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HP-UX 11i Version 3: February 2007

Technical documentation

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shl — shell layer manager




shl provides a means for interacting with more than one shell from a single terminal by using shell layers. A layer is a shell that is bound to a virtual device. The virtual device can be manipulated like an actual terminal by using stty and ioctl() (see stty(1) and ioctl(2)). Each layer has its own process group ID. The user controls these layers by using the commands described below.

The current layer is the layer that can receive input from the keyboard. Other layers attempting to read from the keyboard are blocked. Output from multiple layers is multiplexed onto the terminal. To block the output of a layer when it is not current, the stty option loblk can be set within the layer.

The stty character swtch (set to ^Z if NUL) is used to switch control to shl from a layer. shl has its own prompt, >>>, to distinguish it from a layer.


A name is a sequence of characters delimited by a space, tab, or new-line character. Only the first eight characters are significant. When provided as an argument to the create or name commands, name cannot be of the form n or (n), where n is a decimal number.


The following commands can be issued from the shl prompt level. Any unique prefix is accepted.

create [-[name] | name [command]]

Create a layer called name and make it the current layer. If no argument is given, a layer is created with a name of the form (n), where n is the number of the next available slot in an internal table. Future references to this layer can be made with or without the parentheses. If name is followed by a command, that command is executed in the layer instead of a shell. If - is the first argument, a "login shell" is created in the layer. The shell prompt variable PS1 is set to the name of the layer followed by a space.

name [oldname] newname

Rename the layer oldname, calling it newname. If oldname is not specified, the current layer name is changed.

! [command]

Invoke a sub-shell and execute command. If no command is given, a shell is executed according to the SHELL environment variable.

block name [name]...

For each name, block the output of the corresponding layer when it is not the current layer. This is equivalent to setting the sttyloblk option within the layer.

delete name [name]...

For each name, delete the corresponding layer. All processes in the process group of the layer are sent the SIGHUP signal (see signal(5)).

help | ?

Print the syntax of the shl commands.

layers [-l] [name]...

For each name, list the layer name and its process group. The -l option produces a ps-like listing. If no arguments are given, information is presented for all existing layers.

resume [name]

Change the status of the layer referred to by name to that of current layer. If no argument is given, the last existing current layer is changed.


Change the status of the previous current layer to that of current layer.

unblock name [name]...

For each name, do not block the output of the corresponding layer when it is not the current layer. This is equivalent to setting the stty-loblk option within the layer.


Exit shl. All layers are sent the SIGHUP signal.


Change the status of the layer referred to by name to that of current layer. Any unique prefix is accepted.



The behavior of the block and unblock shl commands is not guaranteed when the SHELL environment variable is set to /usr/bin/csh (for csh(1)) or /usr/bin/ksh (for ksh(1)), or when the shell saves and restores the tty state (defined in termio(7)) before and after each command is invoked interactively from that shell. For both /usr/bin/csh and /usr/bin/ksh, the loblk or -loblk options of stty can be used from within the layer to block or unblock the output of that layer.


For shl to function properly, the ptydaemon process must be running on the system. If your system has been installed with the Desktop HP-UX product, then ptydaemon will not be started by default. In order to start this daemon, change PTYDAEMON_START from a "0" to a "1" in the /etc/rc.config.d/ptydaemon file. The system must either be rebooted for this change to take effect, or you can manually start this daemon by typing :


Note that ptydaemon will also be disabled if the DesktopConfig.LITECONFIG fileset has been installed on the system, or if the system administrator has previously run the SAM utility and selected the Apply Lite HP-UX Configuration Action from within any of SAM's Kernel Configuration screens.

shl does not support long user and group names on the current release, HP-UX 11i V3.



Variable containing path name of the shell to use (default is /usr/bin/sh).


shl: SVID2, SVID3, XPG2

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