IP
Decrease the logging verbosity level.
.IP "\(bu" 3
\fB\f(CR--verbose\fR\fR / \f(CR-v\fR
.IP
Increase the logging verbosity level.
.SS
Miscellaneous global options
.IP "\(bu" 3
\fB\f(CR--help\fR\fR
.IP
Show usage information.
.IP "\(bu" 3
\fB\f(CR--offline\fR\fR
.IP
Disable substituters and consider all previously downloaded files up-to-date.
.IP "\(bu" 3
\fB\f(CR--option\fR\fR \fIname\fR \fIvalue\fR
.IP
Set the Nix configuration setting \fIname\fR to \fIvalue\fR (overriding \f(CRnix.conf\fR).
.IP "\(bu" 3
\fB\f(CR--refresh\fR\fR
.IP
Consider all previously downloaded files out-of-date.
.IP "\(bu" 3
\fB\f(CR--repair\fR\fR
.IP
During evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths.
.IP "\(bu" 3
\fB\f(CR--version\fR\fR
.IP
Show version information.
.SS
Options that change the interpretation of \fBinstallables\fR
.IP "\(bu" 3
\fB\f(CR--expr\fR\fR \fIexpr\fR
.IP
Interpret \fB\fIinstallables\fR\fR as attribute paths relative to the Nix expression \fIexpr\fR.
.IP "\(bu" 3
\fB\f(CR--file\fR\fR / \f(CR-f\fR \fIfile\fR
.IP
Interpret \fB\fIinstallables\fR\fR as attribute paths relative to the Nix expression stored in \fIfile\fR. If \fIfile\fR is the character -, then a Nix expression will be read from standard input. Implies \f(CR--impure\fR.
.RS
.LP
\fBNote\fR
.PP
See \fB\f(CRman nix.conf\fR\fR for overriding configuration settings with command line flags.
.RE