5 Linux / Unix Commands For Connecting To The Serial Console

Most embedded Linux / BSD systems such as routers, servers and nas devices comes with console interface (serial port with RS-232). BIOS can uses this, and after boot BIOS screen I/O is redirected so that you can use the device. RS-232 is also used for communicating to headless server, where no monitor or keyboard is installed, during boot when operating system is not running yet and therefore no network connection is possible. You need to use a serial cable between your computer and embedded system or server. In this post I will cover five conman utilities used for serial communication under Linux / Unix / *BSD and Mac OS X.

Type the following dmesg command:
$ dmesg | egrep --color 'serial|ttyS'
Sample outputs:

[ 1.245258] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A[ 1.265727] serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A[ 1.286713] 00:07: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A[ 1.307321] 00:08: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A

You can use setserial command which is designed to set and/or report the configuration information associated with a serial port:
$ setserial -g /dev/ttyS[0123]
Sample outputs:

/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4/dev/ttyS1, UART: 16550A, Port: 0x02f8, IRQ: 3/dev/ttyS2, UART: unknown, Port: 0x03e8, IRQ: 4/dev/ttyS3, UART: unknown, Port: 0x02e8, IRQ: 3

The cu command is used to call up another system and act as a dial in terminal. cu command is installed on many Unix like systems including OpenBSD/AIX/Solaris and so on. You can use it as follows:

 cu -l /dev/device -s baud-rate-speed 

In this example, I'm using /dev/ttyS0 with 19200 baud-rate:

 cu -l /dev/ttyS0 -s 19200 

Sample outputs:

Fig.01: cu command in action

To exit enter tilde dot (~.).

Use the screen command as follows:
$ screen /dev/device baud-rate
$ screen /dev/ttyS0 19200
$ screen /dev/ttySUSB0 19200,cs8
Sample outputs:

Fig.02: serial command connected to my personal router powered by dd-wrt

minicom command is a communication program which somewhat resembles the shareware program TELIX from old good MS-DOS days. Type minicom to connect:
$ minicom
Sample outputs:

Fig.03: minicom connected to my m0n0wall embedded firewall

See how to install and use minicom under Linux operating systems.

PuTTY is a free and open source gui X based terminal emulator client for the SSH, Telnet, rlogin, and raw TCP computing protocols and as a serial console client. It works under Linux, Unix, Mac OS X, MS-Windows and few other operating systems. See how to configure and use putty for cisco routers:

Fig.04: putty in action

You can use the apt-get or yum command to install putty:
$ sudo apt-get install putty

The tip command is used as a serial terminal. tip command can be found under AIX/HP-UX/Solairs/*BSD/Linux operating systems. The syntax is:
tip -19200 device
tip -19200 s0
tip -19200 ttys0

See our previous howtos about setting and redirecting console output over serial port:

OpenBSD: Set a serial consoleFreeBSD: Set a serial consoleDebian / Ubuntu Linux: Set a serial consoleRHEL / CentOS / Fedora Linux: Set a serial console

You can also use the KVM over IP client as described here for serial console redirection over IP (LAN/WAN).

If you forced to use MS-Windows, try the following free and open source serial communication programs:

Here is a quick demo of my Soekris router running monowall embedded version. The minicom is used to connect to the device and access BIOS and boot the system:

(Video.01: minicom in action)

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