9.1 POSIX Shell Capabilities

   

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The POSIX shell is the default shell for HP-UX users. Other shells are also available on HP-UX. Important shells and their corresponding executable files are listed in Table 9-1.

Table 9-1. Important UNIX Shells

Name of Shell	Path to Shell File
POSIX shell	/usr/bin/sh
Bourne shell	/usr/old/bin/sh
Korn shell	/usr/bin/ksh
C shell	/usr/bin/csh

There are two other shells used in HP-UX. One of these is called the restricted shell ( /usr/bin/rsh ), which is used to provide restricted access to some users. The key shell ( /usr/bin/keysh ) is a context-sensitive shell that may be handy to use with HP terminals.

The capabilities of POSIX and other important shells available on HP-UX systems are presented in Table 9-2.

Table 9-2. Features of Different UNIX Shells

Feature	Description	POSIX	Bourne	Korn	C
History	Allows previous commands to be viewed and reused	Yes	No	Yes	Yes
Line editing	Allows a command line to be edited	Yes	No	Yes	No
File name completion	Enables the user to enter partial file names and complete these automatically by pressing certain keys	Yes	No	Yes	Yes
Command aliases	Allows creating aliases for standard commands	Yes	No	Yes	Yes
Job control	Allows for running jobs in background and moving jobs from background to foreground and vice versa	Yes	No	Yes	Yes

In the rest of this chapter, you will learn more about some of the capabilities of the POSIX shell.

Shell and Subshell

A subshell can be invoked inside a shell at any time by using the shell path as the command line. A subshell inherits all environment variables from the parent shell but doesn't inherit any shell variables . Sometimes a user may need to invoke a subshell to have a particular shell environment. The current shell also invokes subshells if required by a command. Shell scripts are usually executed in subshells. You can exit from a subshell using the exit command at any time.

Controlling Resources (ulimit)

The ulimit command is an intrinsic command of the POSIX shell and is used to limit user resources. If you specify the -a switch with this command, it lists current resource limits. A typical output of the command is as follows .

 $  ulimit -a  time(seconds)        unlimited file(blocks)         unlimited data(kbytes)         135168 stack(kbytes)        8192 memory(kbytes)       unlimited coredump(blocks)     4194303 nofiles(descriptors) 256 $ 

This command is very useful if you want to specify a limit for resources. For example, if you don't want the shell to create a core file larger than 1024 blocks, you can use following command.

 $  ulimit -c 1024  $  ulimit -a  time(seconds)        unlimited file(blocks)         unlimited data(kbytes)         135168 stack(kbytes)        8192 memory(kbytes)       unlimited coredump(blocks)     1024 nofiles(descriptors) 256 $ 

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