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Updated: Wednesday, February 9, 2022

zshtcpsys (1)


zshtcpsys - zsh tcp system


Please see following description for synopsis


ZSHTCPSYS(1)                General Commands Manual               ZSHTCPSYS(1)

       zshtcpsys - zsh tcp system

       A  module  zsh/net/tcp  is  provided to provide network I/O over TCP/IP
       from within the shell; see its description in zshmodules(1).  This man-
       ual page describes a function suite based on the module.  If the module
       is installed, the functions are usually installed at the same time,  in
       which  case they will be available for autoloading in the default func-
       tion search path.  In addition to the zsh/net/tcp module, the  zsh/zse-
       lect  module  is  used  to  implement timeouts on read operations.  For
       troubleshooting tips, consult the corresponding  advice  for  the  zftp
       functions described in zshzftpsys(1).

       There  are  functions  corresponding  to the basic I/O operations open,
       close, read and send, named  tcp_open  etc.,  as  well  as  a  function
       tcp_expect  for pattern match analysis of data read as input.  The sys-
       tem makes it easy to receive data from and send data to multiple  named
       sessions  at once.  In addition, it can be linked with the shell's line
       editor in such a way that input data is automatically shown at the ter-
       minal.   Other  facilities  available  including logging, filtering and
       configurable output prompts.

       To use the system where  it  is  available,  it  should  be  enough  to
       `autoload  -U tcp_open' and run tcp_open as documented below to start a
       session.  The tcp_open function will autoload the remaining functions.

   Basic I/O
       tcp_open [ -qz ] host port [ sess ]
       tcp_open [ -qz ] [ -s sess | -l sess[,...] ] ...
       tcp_open [ -qz ] [ -a fd | -f fd ] [ sess ]
              Open a new session.  In the first and simplest form, open a  TCP
              connection to host host at port port; numeric and symbolic forms
              are understood for both.

              If sess is given, this becomes the name of the session which can
              be used to refer to multiple different TCP connections.  If sess
              is not given, the function will  invent  a  numeric  name  value
              (note  this  is not the same as the file descriptor to which the
              session is attached).  It is recommended that session names  not
              include  `funny'  characters,  where  funny  characters  are not
              well-defined but  certainly  do  not  include  alphanumerics  or
              underscores, and certainly do include whitespace.

              In  the second case, one or more sessions to be opened are given
              by name.  A  single  session  name  is  given  after  -s  and  a
              comma-separated  list  after -l; both options may be repeated as
              many times as necessary.  A failure to open any  session  causes
              tcp_open  to  abort.   The  host and port are read from the file
              .ztcp_sessions in the same directory as the user's zsh initiali-
              sation  files,  i.e. usually the home directory, but $ZDOTDIR if
              that is set.  The file consists of lines each giving  a  session
              name  and  the  corresponding host and port, in that order (note
              the session name comes first, not  last),  separated  by  white-

              The  third form allows passive and fake TCP connections.  If the
              option -a is used, its argument is a file  descriptor  open  for
              listening for connections.  No function front-end is provided to
              open such a file descriptor, but a call to `ztcp -l  port'  will
              create  one  with  the  file  descriptor stored in the parameter
              $REPLY.  The listening port can be closed with `ztcp -c fd'.   A
              call  to  `tcp_open -a fd' will block until a remote TCP connec-
              tion is made to port on the local machine.   At  this  point,  a
              session  is  created  in  the usual way and is largely indistin-
              guishable from an active connection  created  with  one  of  the
              first two forms.

              If  the  option  -f  is  used, its argument is a file descriptor
              which is used directly as if it were a TCP  session.   How  well
              the remainder of the TCP function system copes with this depends
              on what actually underlies this file descriptor.  A regular file
              is  likely  to be unusable; a FIFO (pipe) of some sort will work
              better, but note that it is not a good idea  for  two  different
              sessions to attempt to read from the same FIFO at once.

              If  the option -q is given with any of the three forms, tcp_open
              will not print informational messages, although it will  in  any
              case exit with an appropriate status.

              If  the line editor (zle) is in use, which is typically the case
              if the shell is interactive, tcp_open installs a handler  inside
              zle  which will check for new data at the same time as it checks
              for keyboard input.  This is convenient as the shell consumes no
              CPU  time  while waiting; the test is performed by the operating
              system.  Giving the option -z to any of the  forms  of  tcp_open
              prevents  the handler from being installed, so data must be read
              explicitly.  Note, however, this is not necessary for  executing
              complete  sets of send and read commands from a function, as zle
              is not active at this point.  Generally speaking, the handler is
              only  active  when  the  shell is waiting for input at a command
              prompt or in the vared builtin.  The option has no effect if zle
              is not active; `[[ -o zle]]' will test for this.

              The  first  session to be opened becomes the current session and
              subsequent calls to tcp_open do not change it.  The current ses-
              sion  is  stored  in the parameter $TCP_SESS; see below for more
              detail about the parameters used by the system.

              The function tcp_on_open, if defined, is called when  a  session
              is opened.  See the description below.

       tcp_close [ -qn ] [ -a | -l sess[,...] | sess ... ]
              Close  the  named  sessions,  or  the current session if none is
              given, or all open sessions if -a is given.  The options -l  and
              -s  are both handled for consistency with tcp_open, although the
              latter is redundant.

              If the session being closed is the  current  one,  $TCP_SESS  is
              unset,  leaving no current session, even if there are other ses-
              sions still open.

              If the session was opened with tcp_open -f, the file  descriptor
              is  closed  so  long  as  it  is  in the range 0 to 9 accessible
              directly from the command line.  If the option -n is  given,  no
              attempt  will  be  made  to close file descriptors in this case.
              The -n option is not used for genuine  ztcp  session;  the  file
              descriptors are always closed with the session.

              If  the  option  -q  is given, no informational messages will be

       tcp_read [ -bdq ] [ -t TO ] [ -T TO ]
                [ -a | -u fd[,...] | -l sess[,...] | -s sess ... ]
              Perform a read operation on the current session, or on a list of
              sessions  if  any  are given with -u, -l or -s, or all open ses-
              sions if the option -a is given.   Any  of  the  -u,  -l  or  -s
              options may be repeated or mixed together.  The -u option speci-
              fies a file descriptor directly (only those managed by this sys-
              tem are useful), the other two specify sessions as described for
              tcp_open above.

              The function checks for new data available on all  the  sessions
              listed.   Unless the -b option is given, it will not block wait-
              ing for new data.  Any one line of data from any of  the  avail-
              able  sessions  will be read, stored in the parameter $TCP_LINE,
              and displayed to standard output unless $TCP_SILENT  contains  a
              non-empty  string.   When  printed to standard output the string
              $TCP_PROMPT will be shown at the start of the line; the  default
              form  for this includes the name of the session being read.  See
              below for more information on these parameters.  In  this  mode,
              tcp_read  can  be  called  repeatedly  until it returns status 2
              which indicates all pending input from  all  specified  sessions
              has been handled.

              With the option -b, equivalent to an infinite timeout, the func-
              tion will block until a line is available to read  from  one  of
              the   specified  sessions.   However,  only  a  single  line  is

              The option  -d  indicates  that  all  pending  input  should  be
              drained.   In  this  case tcp_read may process multiple lines in
              the manner given above; only the last is  stored  in  $TCP_LINE,
              but the complete set is stored in the array $tcp_lines.  This is
              cleared at the start of each call to tcp_read.

              The options -t and -T specify a timeout in seconds, which may be
              a  floating  point  number  for increased accuracy.  With -t the
              timeout is applied before each line read.  With -T, the  timeout
              applies  to  the  overall operation, possibly including multiple
              read operations if  the  option  -d  is  present;  without  this
              option, there is no distinction between -t and -T.

              The  function  does not print informational messages, but if the
              option -q is given, no error message is printed for a  non-exis-
              tent session.

              A  return  status  of  2 indicates a timeout or no data to read.
              Any other non-zero return status indicates some error condition.

              See tcp_log for how to control where data is sent by tcp_read.

       tcp_send [ -cnq ] [ -s sess | -l sess[,...] ] data ...
       tcp_send [ -cnq ] -a data ...
              Send the supplied data strings to all the specified sessions  in
              turn.  The underlying operation differs little from a `print -r'
              to the session's file descriptor, although it attempts  to  pre-
              vent  the  shell  from  dying  owing  to  a SIGPIPE caused by an
              attempt to write to a defunct session.

              The option -c causes tcp_send to  behave  like  cat.   It  reads
              lines  from  standard input until end of input and sends them in
              turn to the specified session(s) exactly as if they  were  given
              as data arguments to individual tcp_send commands.

              The  option  -n  prevents tcp_send from putting a newline at the
              end of the data strings.

              The remaining options all behave as for tcp_read.

              The data arguments are not further processed once they have been
              passed to tcp_send; they are simply passed down to print -r.

              If  the  parameter $TCP_OUTPUT is a non-empty string and logging
              is enabled then the data sent to each session will be echoed  to
              the  log  file(s)  with  $TCP_OUTPUT in front where appropriate,
              much in the manner of $TCP_PROMPT.

   Session Management
       tcp_alias [ -q ] alias=sess ...
       tcp_alias [ -q ] [ alias ... ]
       tcp_alias -d [ -q ] alias ...
              This function is not particularly well tested.

              The first form creates an alias for a session  name;  alias  can
              then  be  used  to  refer to the existing session sess.  As many
              aliases may be listed as required.

              The second form lists any aliases specified, or all  aliases  if

              The  third  form deletes all the aliases listed.  The underlying
              sessions are not affected.

              The option -q suppresses  an  inconsistently  chosen  subset  of
              error messages.

       tcp_log [ -asc ] [ -n | -N ] [ logfile ]
              With an argument logfile, all future input from tcp_read will be
              logged to the named file.  Unless -a  (append)  is  given,  this
              file  will  first  be truncated or created empty.  With no argu-
              ments, show the current status of logging.

              With the option -s, per-session logging is enabled.  Input  from
              tcp_read  is output to the file logfile.sess.  As the session is
              automatically discriminated by the filename,  the  contents  are
              raw   (no  $TCP_PROMPT).   The  option   -a  applies  as  above.
              Per-session logging and logging of all data in one file are  not
              mutually exclusive.

              The  option -c closes all logging, both complete and per-session

              The options -n and -N respectively turn off or restore output of
              data  read  by  tcp_read to standard output; hence `tcp_log -cn'
              turns off all output by tcp_read.

              The function is purely a convenient front  end  to  setting  the
              parameters   $TCP_LOG,  $TCP_LOG_SESS,  $TCP_SILENT,  which  are
              described below.

       tcp_rename old new
              Rename session  old  to  session  new.   The  old  name  becomes

       tcp_sess [ sess [ command [ arg ... ] ] ]
              With  no  arguments,  list  all the open sessions and associated
              file descriptors.  The current session is marked  with  a  star.
              For   use   in   functions,  direct  access  to  the  parameters
              $tcp_by_name, $tcp_by_fd and $TCP_SESS is probably  more  conve-
              nient; see below.

              With  a sess argument, set the current session to sess.  This is
              equivalent to changing $TCP_SESS directly.

              With additional arguments, temporarily set the  current  session
              while  executing  `command arg ...'.  command is re-evaluated so
              as to expand aliases etc., but the  remaining  args  are  passed
              through  as  that  appear  to tcp_sess.  The original session is
              restored when tcp_sess exits.

   Advanced I/O
       tcp_command send-option ... send-argument ...
              This is a convenient front-end to tcp_send.  All  arguments  are
              passed  to  tcp_send, then the function pauses waiting for data.
              While data is arriving at least every $TCP_TIMEOUT (default 0.3)
              seconds,  data  is handled and printed out according to the cur-
              rent settings.  Status 0 is always returned.

              This is generally only useful for interactive  use,  to  prevent
              the display becoming fragmented by output returned from the con-
              nection.  Within a programme or function it is generally  better
              to handle reading data by a more explicit method.

       tcp_expect [ -q ] [ -p var | -P var ] [ -t TO | -T TO ]
                  [ -a | -s sess | -l sess[,...] ] pattern ...
              Wait  for  input  matching any of the given patterns from any of
              the specified sessions.  Input is ignored until  an  input  line
              matches  one of the given patterns; at this point status zero is
              returned, the matching line is stored in $TCP_LINE, and the full
              set of lines read during the call to tcp_expect is stored in the
              array $tcp_expect_lines.

              Sessions are specified in the same way as tcp_read: the  default
              is  to use the current session, otherwise the sessions specified
              by -a, -s, or -l are used.

              Each pattern is a standard zsh extended-globbing  pattern;  note
              that  it  needs  to be quoted to avoid it being expanded immedi-
              ately by filename generation.  It must match the full  line,  so
              to  match  a substring there must be a `*' at the start and end.
              The line matched  against  includes  the  $TCP_PROMPT  added  by
              tcp_read.   It is possible to include the globbing flags `#b' or
              `#m' in the patterns to make  backreferences  available  in  the
              parameters  $MATCH,  $match,  etc., as described in the base zsh
              documentation on pattern matching.

              Unlike tcp_read, the default behaviour of tcp_expect is to block
              indefinitely  until  the  required  input is found.  This can be
              modified by specifying a timeout with -t or -T;  these  function
              as  in  tcp_read,  specifying  a  per-read  or  overall timeout,
              respectively, in seconds, as an integer or  floating-point  num-
              ber.   As  tcp_read,  the function returns status 2 if a timeout

              The function returns as soon as any one of  the  patterns  given
              match.   If  the  caller  needs  to  know  which of the patterns
              matched, the option -p var can be used; on return, $var  is  set
              to  the  number of the pattern using ordinary zsh indexing, i.e.
              the first is 1, and so on.  Note the absence of a `$'  in  front
              of  var.   To  avoid  clashes,  the  parameter cannot begin with
              `_expect'.  The index -1 is used if there is a timeout and 0  if
              there is no match.

              The  option -P var works similarly to -p, but instead of numeri-
              cal indexes the regular arguments must begin with a prefix  fol-
              lowed by a colon: that prefix is then used as a tag to which var
              is set when the argument matches.  The tag timeout  is  used  if
              there  is  a  timeout and the empty string if there is no match.
              Note it is acceptable for different arguments to start with  the
              same prefix if the matches do not need to be distinguished.

              The option -q is passed directly down to tcp_read.

              As  all  input  is  done via tcp_read, all the usual rules about
              output of lines read apply.  One exception is that the parameter
              $tcp_lines  will  only  reflect  the  line  actually  matched by
              tcp_expect; use $tcp_expect_lines for the full set of lines read
              during the function call.

              This  is a simple-minded function to accept a TCP connection and
              execute  a  command  with  I/O  redirected  to  the  connection.
              Extreme  caution should be taken as there is no security whatso-
              ever and this can leave your computer open to the  world.   Ide-
              ally, it should only be used behind a firewall.

              The first argument is a TCP port on which the function will lis-

              The remaining arguments give a command and its arguments to exe-
              cute  with  standard  input,  standard output and standard error
              redirected to the file descriptor on which the TCP  session  has
              been  accepted.   If  no command is given, a new zsh is started.
              This gives everyone  on  your  network  direct  access  to  your
              account, which in many cases will be a bad thing.

              The  command  is  run  in  the background, so tcp_proxy can then
              accept new connections.  It continues to accept new  connections
              until interrupted.

       tcp_spam [ -ertv ] [ -a | -s sess | -l sess[,...] ] cmd [ arg ... ]
              Execute  `cmd  [ arg ... ]' for each session in turn.  Note this
              executes the command and arguments; it does not send the command
              line as data unless the -t (transmit) option is given.

              The sessions may be selected explicitly with the standard -a, -s
              or -l options, or may be chosen  implicitly.   If  none  of  the
              three  options  is  given  the  rules  are:  first, if the array
              $tcp_spam_list is set, this is taken as the  list  of  sessions,
              otherwise all sessions are taken.  Second, any sessions given in
              the array $tcp_no_spam_list are removed from the  list  of  ses-

              Normally,  any  sessions added by the `-a' flag or when all ses-
              sions are chosen implicitly are  spammed  in  alphabetic  order;
              sessions  given  by  the  $tcp_spam_list array or on the command
              line are spammed in the order given.  The -r flag  reverses  the
              order however it was arrived it.

              The  -v  flag specifies that a $TCP_PROMPT will be output before
              each session.  This is output after any modification to TCP_SESS
              by   the  user-defined  tcp_on_spam  function  described  below.
              (Obviously that function is able to generate its own output.)

              If the option -e is present, the line given as `cmd [ arg ... ]'
              is  executed  using  eval,  otherwise it is executed without any
              further processing.

              This is a fairly simple-minded attempt to  force  input  to  the
              line editor to go straight to the default TCP_SESS.

              An  escape  string,  $TCP_TALK_ESCAPE,  default  `:', is used to
              allow access to normal shell operation.  If it is on its own  at
              the  start of the line, or followed only by whitespace, the line
              editor returns to normal operation.  Otherwise, the  string  and
              any  following  whitespace  are skipped and the remainder of the
              line executed as shell input without any change of the line edi-
              tor's operating mode.

              The current implementation is somewhat deficient in terms of use
              of the command history.  For this reason, many users will prefer
              to use some form of alternative approach for sending data easily
              to the current session.  One simple approach is  to  alias  some
              special character (such as `%') to `tcp_command --'.

              The  sole  argument is an integer or floating point number which
              gives the seconds to delay.  The shell will do nothing for  that
              period  except  wait  for  input  on all TCP sessions by calling
              tcp_read -a.  This is similar to the  interactive  behaviour  at
              the command prompt when zle handlers are installed.

   `One-shot' file transfer
       tcp_point port
       tcp_shoot host port
              This  pair  of functions provide a simple way to transfer a file
              between two hosts within the shell.  Note,  however,  that  bulk
              data  transfer is currently done using cat.  tcp_point reads any
              data arriving at port and sends it to standard output; tcp_shoot
              connects  to  port  on  host  and sends its standard input.  Any
              unused port may be used; the standard mechanism  for  picking  a
              port  is to think of a random four-digit number above 1024 until
              one works.

              To transfer a file from  host  woodcock  to  host  springes,  on

                     tcp_point 8091 >output_file

              and on woodcock:

                     tcp_shoot springes 8091 <input_file

              As  these  two functions do not require tcp_open to set up a TCP
              connection first, they may need to be autoloaded separately.

       Certain functions, if defined by the user, will be called by the  func-
       tion  system  in certain contexts.  This facility depends on the module
       zsh/parameter, which is usually available in interactive shells as  the
       completion  system  depends  on  it.   None  of  the  functions need be
       defined; they simply provide convenient hooks when necessary.

       Typically, these are called after the requested action has been  taken,
       so that the various parameters will reflect the new state.

       tcp_on_alias alias fd
              When  an alias is defined, this function will be called with two
              arguments: the name of the alias, and the file descriptor of the
              corresponding session.

       tcp_on_awol sess fd
              If  the  function tcp_fd_handler is handling input from the line
              editor and detects that the file descriptor is no  longer  reus-
              able, by default it removes it from the list of file descriptors
              handled by this method and prints a message.   If  the  function
              tcp_on_awol  is  defined  it  is  called immediately before this
              point.  It may return status 100, which indicates that the  nor-
              mal  handling should still be performed; any other return status
              indicates that  no  further  action  should  be  taken  and  the
              tcp_fd_handler  should return immediately with the given status.
              Typically the action of tcp_on_awol will be to  close  the  ses-

              The variable TCP_INVALIDATE_ZLE will be a non-empty string if it
              is necessary to invalidate the line editor  display  using  `zle
              -I' before printing output from the function.

              (`AWOL'  is  military  jargon for `absent without leave' or some
              variation.  It has no pre-existing technical  meaning  known  to
              the author.)

       tcp_on_close sess fd
              This  is  called with the name of a session being closed and the
              file descriptor which corresponded to that session.   Both  will
              be invalid by the time the function is called.

       tcp_on_open sess fd
              This  is  called  after  a new session has been defined with the
              session name and file descriptor as arguments.  If it returns  a
              non-zero  status, opening the session is assumed to fail and the
              session is closed again;  however,  tcp_open  will  continue  to
              attempt  to  open  any  remaining  sessions given on the command

       tcp_on_rename oldsess fd newsess
              This is called after a session has been renamed with  the  three
              arguments old session name, file descriptor, new session name.

       tcp_on_spam sess command ...
              This is called once for each session spammed, just before a com-
              mand is executed for a session by tcp_spam.  The  arguments  are
              the  session  name  followed by the command list to be executed.
              If tcp_spam was called with the option  -t,  the  first  command
              will be tcp_send.

              This  function  is  called after $TCP_SESS is set to reflect the
              session to be spammed, but before any use of it is made.   Hence
              it is possible to alter the value of $TCP_SESS within this func-
              tion.  For example, the  session  arguments  to  tcp_spam  could
              include  extra  information  to be stripped off and processed in

              If the function sets the parameter $REPLY to `done', the command
              line  is not executed; in addition, no prompt is printed for the
              -v option to tcp_spam.

       tcp_on_unalias alias fd
              This is called with the name of an alias and  the  corresponding
              session's file descriptor after an alias has been deleted.

       The  following  functions  are used by the TCP function system but will
       rarely if ever need to be called directly.

              This is the function installed by tcp_open  for  handling  input
              from  within the line editor, if that is required.  It is in the
              format documented for the builtin `zle -F' in zshzle(1) .

              While active, the function sets the parameter TCP_HANDLER_ACTIVE
              to 1.  This allows shell code called internally (for example, by
              setting tcp_on_read) to tell if is being called when  the  shell
              is otherwise idle at the editor prompt.

       tcp_output [ -q ] -P prompt -F fd -S sess
              This  function  is  used for both logging and handling output to
              standard output, from within tcp_read  and  (if  $TCP_OUTPUT  is
              set) tcp_send.

              The  prompt  to use is specified by -P; the default is the empty
              string.  It can contain:
              %c     Expands to 1 if the session is the current session,  oth-
                     erwise   0.    Used  with  ternary  expressions  such  as
                     `%(c.-.+)' to output `+' for the current session and  `-'

              %f     Replaced by the session's file descriptor.

              %s     Replaced by the session name.

              %%     Replaced by a single `%'.

              The  option  -q suppresses output to standard output, but not to
              any log files which are configured.

              The -S and -F options are used to pass in the session  name  and
              file descriptor for possible replacement in the prompt.

       Parameters  follow  the  usual  convention  that  uppercase is used for
       scalars and integers, while lowercase is used for normal  and  associa-
       tive  array.  It is always safe for user code to read these parameters.
       Some parameters may also be set; these are  noted  explicitly.   Others
       are  included  in this group as they are set by the function system for
       the user's benefit, i.e. setting them is typically not  useful  but  is

       It  is  often  also useful to make settable parameters local to a func-
       tion.  For example, `local TCP_SILENT=1' specifies that data read  dur-
       ing  the  function call will not be printed to standard output, regard-
       less  of  the  setting  outside   the   function.    Likewise,   `local
       TCP_SESS=sess'  sets  a  session  for  the  duration of a function, and
       `local TCP_PROMPT=' specifies that no prompt is used for  input  during
       the function.

              Array.    The  set  of  lines  read  during  the  last  call  to
              tcp_expect, including the last ($TCP_LINE).

              Array. May be set directly.  A set of extended globbing patterns
              which,  if  matched in tcp_output, will cause the line not to be
              printed to standard output.  The patterns should be  defined  as
              described  for  the  arguments to tcp_expect.  Output of line to
              log files is not affected.

              Scalar.  Set to 1 within tcp_fd_handler to indicate to functions
              called  recursively  that they have been called during an editor
              session.  Otherwise unset.

              The last line read by tcp_read, and hence also tcp_expect.

              The   file   descriptor   from   which   $TCP_LINE   was   read.
              ${tcp_by_fd[$TCP_LINE_FD]}  will  give the corresponding session

              Array. The set of lines read during the last call  to  tcp_read,
              including the last ($TCP_LINE).

              May  be set directly, although it is also controlled by tcp_log.
              The name of a file to which output from  all  sessions  will  be
              sent.   The output is proceeded by the usual $TCP_PROMPT.  If it
              is not an absolute path name, it will follow the user's  current

              May  be set directly, although it is also controlled by tcp_log.
              The prefix for a set of files to which output from each  session
              separately    will    be    sent;    the    full   filename   is
              ${TCP_LOG_SESS}.sess.  Output to each file is raw; no prompt  is
              added.   If  it is not an absolute path name, it will follow the
              user's current directory.

              Array.  May be set directly.  See tcp_spam for how this is used.

              May be set directly.  If a non-empty string, any data sent to  a
              session  by  tcp_send  will be logged.  This parameter gives the
              prompt to be used in a file specified by $TCP_LOG but not  in  a
              file  generated  from  $TCP_LOG_SESS.  The prompt string has the
              same format as TCP_PROMPT and the same rules for its use apply.

              May be set directly.  Used  as  the  prefix  for  data  read  by
              tcp_read  which is printed to standard output or to the log file
              given by $TCP_LOG, if any.  Any `%s', `%f' or `%%' occurring  in
              the string will be replaced by the name of the session, the ses-
              sion's underlying file descriptor,  or  a  single  `%',  respec-
              tively.   The  expression `%c' expands to 1 if the session being
              read is the current session, else 0;  this  is  most  useful  in
              ternary  expressions such as `%(c.-.+)' which outputs `+' if the
              session is the current one, else `-'.

              If the prompt starts with %P, this is stripped and the  complete
              result  of  the previous stage is passed through standard prompt
              %-style formatting before being output.

              May be set directly.  If this has non-zero length, tcp_read will
              give some limited diagnostics about data being read.

              This value is created and initialised to zero by tcp_open.

              The  functions  tcp_read  and tcp_expect use the shell's SECONDS
              parameter for their own timing purposes.  If that  parameter  is
              not  of floating point type on entry to one of the functions, it
              will create a local parameter SECONDS which  is  floating  point
              and set the parameter TCP_SECONDS_START to the previous value of
              $SECONDS.  If the parameter is already  floating  point,  it  is
              used without a local copy being created and TCP_SECONDS_START is
              not set.  As the global value is zero, the shell elapsed time is
              guaranteed to be the sum of $SECONDS and $TCP_SECONDS_START.

              This  can  be  avoided by setting SECONDS globally to a floating
              point value using `typeset -F SECONDS'; then the  TCP  functions
              will  never make a local copy and never set TCP_SECONDS_START to
              a non-zero value.

              May be set directly.  The current session; must refer to one  of
              the sessions established by tcp_open.

              May  be set directly, although it is also controlled by tcp_log.
              If of non-zero length, data read by tcp_read will not be written
              to standard output, though may still be written to a log file.

              Array.   May  be set directly.  See the description of the func-
              tion tcp_spam for how this is used.

              May be set  directly.   See  the  description  of  the  function
              tcp_talk for how this is used.

              May  be  set directly.  Currently this is only used by the func-
              tion tcp_command, see above.

       The following parameters are not set by the function system, but have a
       special effect if set by the user.

              This should be an associative array; if it is not, the behaviour
              is undefined.  Each key is the name of a shell function or other
              command,  and  the corresponding value is a shell pattern (using
              EXTENDED_GLOB).  Every line read from a TCP session directly  or
              indirectly   using   tcp_read  (which  includes  lines  read  by
              tcp_expect) is  compared  against  the  pattern.   If  the  line
              matches,  the  command given in the key is called with two argu-
              ments: the name of the session from which the line was read, and
              the line itself.

              If  any function called to handle a line returns a non-zero sta-
              tus, the line is not output.  Thus a  tcp_on_read  handler  con-
              taining  only the instruction `return 1' can be used to suppress
              output of particular lines  (see,  however,  tcp_filter  above).
              However,  the  line  is  still stored in TCP_LINE and tcp_lines;
              this occurs after all tcp_on_read processing.

       These parameters are controlled by the function  system;  they  may  be
       read directly, but should not usually be set by user code.

              Associative  array.   The  keys are the names of sessions estab-
              lished with tcp_open; each value is a  space-separated  list  of
              aliases which refer to that session.

              Associative  array.  The keys are session file descriptors; each
              value is the name of that session.

              Associative array.  The keys are the  names  of  sessions;  each
              value is the file descriptor associated with that session.

       Here is a trivial example using a remote calculator.

       To  create a calculator server on port 7337 (see the dc manual page for
       quite how infuriating the underlying command is):

              tcp_proxy 7337 dc

       To connect to this from the same host with a session also named `dc':

              tcp_open localhost 7337 dc

       To send a command to the remote session and wait a short while for out-
       put (assuming dc is the current session):

              tcp_command 2 4 + p

       To close the session:


       The  tcp_proxy  needs  to  be killed to be stopped.  Note this will not
       usually kill any connections which have already been accepted, and also
       that the port is not immediately available for reuse.

       The  following  chunk  of  code  puts  a list of sessions into an xterm
       header, with the current session followed by a star.

              print -n "\033]2;TCP:" ${(k)tcp_by_name:/$TCP_SESS/$TCP_SESS\*} "\a"

       The function tcp_read uses the shell's normal read  builtin.   As  this
       reads a complete line at once, data arriving without a terminating new-
       line can cause the function to block indefinitely.

       Though the function suite works well for interactive use and  for  data
       arriving  in  small amounts, the performance when large amounts of data
       are being exchanged is likely to be extremely poor.

zsh 5.8                        February 14, 2020                  ZSHTCPSYS(1)