Synopsis

sh [file]

If a file is named as a command line argument, that file is the source of input; otherwise, standard input is read.

There are features that allow input/output redirection, creating pipelines, and performing tasks in background.

Starting the Shell

Command Line Syntax

Command pipelines are not allowed to span lines.

Each command pipeline consists of one or more commands separated by a vertical bar (|) character. The standard output of one command is made the standard input of the next command to the right.

Re-direction of input/output to pipes takes precedence over re-direction from/to files.

In the limit case, a command pipeline consists of a single command with no pipes.

A command consists of one or more fields. The first (leftmost) field is the command field. It is used to determined the executable file to be loaded and run; see below. The remaining fields are parsed and become command line arguments that are passed to the module's init function as a list of strings.

Finding the Module

As a convenience, the user need not specify the .dis suffix to the filename. If missing, it will be added by the shell.

If the load fails there is, in general, a second attempt to load the module by resolving the pathname relative to the /dis directory (the general system repository of Dis files).

There are two exceptions to this second attempt. The second load attempt is not performed if the command field provides an absolute pathname or a relative pathname starting with dot-slash (./). Such explicit naming is taken to mean that the user will accept no substitutions.

The shell requires that the Dis file implement a module with an interface equivalent to the Command module as specified in /module/sh.m. Otherwise, the named file will not load.

File Name Expansion

This expansion is not applied to the filenames used for input/output redirection.

Comments

Quoting

Inside a quoted string, the special meaning of the single quote can be removed by immediately following it with another single quote. Command lines with un-terminated quoted strings are rejected and cause an error message.

Examples:

pcsrb.mh.lucent.com$ echo ''''
'
pcsrb.mh.lucent.com$ echo 'don''t'
don't
pcsrb.mh.lucent.com$ echo 'hello' 'world
sh: unmatched quote
pcsrb.mh.lucent.com$ echo 'a'b
ab
pcsrb.mh.lucent.com$ echo a'b'
ab
pcsrb.mh.lucent.com$

Input/Output Re-directions

Attempts to redirect standard input to a non-existing file will fail. Redirecting standard output to an non-existing file will cause the creation of that file. The destination file already exists, it will be overwritten. Any previous contents are lost.

In cases of competing re-direction mechanisms (re-direct to a file and to a pipe), the pipe has precedence.

Background Tasks

Name Space Concerns

Command pipelines that are started synchronously by the shell are executed by threads that share the shell's name space. If those commands modify the file name space (and they have not mimicked the shell in creating their own independent name space), those modifications will be perceived by the shell when it continues execution. See bind and pctl.

Command pipelines that are started asynchronously by the shell (background tasks) are implicitly given an independent, duplicate file name space.

Files

See Also

bind	Modifying the file name space.
filepat	Details of pattern expansion of file names.
pipe	Details of pipe facilities.
sh	Description of the interface for 'command' modules.
pctl	See Notes on name space management.
cons	Details of /dev/sysname used in the shell prompt.
prog	Details of the /prog/n/wait interface.