libexpect(3) — Linux manual page


LIBEXPECT(3)              Library Functions Manual              LIBEXPECT(3)

NAME         top

       libexpect - programmed dialogue library with interactive programs

DESCRIPTION         top

       This library contains functions that allow Expect to be used as a Tcl
       extension or to be used directly from C or C++ (without Tcl).  Adding
       Expect as a Tcl extension is very short and simple, so that will be
       covered first.

SYNOPSIS         top

       #include expect_tcl.h

       cc files... -lexpect5.20 -ltcl7.5 -lm

       Note: library versions may differ in the actual release.

       The Expect_Init function adds expect commands to the named
       interpreter.  It avoids overwriting commands that already exist,
       however aliases beginning with "exp_" are always created for expect
       commands.  So for example, "send" can be used as "exp_send".

       Generally, you should only call Expect commands via Tcl_Eval.
       Certain auxiliary functions may be called directly.  They are
       summarized below.  They may be useful in constructing your own main.
       Look at the file exp_main_exp.c in the Expect distribution as a
       prototype main.  Another prototype is tclAppInit.c in the Tcl source
       distribution.  A prototype for working with Tk is in exp_main_tk.c in
       the Expect distribution.

       int exp_cmdlinecmds;
       int exp_interactive;
       FILE *exp_cmdfile;
       char *exp_cmdfilename;
       int exp_tcl_debugger_available;

       void exp_parse_argv(Tcl_Interp *,int argc,char **argv);
       int  exp_interpreter(Tcl_Interp *);
       void exp_interpret_cmdfile(Tcl_Interp *,FILE *);
       void exp_interpret_cmdfilename(Tcl_Interp *,char *);
       void exp_interpret_rcfiles(Tcl_Interp *,int my_rc,int sys_rc);
       char *    exp_cook(char *s,int *len);
       void (*exp_app_exit)EXP_PROTO((Tcl_Interp *);
       void exp_exit(Tcl_Interp *,int status);
       void exp_exit_handlers(Tcl_Interp *);
       void exp_error(Tcl_Interp,char *,...);

       exp_cmdlinecmds is 1 if Expect has been invoked with commands on the
       program command-line (using "-c" for example).  exp_interactive is 1
       if Expect has been invoked with the -i flag or if no commands or
       script is being invoked.  exp_cmdfile is a stream from which Expect
       will read commands.  exp_cmdfilename is the name of a file which
       Expect will open and read commands from.  exp_tcl_debugger_available
       is 1 if the debugger has been armed.

       exp_parse_argv reads the representation of the command line.  Based
       on what is found, any of the other variables listed here are
       initialized appropriately.  exp_interpreter interactively prompts the
       user for commands and evaluates them.  exp_interpret_cmdfile reads
       the given stream and evaluates any commands found.
       exp_interpret_cmdfilename opens the named file and evaluates any
       commands found.  exp_interpret_rcfiles reads and evalutes the .rc
       files.  If my_rc is zero, then ~/.expectrc is skipped.  If sys_rc is
       zero, then the system-wide expectrc file is skipped.  exp_cook
       returns a static buffer containing the argument reproduced with
       newlines replaced by carriage-return linefeed sequences.  The primary
       purpose of this is to allow messages to be produced without worrying
       about whether the terminal is in raw mode or cooked mode.  If length
       is zero, it is computed via strlen.  exp_error is a printf-like
       function that writes the result to interp->result.

SYNOPSIS         top

       #include <expect.h>

       exp_spawnl(file, arg0 [, arg1, ..., argn] (char *)0);
       char *file;
       char *arg0, *arg1, ... *argn;

       char *file, *argv[ ];

       int fd;

       FILE *
       char *command;

       extern int exp_pid;
       extern int exp_ttyinit;
       extern int exp_ttycopy;
       extern int exp_console;
       extern char *exp_stty_init;
       extern void (*exp_close_in_child)();
       extern void (*exp_child_exec_prelude)();
       extern void exp_close_tcl_files();

       cc files... -lexpect -ltcl -lm

DESCRIPTION         top

       exp_spawnl and exp_spawnv fork a new process so that its stdin,
       stdout, and stderr can be written and read by the current process.
       file is the name of a file to be executed.  The arg pointers are
       null-terminated strings.  Following the style of execve(), arg0 (or
       argv[0]) is customarily a duplicate of the name of the file.

       Four interfaces are available, exp_spawnl is useful when the number
       of arguments is known at compile time.  exp_spawnv is useful when the
       number of arguments is not known at compile time.  exp_spawnfd is
       useful when an open file descriptor is already available as a source.
       exp_popen is explained later on.

       If the process is successfully created, a file descriptor is returned
       which corresponds to the process's stdin, stdout and stderr.  A
       stream may be associated with the file descriptor by using fdopen().
       (This should almost certainly be followed by setbuf() to unbuffer the

       Closing the file descriptor will typically be detected by the process
       as an EOF.  Once such a process exits, it should be waited upon (via
       wait) in order to free up the kernel process slot.  (Some systems
       allow you to avoid this if you ignore the SIGCHLD signal).

       exp_popen is yet another interface, styled after popen().  It takes a
       Bourne shell command line, and returns a stream that corresponds to
       the process's stdin, stdout and stderr.  The actual implementation of
       exp_popen below demonstrates exp_spawnl.

       FILE *
       char *program;
            FILE *fp;
            int ec;

            if (0 > (ec = exp_spawnl("sh","sh","-c",program,(char *)0)))
            if (NULL == (fp = fdopen(ec,"r+")) return(0);
            setbuf(fp,(char *)0);

       After a process is started, the variable exp_pid is set to the
       process-id of the new process.  The variable exp_pty_slave_name is
       set to the name of the slave side of the pty.

       The spawn functions uses a pty to communicate with the process.  By
       default, the pty is initialized the same way as the user's tty (if
       possible, i.e., if the environment has a controlling terminal.)  This
       initialization can be skipped by setting exp_ttycopy to 0.

       The pty is further initialized to some system wide defaults if
       exp_ttyinit is non-zero.  The default is generally comparable to
       "stty sane".

       The tty setting can be further modified by setting the variable
       exp_stty_init.  This variable is interpreted in the style of stty
       arguments.  For example, exp_stty_init = "sane"; repeats the default

       On some systems, it is possible to redirect console output to ptys.
       If this is supported, you can force the next spawn to obtain the
       console output by setting the variable exp_console to 1.

       Between the time a process is started and the new program is given
       control, the spawn functions can clean up the environment by closing
       file descriptors.  By default, the only file descriptors closed are
       ones internal to Expect and any marked "close-on-exec".

       If needed, you can close additional file descriptors by creating an
       appropriate function and assigning it to exp_close_in_child.  The
       function will be called after the fork and before the exec.  (This
       also modifies the behavior of the spawn command in Expect.)

       If you are also using Tcl, it may be convenient to use the function
       exp_close_tcl_files which closes all files between the default
       standard file descriptors and the highest descriptor known to Tcl.
       (Expect does this.)

       The function exp_child_exec_prelude is the last function called prior
       to the actual exec in the child.  You can redefine this for effects
       such as manipulating the uid or the signals.


       extern int exp_autoallocpty;
       extern int exp_pty[2];

       The spawn functions use a pty to communicate with the process.  By
       default, a pty is automatically allocated each time a process is
       spawned.  If you want to allocate ptys yourself, before calling one
       of the spawn functions, set exp_autoallocpty to 0, exp_pty[0] to the
       master pty file descriptor and exp_pty[1] to the slave pty file
       descriptor.  The expect library will not do any pty initializations
       (e.g., exp_stty_init will not be used).  The slave pty file
       descriptor will be automatically closed when the process is spawned.
       After the process is started, all further communication takes place
       with the master pty file descriptor.

       exp_spawnl and exp_spawnv duplicate the shell's actions in searching
       for an executable file in a list of directories.  The directory list
       is obtained from the environment.


       While it is possible to use read() to read information from a process
       spawned by exp_spawnl or exp_spawnv, more convenient functions are
       provided.  They are as follows:

       int fd;
       enum exp_type type;
       char *pattern1, *pattern2, ...;
       regexp *re1, *re2, ...;
       int value1, value2, ...;

       FILE *fp;
       enum exp_type type;
       char *pattern1, *pattern2, ...;
       regexp *re1, *re2, ...;
       int value1, value2, ...;

       enum exp_type {

       struct exp_case {
       char *pattern;
       regexp *re;
       enum exp_type type;
       int value;

       int fd;
       struct exp_case *cases;

       FILE *fp;
       struct exp_case *cases;

       extern int exp_timeout;
       extern char *exp_match;
       extern char *exp_match_end;
       extern char *exp_buffer;
       extern char *exp_buffer_end;
       extern int exp_match_max;
       extern int exp_full_buffer;
       extern int exp_remove_nulls;

       The functions wait until the output from a process matches one of the
       patterns, a specified time period has passed, or an EOF is seen.

       The first argument to each function is either a file descriptor or a
       stream.  Successive sets of arguments describe patterns and
       associated integer values to return when the pattern matches.

       The type argument is one of four values.  exp_end indicates that no
       more patterns appear.  exp_glob indicates that the pattern is a glob-
       style string pattern.  exp_exact indicates that the pattern is an
       exact string.  exp_regexp indicates that the pattern is a regexp-
       style string pattern.  exp_compiled indicates that the pattern is a
       regexp-style string pattern, and that its compiled form is also
       provided.  exp_null indicates that the pattern is a null (for
       debugging purposes, a string pattern must also follow).

       If the compiled form is not provided with the functions exp_expectl
       and exp_fexpectl, any pattern compilation done internally is thrown
       away after the function returns.  The functions exp_expectv and
       exp_fexpectv will automatically compile patterns and will not throw
       them away.  Instead, they must be discarded by the user, by calling
       free on each pattern.  It is only necessary to discard them, the last
       time the cases are used.

       Regexp subpatterns matched are stored in the compiled regexp.
       Assuming "re" contains a compiled regexp, the matched string can be
       found in re->startp[0].  The match substrings (according to the
       parentheses) in the original pattern can be found in re->startp[1],
       re->startp[2], and so on, up to re->startp[9].  The corresponding
       strings ends are re->endp[x] where x is that same index as for the
       string start.

       The type exp_null matches if a null appears in the input.  The
       variable exp_remove_nulls must be set to 0 to prevent nulls from
       being automatically stripped.  By default, exp_remove_nulls is set to
       1 and nulls are automatically stripped.

       exp_expectv and exp_fexpectv are useful when the number of patterns
       is not known in advance.  In this case, the sets are provided in an
       array.  The end of the array is denoted by a struct exp_case with
       type exp_end.  For the rest of this discussion, these functions will
       be referred to generically as expect.

       If a pattern matches, then the corresponding integer value is
       returned.  Values need not be unique, however they should be positive
       to avoid being mistaken for EXP_EOF, EXP_TIMEOUT, or EXP_FULLBUFFER.
       Upon EOF or timeout, the value EXP_EOF or EXP_TIMEOUT is returned.
       The default timeout period is 10 seconds but may be changed by
       setting the variable exp_timeout.  A value of -1 disables a timeout
       from occurring.  A value of 0 causes the expect function to return
       immediately (i.e., poll) after one read().  However it must be
       preceded by a function such as select, poll, or an event manager
       callback to guarantee that there is data to be read.

       If the variable exp_full_buffer is 1, then EXP_FULLBUFFER is returned
       if exp_buffer fills with no pattern having matched.

       When the expect function returns, exp_buffer points to the buffer of
       characters that was being considered for matching.  exp_buffer_end
       points to one past the last character in exp_buffer.  If a match
       occurred, exp_match points into exp_buffer where the match began.
       exp_match_end points to one character past where the match ended.

       Each time new input arrives, it is compared to each pattern in the
       order they are listed.  Thus, you may test for absence of a match by
       making the last pattern something guaranteed to appear, such as a
       prompt.  In situations where there is no prompt, you must check for
       EXP_TIMEOUT (just like you would if you were interacting manually).
       More philosophy and strategies on specifying expect patterns can be
       found in the documentation on the expect program itself.  See SEE
       ALSO below.

       Patterns are the usual C-shell-style regular expressions.  For
       example, the following fragment looks for a successful login, such as
       from a telnet dialogue.

            switch (exp_expectl(
                 exp_glob,"invalid password",ABORT,
                 exp_end)) {
            case CONN:     /* logged in successfully */
            case BUSY:     /* couldn't log in at the moment */
            case EXP_TIMEOUT:
            case ABORT:    /* can't log in at any moment! */
            default: /* problem with expect */

       Asterisks (as in the example above) are a useful shorthand for
       omitting line-termination characters and other detail.  Patterns must
       match the entire output of the current process (since the previous
       read on the descriptor or stream).  More than 2000 bytes of output
       can force earlier bytes to be "forgotten".  This may be changed by
       setting the variable exp_match_max.  Note that excessively large
       values can slow down the pattern matcher.


       extern int exp_disconnected;
       int exp_disconnect();

       It is possible to move a process into the background after it has
       begun running.  A typical use for this is to read passwords and then
       go into the background to sleep before using the passwords to do real

       To move a process into the background, fork, call exp_disconnect() in
       the child process and exit() in the parent process.  This
       disassociates your process from the controlling terminal.  If you
       wish to move a process into the background in a different way, you
       must set the variable exp_disconnected to 1.  This allows processes
       spawned after this point to be started correctly.

MULTIPLEXING         top

       By default, the expect functions block inside of a read on a single
       file descriptor.  If you want to wait on patterns from multiple file
       descriptors, use select, poll, or an event manager.  They will tell
       you what file descriptor is ready to read.

       When a file descriptor is ready to read, you can use the expect
       functions to do one and only read by setting timeout to 0.

SLAVE CONTROL         top

       int fd;
       int enable;

       Pty trapping is normally done automatically by the expect functions.
       However, if you want to issue an ioctl, for example, directly on the
       slave device, you should temporary disable trapping.

       Pty trapping can be controlled with exp_slave_control.  The first
       argument is the file descriptor corresponding to the spawned process.
       The second argument is a 0 if trapping is to be disabled and 1 if it
       is to be enabled.

ERRORS         top

       All functions indicate errors by returning -1 and setting errno.

       Errors that occur after the spawn functions fork (e.g., attempting to
       spawn a non-existent program) are written to the process's stderr,
       and will be read by the first expect.

SIGNALS         top

       extern int exp_reading;
       extern jmp_buf exp_readenv;

       expect uses alarm() to timeout, thus if you generate alarms during
       expect, it will timeout prematurely.

       Internally, expect calls read() which can be interrupted by signals.
       If you define signal handlers, you can choose to restart or abort
       expect's internal read.  The variable, exp_reading, is true if (and
       only if) expect's read has been interrupted.
       longjmp(exp_readenv,EXP_ABORT) will abort the read.
       longjmp(exp_readenv,EXP_RESTART) will restart the read.

LOGGING         top

       extern int exp_loguser;
       extern int exp_logfile_all
       extern FILE *exp_logfile;

       If exp_loguser is nonzero, expect sends any output from the spawned
       process to stdout.  Since interactive programs typically echo their
       input, this usually suffices to show both sides of the conversation.
       If exp_logfile is also nonzero, this same output is written to the
       stream defined by exp_logfile.  If exp_logfile_all is non-zero,
       exp_logfile is written regardless of the value of exp_loguser.

DEBUGGING         top

       While I consider the library to be easy to use, I think that the
       standalone expect program is much, much, easier to use than working
       with the C compiler and its usual edit, compile, debug cycle.  Unlike
       typical C programs, most of the debugging isn't getting the C
       compiler to accept your programs - rather, it is getting the dialogue
       correct.  Also, translating scripts from expect to C is usually not
       necessary.  For example, the speed of interactive dialogues is
       virtually never an issue.  So please try the standalone 'expect'
       program first.  I suspect it is a more appropriate solution for most
       people than the library.

       Nonetheless, if you feel compelled to debug in C, here are some tools
       to help you.

       extern int exp_is_debugging;
       extern FILE *exp_debugfile;

       While expect dialogues seem very intuitive, trying to codify them in
       a program can reveal many surprises in a program's interface.
       Therefore a variety of debugging aids are available.  They are
       controlled by the above variables, all 0 by default.

       Debugging information internal to expect is sent to stderr when
       exp_is_debugging is non-zero.  The debugging information includes
       every character received, and every attempt made to match the current
       input against the patterns.  In addition, non-printable characters
       are translated to a printable form.  For example, a control-C appears
       as a caret followed by a C.  If exp_logfile is non-zero, this
       information is also written to that stream.

       If exp_debugfile is non-zero, all normal and debugging information is
       written to that stream, regardless of the value of exp_is_debugging.

CAVEATS         top

       The stream versions of the expect functions are much slower than the
       file descriptor versions because there is no way to portably read an
       unknown number of bytes without the potential of timing out.  Thus,
       characters are read one at a time.  You are therefore strongly
       encouraged to use the file descriptor versions of expect (although,
       automated versions of interactive programs don't usually demand high
       speed anyway).

       You can actually get the best of both worlds, writing with the usual
       stream functions and reading with the file descriptor versions of
       expect as long as you don't attempt to intermix other stream input
       functions (e.g., fgetc).  To do this, pass fileno(stream) as the file
       descriptor each time.  Fortunately, there is little reason to use
       anything but the expect functions when reading from interactive

       There is no matching exp_pclose to exp_popen (unlike popen and
       pclose).  It only takes two functions to close down a connection
       (fclose() followed by waiting on the pid), but it is not uncommon to
       separate these two actions by large time intervals, so the function
       seems of little value.

       If you are running on a Cray running Unicos (all I know for sure from
       experience), you must run your compiled program as root or setuid.
       The problem is that the Cray only allows root processes to open ptys.
       You should observe as much precautions as possible:  If you don't
       need permissions, setuid(0) only immediately before calling one of
       the spawn functions and immediately set it back afterwards.

       Normally, spawn takes little time to execute.  If you notice spawn
       taking a significant amount of time, it is probably encountering ptys
       that are wedged.  A number of tests are run on ptys to avoid
       entanglements with errant processes.  (These take 10 seconds per
       wedged pty.)  Running expect with the -d option will show if expect
       is encountering many ptys in odd states.  If you cannot kill the
       processes to which these ptys are attached, your only recourse may be
       to reboot.

BUGS         top

       The exp_fexpect functions don't work at all under HP-UX - it appears
       to be a bug in getc.  Follow the advice (above) about using the
       exp_expect functions (which doesn't need to call getc).  If you fix
       the problem (before I do - please check the latest release) let me

SEE ALSO         top

       An alternative to this library is the expect program.  expect
       interprets scripts written in a high-level language which direct the
       dialogue.  In addition, the user can take control and interact
       directly when desired.  If it is not absolutely necessary to write
       your own C program, it is much easier to use expect to perform the
       entire interaction.  It is described further in the following

       "expect: Curing Those Uncontrollable Fits of Interactivity" by Don
       Libes, Proceedings of the Summer 1990 USENIX Conference, Anaheim,
       California, June 11-15, 1990.

       "Using expect to Automate System Administration Tasks" by Don Libes,
       Proceedings of the 1990 USENIX Large Installation Systems
       Administration Conference, Colorado Springs, Colorado, October 17-19,

       expect(1), alarm(3), read(2), write(2), fdopen(3), execve(2),
       execvp(3), longjmp(3), pty(4).

       There are several examples C programs in the test directory of
       expect's source distribution which use the expect library.

AUTHOR         top

       Don Libes,, National Institute of Standards and


       Thanks to John Ousterhout (UCBerkeley) for supplying the pattern

       Design and implementation of the expect library was paid for by the
       U.S. government and is therefore in the public domain.  However the
       author and NIST would like credit if this program and documentation
       or portions of them are used.

COLOPHON         top

       This page is part of the expect (programmed dialogue with interactive
       programs) project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for this
       manual page, see ⟨⟩.  This page
       was obtained from the tarball expect5.45.3.tar.gz fetched from
       ⟨⟩ on 2020-11-01.
       If you discover any rendering problems in this HTML version of the
       page, or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the information
       in this COLOPHON (which is not part of the original manual page),
       send a mail to

                              12 December 1991                  LIBEXPECT(3)

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