empty - run processes under pseudo-terminal sessions
empty -f [-i fifo1 -o fifo2] [-p file.pid] [-L file.log] command [command
empty -w [-Sv] [-t n] [-i fifo2 -o fifo1] key1 [answer1] ... [keyX
empty -s [-Sc] -o fifo1 [request]
empty -r [-b size] [-t n] [-i fifo2]
empty -k [pid] [signal]
empty is an utility that provides a simple interface to execute and/or
interact with processes under pseudo-terminal sessions. This tool is
definitely useful in programming of shell scripts which are used to
communicate with interactive programs like telnet or ftp. In some cases
empty can be a substitution for TCL/expect or other similar programming
There are several common forms of command lines for empty.
But the first execution of empty is usually a start in the daemon
mode to fork a new command (process) under PTY-session. This
can be done with -f key. An interface for the input and output data
channels of the forked process is performed by two fifo files which names
may be specified with -i and -o keys. These files are
automatically created/deleted any time you start/exit empty daemon,
so you must not create them manually. If you did not specify these fifo
files in the command line, empty names them by itself basing on its
PID and PID of forked PTY process.
At this point any application can easily communicate with forked
process by writing data to the input fifo and reading answers from the
output fifo, see EXAMPLES section for the details. To simplify this
operations, empty offers an interfase to just send any data
(use -s key), or even to watch the output fifo for multiple
keyphrases and reply to the input fifo with one of the
responses (see -w key).
Note! Input fifo for empty -f ... is usually an
output fifo for empty -w and empty -s forms. And output fifo
of empty -f ... is an input fifo for empty -w ...
If something goes wrong the forked process may be killed by
the standard kill command, or using -k key of empty. See
-p option to save PID of empty daemon process.
The following options are available:
- fork, spawn, start or execute a new process specified by the
command and its arguments. If you omit fifo files, empty
with its job control algorithm will create them under /tmp directory using
this templates: empty.PPID.PID.in and empty.PPID.PID.out, here PPID is
usually your shell system process ID and PID is system process ID of
- send data (request) to the forked process. If fifo file was not specitied
with -o key, empty will try to find an automatically created
fifo file it in /tmp directory. Instead of command line you can send your
request or data directly to standard input (stdin) of
- watch for one or more keyphrases and if specified send the
appropriated response to the input fifo. If response is not
set, empty waits for the proper keyphrase then exits. With
-w key empty returns the number of matched
keyphrase-response pair, or 255 if fails to find this match (see -t key
for details of possible exit on timeout).
- read from output FIFO one line (default) or one block of data (if -b
size was specified). If -t n key was placed, exit on
- list automatically created jobs by your shell. NB! Your custom
jobs, which fifo files you specified with -i and -o keys,
are not displayed. So if you did not specify fifo files with -i and
-o keys all operations are done under the job marked
- send signal to the process with pid. If you did not specify
pid, empty tries to find it within the list of automatically
created jobs. If signal is omitted the default SIGTERM is
- print short help message and exit
- -i fifo1
- a fifo file, which is used as input for a forked process.
- -o fifo2
- a fifo file, which is used as output for a forked process.
- -L file.log
- This option allows to log the whole empty session to a file. Marks
>>> and <<< show the directions of data flow.
- -p file.pid
- Save PID of empty daemon process to a file
- -t n
- If input FIFO is empty, wait for n seconds (default is 10) to receive the
keyphrase then exit on timeout with 255 code.
- force empty to use stdin for data or requests.
- Strip the last character from the input. Works with -s and -w keys
- kvazi verbose mode. Show all contents of received buffer.
It is considered insecure to send a password in the command line like this:
- Start a new PTY-session with telnet to localhost:
empty -f -i in.fifo -o out.fifo -p empty.pid -L empty.log telnet localhost
- Interact with telnet:
empty -w -i out.fifo -o in.fifo ogin 'my_user\n'
empty -w -i out.fifo -o in.fifo assword 'my_password\n'
- Send commands to telnet with empty:
empty -s -o in.fifo who
empty -s -o in.fifo "ls -la /\n"
- The same using STDIN:
echo who | empty -s -o in.fifo
echo "ls -la /" | empty -s -o in.fifo
- Just cat output from telnet:
- Read one line from out.fifo:
empty -r -i out.fifo
- Send commands to telnet with ordinary echo:
echo "who am i" > in.fifo
echo "uname -a" > in.fifo
- Kill a process with PID 1234:
empty -k 1234
- Telnet session with automatically created jobs:
empty -f telnet localhost
- Interact with telnet using job control:
empty -w ogin 'my_user\n'
empty -w assword 'my_password\n'
- List automatically created jobs:
PPID PID TYPE FILENAME
479 706 in /tmp/empty.479.706.in
479 706 out /tmp/empty.479.706.out
479 711 in /tmp/empty.479.711.in
479 711 out /tmp/empty.479.711.out
479 711 current
empty -w assword 'my_password\n'
or like this:
empty -s 'my_password\n'
The reason is that the command line arguments are visible to the
system while empty is running. Any local user can see them with
ps(1), sometimes they are visible even remotely with finger(1). Also your
server may have some monitoring tools which may store the output from ps(1)
in their logs. There are also other, more complicated ways to compromise
this information. Generally, you should take command line arguments as
(possibly) visible to every one unless you really know what you're
empty with '-s' flag runs quickly in most cases, but still
it can hang for a number of reasons (like fifo overloading), and even if it
runs quick you still cannot be sure that no one will see its command line
arguments even in this short time. empty with '-w' flag is even worse
because it must wait for the keyphrase.
A better way to send the password to the supervised program is to
read it from file:
empty -s [common options] <./password-file
or from a pipe:
get-password-of-user "$user" |empty -s [common options]
You should still make sure that you do not send any password via command line
while creating this file, and certainly you should set some safe permissions
to this file AND its directory (with the parent directories) before reading
the password from the file OR writing the password to it.
Another possible way is to use your shell's builtin (but see
echo "$password" |empty -s [common options]
Many shells like bash(1), csh(1) and FreeBSD's sh(1) do not call external
echo(1) command but use their own builtin echo command. Since no external
command is started (the shell itself does all that echo(1) must do), nothing
is shown in the process list. It is beyond this manual page to discuss the way
to make sure that your shell uses the builtin command.
If any error occurs empty usually exits with code 255. Otherwise zero or
some positive value (see -w key) is returned.
empty was made by Mikhail E. Zakharov. This software was based on the
basic idea of pty version 4.0 Copyright (c) 1992, Daniel J. Bernstein but no
code was ported from pty4. SECURITY section of this manual page was
contributed by Sergey Redin.