2. Configuration and Initialization

2.1. Introduction

This chapter provides information on how the application configures and initializes the RTEMS shell.

2.2. Configuration

The command set available to the application is user configurable. It is configured using a mechanism similar to the confdefs.h mechanism used to specify application configuration.

In the simplest case, if the user wishes to configure a command set with all commands available that are neither filesystem management (e.g. mounting, formating, etc.) or network related, then the following is all that is required:

#include <rtems/shellconfig.h>

In a slightly more complex example, if the user wishes to include all networking commands as well as support for mounting MS-DOS and NFS filesystems, then the following is all that is required:

#include <rtems/shellconfig.h>

The shell uses a POSIX key to reference the shell’s per thread environment. A user’s application needs to account for this key. If the application has a configuration for POSIX keys add one extra for the shell. If there is no entry add to the configuration:


2.2.1. Customizing the Command Set

The user can configure specific command sets by either building up the set from individual commands or starting with a complete set and disabling individual commands. Each command has two configuration macros associated with it.


Each command has a constant of this form which is defined when building a command set by individually enabling specific commands.


In contrast, each command has a similar command which is defined when the application is configuring a command set by disabling specific commands in the set.

2.2.2. Adding Custom Commands

One of the design goals of the RTEMS Shell was to make it easy for a user to add custom commands specific to their application. We believe this design goal was accomplished. In order to add a custom command, the user is required to do the following:

  • Provide a main-style function which implements the command. If that command function uses a getopt related function to parse arguments, it MUST use the reentrant form.

  • Provide a command definition structure of type rtems_shell_cmd_t.

  • Configure that command using the CONFIGURE_SHELL_USER_COMMANDS macro.

Custom aliases are configured similarly but the user only provides an alias definition structure of type rtems_shell_alias_t and configures the alias via the CONFIGURE_SHELL_USER_ALIASES macro.

In the following example, we have implemented a custom command named usercmd which simply prints the arguments it was passed. We have also provided an alias for usercmd named userecho.

#include <rtems/shell.h>
int main_usercmd(int argc, char **argv)
    int i;
    printf( "UserCommand: argc=%d\n", argc );
    for (i=0 ; i<argc ; i++ )
        printf( "argv[%d]= %s\n", i, argv[i] );
    return 0;
rtems_shell_cmd_t Shell_USERCMD_Command = {
    "usercmd",                                                   /* name */
    "usercmd n1 [n2 [n3...]]",                                   /* usage */
    "user",                                                      /* topic */
    main_usercmd,                                                /* command */
    NULL,                                                        /* alias */
    NULL,                                                        /* next */
    S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH,   /* mode */
    0,                                                           /* uid */
    0                                                            /* gid */
rtems_shell_alias_t Shell_USERECHO_Alias = {
    "usercmd",                   /* command */
    "userecho"                   /* alias */
#include <rtems/shellconfig.h>

Notice in the above example, that the user wrote the main for their command (e.g. main_usercmd) which looks much like any other main(). They then defined a rtems_shell_cmd_t structure named Shell_USERCMD_Command which describes that command. This command definition structure is registered into the static command set by defining CONFIGURE_SHELL_USER_COMMANDS to &Shell_USERCMD_Command.

Similarly, to add the userecho alias, the user provides the alias definition structure named Shell_USERECHO_Alias and defines CONFIGURE_SHELL_USER_ALIASES to configure the alias.

The user can configure any number of commands and aliases in this manner.

2.3. Initialization

The shell may be easily attached to a serial port or to the telnetd server. This section describes how that is accomplished.

2.3.1. Attached to a Serial Port

Starting the shell attached to the console or a serial port is very simple. The user invokes rtems_shell_init with parameters to indicate the characteristics of the task that will be executing the shell including name, stack size, and priority. The user also specifies the device that the shell is to be attached to.

This example is taken from the fileio sample test. This shell portion of this test can be run on any target which provides a console with input and output capabilities. It does not include any commands which cannot be supported on all BSPs. The source code for this test is in testsuites/samples/fileio with the shell configuration in the init.c file.

#include <rtems/shell.h>
void start_shell(void)
    printf(" =========================\n");
    printf(" starting shell\n");
    printf(" =========================\n");
        "SHLL",                       /* task name */
        RTEMS_MINIMUM_STACK_SIZE * 4, /* task stack size */
        100,                          /* task priority */
        "/dev/console",               /* device name */
        false,                        /* run forever */
        true,                         /* wait for shell to terminate */
        rtems_shell_login_check       /* login check function,
        use NULL to disable a login check */

In the above example, the call to rtems_shell_init spawns a task to run the RTEMS Shell attached to /dev/console and executing at priority 100. The caller suspends itself and lets the shell take over the console device. When the shell is exited by the user, then control returns to the caller.

2.3.2. Attached to a Socket


2.4. Access Control

2.4.1. Login Checks

Login checks are optional for the RTEMS shell and can be configured via a login check handler passed to rtems_shell_init(). One login check handler is rtems_shell_login_check().

2.4.2. Configuration Files

The following files are used by the login check handler rtems_shell_login_check() to validate a passphrase for a user and to set up the user environment for the shell command execution.


The format for each line is


with colon separated fields. For more information refer to the Linux PASSWD(5) man page. Use a password of * to disable the login of the user. An empty password allows login without a password for this user. In contrast to standard UNIX systems, this file is only readable and writeable for the user with an UID of zero by default. The directory is used to perform a filesystem change root operation in rtems_shell_login_check() in contrast to a normal usage as the HOME directory of the user. The default content is:


so there is no password required for the root user.


The format for each line is:


with colon separated fields. The user_list is comma separated. For more information refer to the Linux GROUP(5) man page. In contrast to standard UNIX systems, this file is only readable and writeable for the user with an UID of zero by default. The default content is


2.4.3. Command Visibility and Execution Permission

Each command has:

  • an owner,

  • a group, and

  • a read permission flag for the owner, the group and all other users, and

  • an execution permission flag for the owner, the group and all other users.

The read and write permission flags are stored in the command mode. The read permission flags determine the visibility of the command for the current user. The execution permission flags determine the ability to execute a command for the current user. These command properties can be displayed and changed with the:

  • cmdls,

  • cmdchown, and

  • cmdchmod

commands. The access is determined by the effective UID, the effective GID and the supplementary group IDs of the current user and follows the standard filesystem access procedure.

2.4.4. Add CRYPT(3) Formats

By default the crypt_r() function used by rtems_shell_login_check() supports only plain text passphrases. Use crypt_add_format() to add more formats. The following formats are available out of the box:

  • crypt_md5_format,

  • crypt_sha256_format, and

  • crypt_sha512_format.

An example follows:

#include <crypt.h>
void add_formats( void )
    crypt_add_format( &crypt_md5_format );
    crypt_add_format( &crypt_sha512_format );

2.5. Functions

This section describes the Shell related C functions which are publicly available related to initialization and configuration.

2.5.1. rtems_shell_init - Initialize the shell

rtems_status_code rtems_shell_init(
    const char          *task_name,
    size_t               task_stacksize,
    rtems_task_priority  task_priority,
    const char          *devname,
    bool                 forever,
    bool                 wait,
    rtems_login_check    login_check

RTEMS_SUCCESSFUL - Shell task spawned successfully others - to indicate a failure condition


This service creates a task with the specified characteristics to run the RTEMS Shell attached to the specified devname.


This method invokes the rtems_task_create and rtems_task_start directives and as such may return any status code that those directives may return.

There is one POSIX key necessary for all shell instances together and one POSIX key value pair per instance. You should make sure that your RTEMS configuration accounts for these resources.

2.5.2. rtems_shell_login_check - Default login check handler

bool rtems_shell_login_check(
  const char *user,
  const char *passphrase

true - login is allowed, and false - otherwise.


This function checks if the specified passphrase is valid for the specified user.


As a side-effect if the specified passphrase is valid for the specified user, this function:

  • performs a filesystem change root operation to the directory of the specified user if the directory path is non-empty,

  • changes the owner of the current shell device to the UID of the specified user,

  • sets the real and effective UID of the current user environment to the UID of the specified user,

  • sets the real and effective GID of the current user environment to the GID of the specified user, and

  • sets the supplementary group IDs of the current user environment to the supplementary group IDs of the specified user.

In case the filesystem change root operation fails, then the environment setup is aborted and false is returned.