Overview
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)
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:
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.
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.
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).
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 |
|
Delete a single character at the cursor position |
|
Move cursor to end of current line |
|
Move right one character |
|
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 |
|
Background command |
|
Re-directed background command |
|
Where:
<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 $BAR`Will execute the program named
myprogrampassing it the value of the environment variableBAR. The value of the environment variableFOOis then set output ofmyprogramonstdout.
CONFIG_NSH_ARGCAT: Support concatenation of strings with environment variables or command output. For example:set FOO XYZ set BAR 123 set FOOBAR ABC_${FOO}_${BAR}would set the environment variable
FOOtoXYZ,BARto123andFOOBARtoABC_XYZ_123. IfCONFIG_NSH_ARGCATis not selected, then a slightly smaller FLASH footprint results but then also only simple environment variables like$FOOcan 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 isCONFIG_NSH_ARGCATis 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>
then
[sequence of <cmd>]
else
[sequence of <cmd>]
fi
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.
Examples:
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
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> do [sequence of <cmd>] done
until-do-done: Execute[sequence of <cmd>]as long as<cmd>has a non-zero exit status. The syntax is as follows:until <cmd> do [sequence of <cmd>] done
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. |
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 |
|
The current working directory |
|
The previous working directory |