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# 8 Command-Line Interface Overview

A CmdStan executable is built from the Stan model concept and the CmdStan command line parser. The command line argument syntax consists of sets of keywords and keyword-value pairs. Arguments are grouped by the following keywords:

• method - specifies the kind of inference done on the model.
Each kind of inference requires further configuration via sub-arguments. The method argument is required. It can be specified overtly as the a keyword-value pair method=<inference> or implicitly as one of the following:

• sample - obtain a sample from the posterior using HMC
• optimize - penalized maximum likelihood estimation
• variational - automatic variational inference
• generate_quantities - run model’s generated quantities block on existing sample to obtain new quantities of interest.
• diagnose - compute and compare sampler gradient calculations to finite differences.
• data - specifies the input data file, if any.

• output - specifies program outputs, both disk files and terminal window outputs.

• init - specifies initial values for the model parameters, if any.

• random - specifies the seed for the pseudo-random number.

The remainder of this chapter covers the general configuration options used for all processing. The following chapters cover the per-inference configuration options.

## 8.1 Input data argument

The values for all variables declared in the data block of the model are read in from an input data file in either JSON or Rdump format. The syntax for the input data argument is:

data file=<filepath>

The keyword data must be followed directly by the keyword-value pair file=<filepath>. If the model doesn’t declare any data variables, this argument is ignored.

The input data file must contain definitions for all data variables declared in the data block. If one or more data block variables are missing from the input data file, the program prints an error message to stderr and returns a non-zero return code. For example, the model bernoulli.stan defines two data variables N and y. If the input data file doesn’t include both variables, or if the data variable doesn’t match the declared type and dimensions, the program will exit with an error message at the point where it first encounters missing data.

For example if the input data file doesn’t include the definition for variable y, the executable exits with the following message:

Exception: variable does not exist; processing stage=data initialization; variable name=y; base type=int (in 'examples/bernoulli/bernoulli.stan', line 3, column 2 to column 28)

## 8.2 Output control arguments

The output keyword is used to specify non-default options for output files and messages written to the terminal window. The output keyword takes several keyword-value pair sub-arguments.

The keyword value pair file=<filepath> specifies the location of the Stan CSV output file. If unspecified, the output file is written to a file named output.csv in the current working directory.

The keyword value pair diagnostic_file=<filepath> specifies the location of the auxiliary output file. By default, no auxiliary output file is produced. This option is only valid for the iterative algorithms sample and variational.

The keyword value pair refresh=<int> specifies the number of iterations between progress messages written to the terminal window. The default value is 100 iterations.

The keyword value pair sig_figs=<int> specifies the number of significant digits for all numerical values in the output files. Allowable values are between 1 and 18, which is the maximum amount of precision available for 64-bit floating point arithmetic. The default value is 6.   Note: increasing sig_figs above the default will increase the size of the output CSV files accordingly.

The keyword value pair profile_file=<filepath> specifies the location of the output file for profiling data. If the model uses no profiling, the output profile file is not produced. If the model uses profiling and profile_file is unspecified, the profiling data is written to a file named profile.csv in the current working directory.

## 8.3 Initialize model parameters argument

Initialization is only applied to parameters defined in the parameters block. By default, all parameters are initialized to random draws from a uniform distribution over the range $$[-2, 2]$$. These values are on the unconstrained scale, so must be inverse transformed back to satisfy the constraints declared for parameters. Because zero is chosen to be a reasonable default initial value for most parameters, the interval around zero provides a fairly diffuse starting point. For instance, unconstrained variables are initialized randomly in $$(-2, 2)$$, variables constrained to be positive are initialized roughly in $$(0.14, 7.4)$$, variables constrained to fall between 0 and 1 are initialized with values roughly in $$(0.12, 0.88)$$.

The initialization argument is specified as keyword-value pair with keyword init. The value can be one of the following:

• positive real number $$x$$. All parameters will be initialized to random draws from a uniform distribution over the range $$[-x, x]$$.

• $$0$$ - All parameters will be initialized to zero values on the unconstrained scale. The transforms are arranged in such a way that zero initialization provides reasonable variable initializations: $$0$$ for unconstrained parameters; $$1$$ for parameters constrained to be positive; $$0.5$$ for variables to constrained to lie between $$0$$ and $$1$$; a symmetric (uniform) vector for simplexes; unit matrices for both correlation and covariance matrices; and so on.

• filepath - A data file in JSON or Rdump format containing initial parameters values for some or all of the model parameters. User specified initial values must satisfy the constraints declared in the model (i.e., they are on the constrained scale). Parameters which aren’t explicitly initialized will be initialized randomly over the range $$[-2, 2]$$.

## 8.4 Random number generator arguments

The random-number generator’s behavior is determined by the unsigned seed (positive integer) it is started with. If a seed is not specified, or a seed of 0 or less is specified, the system time is used to generate a seed. The seed is recorded and included with Stan’s output regardless of whether it was specified or generated randomly from the system time.

The syntax for the random seed argument is:

random seed=<int>

The keyword random must be followed directly by the keyword-value pair seed=<int>.

## 8.5 Chain identifier argument: id

The chain identifier argument is used in conjunction with the random seed argument when running multiple Markov chains for sampling. The chain identifier is used to advance the random number generator a very large number of random variates so that two chains with the same seed and different identifiers draw from non-overlapping subsequences of the random-number sequence determined by the seed. Together, the seed and chain identifier determine the behavior of the random number generator.

The syntax for the random seed argument is:

id=<int>

The default value is 0.

When running a set of chains from the command line with a specified seed, this argument should be set to the chain index. E.g., when running 4 chains, the value should be 1,..,4, successively. When running multiple chains from a single command, Stan’s interfaces manage the chain identifier arguments automatically.

For complete reproducibility, every aspect of the environment needs to be locked down from the OS and version to the C++ compiler and version to the version of Stan and all dependent libraries. See the Stan Reference Manual Reproducibility chapter for further details.

## 8.6 Command line help

CmdStan provides a help and help-all mechanism that displays either the available top-level or keyword-specific key-value argument pairs. To display top-level help, call the CmdStan executable with keyword help:

> ./bernoulli help
Usage: ./bernoulli <arg1> <subarg1_1> ... <subarg1_m> ... <arg_n> <subarg_n_1> ... <subarg_n_m>

Begin by selecting amongst the following inference methods and diagnostics,
sample      Bayesian inference with Markov Chain Monte Carlo
optimize    Point estimation
variational  Variational inference
diagnose    Model diagnostics
generate_quantities  Generate quantities of interest

Or see help information with
help        Prints help
help-all    Prints entire argument tree

Additional configuration available by specifying
id          Unique process identifier
data        Input data options
init        Initialization method: "x" initializes randomly between [-x, x], "0" initializes to 0, anything else identifies a file of values
random      Random number configuration
output      File output options

See ./bernoulli <arg1> [ help | help-all ] for details on individual arguments.

## 8.7 Error messages and return codes

CmdStan executables and utility programs use streams standard output (stdout) and standard error (stderr) to report information and error messages, respectively. Some methods also generate warning messages when the algorithm detects potential problems with the inference. Depending on the method, these messages are sent to either standard out or standard error.

All program executables provide a return code between 0 and 255:

• 0 - Program ran to termination as expected.

• value in range [1 : 125] - Method invoked could not run due to problems with model or data.

• value > 128 - Fatal error during execution, process terminated by signal. To determine the signal number, subtract 128 from the return value, e.g. return code 139 results from termination signal 11 (segmentation violation).

A non-zero return code or outputs sent to stderr indicate problems with the inference. However, a return code of zero and absense of error messages doesn’t necessarily mean that the inference is valid, it is still necessary to validate the inferences using all available summary and diagnostic techniques.