empty - run processes under pseudo-terminal sessions
-f [-i fifo1 -o fifo2] [-p file.pid] [-L file.log] command [command
-w [-Sv] [-t n] [-i fifo2 -o fifo1] key1 [answer1] ... [keyX
-s [-Sc] -o fifo1 [request]
-r [-b size] [-t n] [-i fifo2]
-k [pid] [signal]
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
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
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
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
names them by itself basing on its PID and PID of forked
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
section for the details. To simplify this operations,
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
Input fifo for empty -f ...
is usually an output fifo for
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.
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.
- 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
It is considered insecure to send a password in the command line like this:
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
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 doing.
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
empty -s [common options] <./password-file
or from a pipe:
get-password-of-user "$user" |empty -s [common options]
You should still make sure that you do not send any password via command line
while creating this file, and certainly you should set some safe permissions
to this file AND its directory (with the parent directories) before reading
the password from the file OR writing the password to it.
Another possible way is to use your shell's builtin (but see below):
echo "$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.
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.