Linux/UNIX Pipes File Descriptors, and Process Substitution

• POSIX Shell – A logical container providing an environment and a set of commands to interact with a UNIX and/or UNIX-like system.
  Examples include: sh(Bourne shell), bash, ash, csh, etc.
• Pipe – a unidirectional connection between two processes utilized for inter-process communications.
• File Descriptor (fd) – A dynamically created, locally significant and non-persistent file representation of a FIFO buffer which can be connected to a pipe, or by default to the parent process’ standard input, output, and error buffers. As an example, a (virtual) terminal’s input device (keyboard, mouse, etc.) and output device (display).
• Process substitution – Allows the POSIX shell user the ability create scripted constructs that mimic functional and/or imperative programming syntax in-line.

Every interactive process on startup within a POSIX shell is attached dynamically to three pipes local to its memory space. Standard Input(0), Standard Output(1), and Standard Error(2). If the POSIX system supports file descriptors they are then created and attached to the pipes at ‘/dev/fd/’.

Name	Abbrev.	File	Symbols
standard input	stdin	/dev/fd/0	< or <<
standard output	stdout	/dev/fd/1	1> or 1>>
standard error	stderr	/dev/fd/2	2> or 2>>

POSIX Shell Plumbing

The POSIX (Bourne) shell and its derivatives provides the ability to utilize various symbols to plumb processes together by their file descriptors.

Symbol Name	_Symbol_Syntax_	________Function________
pipe	{ block } | { block }	attach stdout of left script block to  stdin of right script block
success exec	cmd && { block }	if cmd exits with success (0) then execute block
failure exec	cmd || { block }	if cmd exits with error (not 0) then execute block
write file	{ block } #>file	attach /dev/fd/# of left script block to file and overwrite with bytestream
write fd	{ block } #>&?	attach /dev/fd/# of left script block to /dev/fd/? and overwrite with bytestream
append	{ block } #>>file	attach /dev/fd/# of left script block to file and append bytestream
close stdout	#>&-	close /dev/fd/# (no # specified defaults to stdout)
read file	{ block } <file	attach stdin of left script block to file and read bytestream
read until	{ block } <&?	attach stdin of left script block to /dev/fd/? and read bytstream
read until	{ block } <<word	attach stdin of left script block to stdin of proc and read until word
close stdin	<&-	close stdin

Process Substitution

The POSIX shell also provides the ability to perform inline substitution utilizing two methods:

_Symbol_ Name	Symbol_Syntax	________Function________
sub-shell exec	( block )	execute process proc without environment persistance
sub-block exec	{ block; }	execute proc resulting inline at location within script with evnironment persistance

…for examples see below

Continue reading →

Bert Testing

loopback local

Puts the locally attached DSU (Internal to the Router) in Loop. This allows the testing of the local Card for errors with the line driver or modulator

loopback remote

Requests a loopback from the remote DSU. This allows the testing of the full circuit up to the DSU at the far end.

loopback network

Sets the local controller to loop towards the network allowing the far end to run test patterns across the entire path.

Testing Examples:

controller t3 1/0 !-- test the local hardware
 loopback local
 !
 bert pattern 0s interval 1
 do show controller t1 0/0
 !
 bert pattern 1s interval 1
 do show controller t1 0/0
 !
 bert pattern alt-0-1 interval 1
 do show controller t1 0/0
 !
 no loopback

controller t3 1/0 !-- test to the remote DSU.
 loopback remote
 bert pattern 0s interval 1
 do show controller t3 1/0
 !
 bert pattern 1s interval 1
 do show controller t3 1/0
 !
 bert pattern alt-0-1 interval 1
 do show controller t3 1/0
 !
 no loopback

controller t3 1/0 !-- allow the far end to perform end-to-end testing.
 loopback network
 !
 !-- WAIT FOR TESTING AT FAR END TO COMPLETE..
 !
 no loopback
!
end

Circuit Locating

You can transmit endpoint location information across a circuit using the ANSI Maintenance Description Set. The following example shows how this is specified:

Circuit Labeling Example:

controller t3 1/0 !-- Identification notation transmited via ANSI MDL Frames
 mdl string eic RouterA     !-- Equiptment ID Code
 mdl string fic 0x01        !-- Frame ID Code
 mdl string generator gen-1 !-- Generator Number in MDL Test Signal Message
 mdl string lic OKC         !-- Location ID Code
 mdl string pfi DT          !-- Facility ID Code
 mdl string port 1-0        !-- Port Number in MDL Idle Signal Message
 mdl string unit 2          !-- Unit Code
 !
 mdl transmit test-signal
!
end

Viewing example at far end of circuit:

show controllers t3 1/0

Results:
T3 1/0 is up.
  Applique type is Subrate T3
  Transmitter is sending AIS.
  Receiver has no alarms.
  MDL transmission is enabled
     EIC: Router-B, LIC: TUL, FIC: FO, UNIT: 1
  Far-End MDL Information Received
     EIC: Router-A, LIC: OKC, FIC: DT, UNIT: 1
  FEAC code received: No code is being received
  Framing is C-BIT Parity, Line Code is B3ZS, Clock Source is Line
  Data in current interval (30 seconds elapsed):
     0 Line Code Violations, 0 P-bit Coding Violation
     0 C-bit Coding Violation, 0 P-bit Err Secs
     0 P-bit Severely Err Secs, 0 Severely Err Framing Secs
     0 Unavailable Secs, 0 Line Errored Secs
     0 C-bit Errored Secs, 0 C-bit Severely Errored Secs
!
end