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Real-time embedded systems vary widely based upon their operational and maintenance requirements. Some of these systems provide ways for the user or developer to interact with them. This interaction could be used for operational, diagnostic, or configuration purposes. The capabilities described in this manual are those provided with RTEMS to provide a command line interface for user access. Some of these commands will be familiar as standard POSIX utilities while others are RTEMS specific or helpful in debugging and analyzing an embedded system. As a simple example of the powerful and very familiar capabilities that the RTEMS Shell provides to an application, consider the following example which hints at some of the capabilities available:

Welcome to rtems-
COPYRIGHT (c) 1989-2011.
On-Line Applications Research Corporation (OAR).
Login into RTEMS
login: rtems
RTEMS SHELL (Ver.1.0-FRC):/dev/console. Feb 28 2008. 'help' to list commands.
SHLL [/] $ cat /etc/passwd
rtems:*:1:1:RTEMS Application::/:/bin/sh
tty:!:2:2:tty owner::/:/bin/false
SHLL [/] $ ls /dev
-rwxr-xr-x   1  rtems   root           0 Jan 01 00:00 console
-rwxr-xr-x   1   root   root           0 Jan 01 00:00 console_b
2 files 0 bytes occupied
SHLL [/] $ stackuse
Stack usage by thread
ID      NAME    LOW          HIGH     CURRENT     AVAILABLE     USED
0x09010001  IDLE 0x023d89a0 - 0x023d99af 0x023d9760      4096        608
0x0a010001  UI1  0x023d9f30 - 0x023daf3f 0x023dad18      4096       1804
0x0a010002  SHLL 0x023db4c0 - 0x023df4cf 0x023de9d0     16384       6204
0xffffffff  INTR 0x023d2760 - 0x023d375f 0x00000000      4080        316
SHLL [/] $ mount -L
File systems: msdos
SHLL [/] $

In the above example, the user rtems logs into a SPARC based RTEMS system. The first command is cat /etc/passwd. This simple command lets us know that this application is running the In Memory File System (IMFS) and that the infrastructure has provided dummy entries for /etc/passwd and a few other files. The contents of /etc/passwd let us know that the user could have logged in as root. In fact, the root user has more permissions than rtems who is not allowed to write into the filesystem.

The second command is ls /dev which lets us know that RTEMS has POSIX-style device nodes which can be accesses through standard I/O function calls.

The third command executed is the RTEMS specific stackuse which gives a report on the stack usage of each thread in the system. Since stack overflows are a common error in deeply embedded systems, this is a surprising simple, yet powerful debugging aid.

Finally, the last command, mount -L hints that RTEMS supports a variety of mountable filesystems. With support for MS-DOS FAT on IDE/ATA and Flash devices as well as network-based filesystens such as NFS and TFTP, the standard free RTEMS provides a robuse infrastructure for embedded applications.

This manual describes the RTEMS Shell and its command set. In our terminology, the Shell is just a loop reading user input and turning that input into commands with argument. The Shell provided with RTEMS is a simple command reading loop with limited scripting capabilities. It can be connected to via a standard serial port or connected to the RTEMS telnetd server for use across a network.

Each command in the command set is implemented as a single subroutine which has a main-style prototype. The commands interpret their arguments and operate upon stdin, stdout, and stderr by default. This allows each command to be invoked independent of the shell.

The described separation of shell from commands from communications mechanism was an important design goal. At one level, the RTEMS Shell is a complete shell environment providing access to multiple POSIX compliant filesystems and TCP/IP stack. The subset of capabilities available is easy to configure and the standard Shell can be logged into from either a serial port or via telnet. But at another level, the Shell is a large set of components which can be integrated into the user's developed command interpreter. In either case, it is trivial to add custom commands to the command set available.


The Institute of Electrical and Electronics Engineers, Inc and The Open Group, have given us permission to reprint portions of their documentation.

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (c) 2001-2004 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at This notice shall appear on any product containing this material.
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