The NSH Library. The apps/nshlib sub-directory contains the NuttShell (NSH) library. This library can easily to linked to produce a NSH application (See as an example apps/examples/nsh). The NSH Library provides a simple shell application for NuttX.

Console/NSH Front End

NSH Consoles. Using settings in the configuration file, NSH may be configured to use (1) the serial stdin/out, (2) a USB serial device (such as CDC/ACM), or (3) a telnet connection as the console. Or, perhaps even all at once since or BOTH. An indefinite number of telnet sessions are supported.

Start-Up prompt. When NSH is started, you will see the a welcome message such the following on the selected console:

NuttShell (NSH)

The greeting may also include NuttX versioning information if you are using a versioned copy of NuttX. nsh> is the NSH prompt and indicates that you may enter a command from the console.

USB console startup. When using a USB console, the start-up sequence differs a little: In this case, you are required to press ENTER three times. Then NSH prompt will appear as described above. This is required for the following reasons:

  1. This assures that the USB connection is stable. The USB connection may be made, broken, and re-established a few times if the USB cable is not yet fully seated. Waiting for ENTER to be pressed three times assures that the connection is stable.

  2. The establishment of the connection is two step process: First, the USB serial connection is made with the host PC. Then the application that uses the serial interface is started on the host. When the serial connection is established on the host, the host operating system may send several AT modem commands to the host depending upon how the host serial port is configured. By waiting for ENTER to be pressed three consecutive times, all of these modem commands will go to the bit-bucket and will not be interpreted as NSH command input.

  3. Similarly, in the second step when the applications is started, there may be additional AT modem commands sent out the serial port. Most serial terminal programs will do this unless they are specifically configured to suppress the modem command output. Waiting for the ENTER input eliminates the invalid command errors from both (2) and (3).

  4. Finally, if NSH did not wait for some positive indication that the serial terminal program is up and running, then the output of the NSH greeting and initial NSH prompt would be lost.

Extended Command Line Editing. By default, NuttX uses a simple command line editor that allows command entry after the nsh> and supports only the backspace key for editing. However, a more complete command line editor can be selected by setting CONFIG_NSH_CLE=y in the NuttX configuration file. When that option is selected, the following EMACS-like line editing commands are supported:

Key Binding

Editor Action


Move cursor to start of the line


Move left one character

^D or Del

Delete a single character at the cursor position


Move cursor to end of current line


Move right one character

^H or Backspace

Delete character, left (backspace)


Delete to the end of the line


Delete the entire line

Command Overview

Simple, Re-directed, and Background Commands. The NuttShell (NSH) is a simple shell application. NSH supports the following commands forms:

Simple command


Command with re-directed output

<cmd> > <file> <cmd> >> <file>

Background command

<cmd> &

Re-directed background command

<cmd> > <file> & <cmd> >> <file> &


  • <cmd> is any one of the simple commands listed later.

  • <file> is the full or relative path to any writable object in the file system name space (file or character driver). Such objects will be referred to simply as files throughout this document.

nice ‘d Background Commands. NSH executes at the mid-priority (128). Backgrounded commands can be made to execute at higher or lower priorities using nice:

[nice [-d <niceness>>]] <cmd> [> <file>|>> <file>] [&]

Where <niceness> is any value between -20 and 19 where lower (more negative values) correspond to higher priorities. The default niceness is 10.

Multiple commands per line. NSH will accept multiple commands per command line with each command separated with the semi-colon character (;).

Optional Syntax Extensions Because these features commit significant resources, they are disabled by default.

  • CONFIG_NSH_CMDPARMS: If selected, then the output from commands, from file applications, and from NSH built-in commands can be used as arguments to other commands. The entity to be executed is identified by enclosing the command line in back quotes. For example,

    set FOO `myprogram $BAR`

    Will execute the program named myprogram passing it the value of the environment variable BAR. The value of the environment variable FOO is then set output of myprogram on stdout.

  • CONFIG_NSH_ARGCAT: Support concatenation of strings with environment variables or command output. For example:

    set FOO XYZ
    set BAR 123
    set FOOBAR ABC_${FOO}_${BAR}

    would set the environment variable FOO to XYZ, BAR to 123 and FOOBAR to ABC_XYZ_123. If CONFIG_NSH_ARGCAT is not selected, then a slightly smaller FLASH footprint results but then also only simple environment variables like $FOO can be used on the command line.

  • CONFIG_NSH_QUOTE: Enables back-slash quoting of certain characters within the command. This option is useful for the case where an NSH script is used to dynamically generate a new NSH script. In that case, commands must be treated as simple text strings without interpretation of any special characters. Special characters such as $, `, ", and others must be retained intact as part of the test string. This option is currently only available is CONFIG_NSH_ARGCAT is also selected.

Conditional Command Execution

An if-then[-else]-fi construct is also supported in order to support conditional execution of commands. This works from the command line but is primarily intended for use within NSH scripts (see the `sh <#cmdsh>`__ command). The syntax is as follows:

if [!] <cmd>
  [sequence of <cmd>]
  [sequence of <cmd>]

Where <cmd> is a simple command. The command success value of zero is treated true; a non-zero command failure value is treated false. The `test <#cmdtest>`__ command is frequently used for comparisons.


nsh> if test 1 -eq 1; then echo "1 is equal 1"; else echo "1 is not equal 1"; fi
1 is equal 1

nsh> if test 1 -eq 0; then echo "1 is equal 0"; else echo "1 is not equal 0"; fi
1 is not equal 0


Looping Constructs. while-do-done and until-do-done looping constructs are also supported. These work from the command line but are primarily intended for use within NSH scripts (see the `sh <#cmdsh>`__ command).

  • while-do-done: Execute [sequence of <cmd>] as long as <cmd> has an exit status of zero. The syntax is as follows:

    while <cmd>
      [sequence of <cmd>]
  • until-do-done: Execute [sequence of <cmd>] as long as <cmd> has a non-zero exit status. The syntax is as follows:

    until <cmd>
      [sequence of <cmd>]

Where <cmd> is a simple command. The command success value of zero is treated true; a non-zero command failure value is treated false. The `test <#cmdtest>`__ command is frequently used for comparisons.

The break Command. A break command is also supported. The break command is only meaningful within the body of the a while or until loop, between the do and done tokens. If the break command is executed within the body of a loop, the loop will immediately terminate and execution will continue with the next command immediately following the done token.

Built-In Variables


The result of the last simple command execution.
On backgrounded commands, this variable holds only
the result of spawning the background command.

Current Working Directory

cd and pwd. All path arguments to commands may be either an absolute path or a path relative to the current working directory. The current working directory is set using the cd command and can be queried either by using the pwd command or by using the echo $PWD command.

Environment Variables


The default path in the file systems to look
for executable, binary programs working directory


The current working directory


The previous working directory