.\" -*- mode: troff; coding: utf-8 -*-
.TH "nix3-eval" "1" ""
.PP
\fBWarning\fR: This program is \fBexperimental\fR and its interface is subject to change.
.SH Name
\fCnix eval\fR - evaluate a Nix expression
.SH Synopsis
\fCnix eval\fR [\fIoption\fR\[u2026]] \fIinstallable\fR
.SH Examples
.IP "\(bu" 2
Evaluate a Nix expression given on the command line:
.LP
.EX
# nix eval --expr '1 + 2'
.EE
.IP "\(bu" 2
Evaluate a Nix expression to JSON:
.LP
.EX
# nix eval --json --expr '{ x = 1; }'
{\(dqx\(dq:1}
.EE
.IP "\(bu" 2
Evaluate a Nix expression from a file:
.LP
.EX
# nix eval -f ./my-nixpkgs hello.name
.EE
.IP "\(bu" 2
Get the current version of the \fCnixpkgs\fR flake:
.LP
.EX
# nix eval --raw nixpkgs#lib.version
.EE
.IP "\(bu" 2
Print the store path of the Hello package:
.LP
.EX
# nix eval --raw nixpkgs#hello
.EE
.IP "\(bu" 2
Get a list of checks in the \fCnix\fR flake:
.LP
.EX
# nix eval nix#checks.x86_64-linux --apply builtins.attrNames
.EE
.IP "\(bu" 2
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
This command evaluates the Nix expression \fIinstallable\fR and prints the
result on standard output.
.SH Output format
\fCnix eval\fR can produce output in several formats:
.IP "\(bu" 2
By default, the evaluation result is printed as a Nix expression.
.IP "\(bu" 2
With \fC--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" 2
With \fC--raw\fR, the evaluation result must be a string, which is
printed verbatim, without any quoting.
.IP "\(bu" 2
With \fC--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" 2
\fC--apply\fR \fIexpr\fR
.br
Apply the function \fIexpr\fR to each argument.
.IP "\(bu" 2
\fC--json\fR
.br
Produce output in JSON format, suitable for consumption by another program.
.IP "\(bu" 2
\fC--raw\fR
.br
Print strings without quotes or escaping.
.IP "\(bu" 2
\fC--read-only\fR
.br
Do not instantiate each evaluated derivation. This improves performance, but can cause errors when accessing store paths of derivations during evaluation.
.IP "\(bu" 2
\fC--write-to\fR \fIpath\fR
.br
Write a string or attrset of strings to \fIpath\fR.
.LP
\fBCommon evaluation options:\fR
.IP "\(bu" 2
\fC--arg\fR \fIname\fR \fIexpr\fR
.br
Pass the value \fIexpr\fR as the argument \fIname\fR to Nix functions.
.IP "\(bu" 2
\fC--argstr\fR \fIname\fR \fIstring\fR
.br
Pass the string \fIstring\fR as the argument \fIname\fR to Nix functions.
.IP "\(bu" 2
\fC--eval-store\fR \fIstore-url\fR
.br
The Nix store to use for evaluations.
.IP "\(bu" 2
\fC--impure\fR
.br
Allow access to mutable paths and repositories.
.IP "\(bu" 2
\fC--include\fR / \fC-I\fR \fIpath\fR
.br
Add \fIpath\fR to the list of locations used to look up \fC<...>\fR file names.
.IP "\(bu" 2
\fC--override-flake\fR \fIoriginal-ref\fR \fIresolved-ref\fR
.br
Override the flake registries, redirecting \fIoriginal-ref\fR to \fIresolved-ref\fR.
.LP
\fBCommon flake-related options:\fR
.IP "\(bu" 2
\fC--commit-lock-file\fR
.br
Commit changes to the flake\[u2019]s lock file.
.IP "\(bu" 2
\fC--inputs-from\fR \fIflake-url\fR
.br
Use the inputs of the specified flake as registry entries.
.IP "\(bu" 2
\fC--no-registries\fR
.br
Don\[u2019]t allow lookups in the flake registries. This option is deprecated; use \fC--no-use-registries\fR.
.IP "\(bu" 2
\fC--no-update-lock-file\fR
.br
Do not allow any updates to the flake\[u2019]s lock file.
.IP "\(bu" 2
\fC--no-write-lock-file\fR
.br
Do not write the flake\[u2019]s newly generated lock file.
.IP "\(bu" 2
\fC--override-input\fR \fIinput-path\fR \fIflake-url\fR
.br
Override a specific flake input (e.g. \fCdwarffs/nixpkgs\fR). This implies \fC--no-write-lock-file\fR.
.IP "\(bu" 2
\fC--recreate-lock-file\fR
.br
Recreate the flake\[u2019]s lock file from scratch.
.IP "\(bu" 2
\fC--update-input\fR \fIinput-path\fR
.br
Update a specific flake input (ignoring its previous entry in the lock file).
.LP
\fBOptions that change the interpretation of installables:\fR
.IP "\(bu" 2
\fC--derivation\fR
.br
Operate on the store derivation rather than its outputs.
.IP "\(bu" 2
\fC--expr\fR \fIexpr\fR
.br
Interpret installables as attribute paths relative to the Nix expression \fIexpr\fR.
.IP "\(bu" 2
\fC--file\fR / \fC-f\fR \fIfile\fR
.br
Interpret installables 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.