The Bourne shell

This command interpreter can be invoked by typing out sh. It can take a filename as input or can execute commands read directly from the terminal (when called without any arguments).

Syntax:
sh [-ceiknrstuvx] [arg] ...

Invocation:

If the first character of arg[0] is a (-), then commands are read from $HOME/.profile, if such a file exists. The options are interpreted as follows:

For the other shell options, refer to the man pages on sh.

Commands:

The syntax of a command and pipeline is same as for the C-shell. In addition, a concept called list is available which is a sequence of one or more pipelines separated by ; & && || and optionally terminated by ; or &.

; causes sequential execution

& causes the preceding pipeline to be executed without waiting for it to finish

&& (||) causes the list following to be executed only if the preceding pipeline returns a 0 (non-0)value

Command Substitution

The standard output from a command enclosed in a pair of backquotes (` `) may be used as part or all of a word; trailing new lines are removed.

Parameter Substitution

The character $ is used to introduce substitutable parameters. Positional parameters may be assigned values by set. Variables may be set by writing:
name=value ...

${name} substitutes the value of the parameter name
The curly braces are required only if another letter immediately follows name.

${name-word} substitutes name's value if it is set;
otherwise substitutes word.

${name=word} sets name to word if not already set and then substitute; otherwise substitute parameter by its value.

There are other ways of substitution listed in the man pages on sh. Note that word is not evaluated unless it is to be used as the substitution string. For example,
% echo ${d-`pwd`}
will only execute pwd if d is not set.

The following parameters are automatically set by the shell:

# the number of positional parameters in decimal

- options supplied to the shell on invocation or by set

? the value returned by the last executed command

$ the process number of this shell

! the process numner of the last background command invoked

The following parameters are used but not set by the shell:

HOME the default argument for the cd command

PATH the search path for commands

MAIL if set to the name of a mail file, the shell informs the user of the arrival of mail in that file

PS1 the primary prompt string (default $)

PS2 the secondary prompt string (defaut >)

IFS internal field separators, normally space, tab and newline

After parameter and command substitution, any results of substitution are scanned for internal field separator characters and split into distinct arguments at such points.

Filename Generation

Following substitution, each command word is scanned for the characters ? [ * . If any of these characters appears, the word is regarded as a pattern and replaces by the filenames that match the pattern (in alphabetically sorted fashion), if there is no match the word is left unchanged. As in the C-shell, . and / must be matched explicitly. The rules for matching are identical to the C-shell.

Quoting

The following characters have a special meaning to the shell and cause termination of a word unless quoted - ; | & ( ) > < newline, s[ace, tab. A character may be quoted by preceding it with a backslash(\). All characters enclosed within a pair of quotes (' ') are quoted. Inside double quotes (" ") parameter and command substitution occurs and \ quotes the characters \ ' " $. For example
"$*" is equivalent to "$1 $2 ..." whereas
"$@" is equivalent to "$1" "$2" ... .

Input-Output

A command's input and output may be redirected using a special notation interpretd by the shell. In the following uses, substitution occurs before word or digit is used.

Environment

The environment is a list of name-value pairs that is passed to an executed program. On invocation the shell scans the environment and creates a parameter for each valid name found, giving it the corresponding value. Executed commands inherit the same environment. If the user modifies the values of these parameters or creates new ones, none of these affects the environment unless the export command is used to bind the shell's parameter to the environment. The environment for any single command may be augmented by prefixing it with one or more assignments to parameters, for eg.,
TERM=450 <cmd> args

Typing set with no arguments gives a list of all the shell parameters that are set along with their values.

Execution

Each time a command is executed the above substitutions are carried out. Except for the special commands listed below, a new process is created and an attempt is made to execute the command via execve. The shell parameter PATH defines the search path for the directory containing the command. Each alternative directory name is separated by a colon(:). The default path is :/usr/ucb:/bin:/usr/bin. If the command name contains a / then the search path is not used. Otherwise each directory in the path is searched for an executable file. A subshell is spawned to read it. To deliberately execute in a subshell, one can parenthesize "( )" the command.

Special Commands

The following commands are executed in the shell process and except where specified no input output redirection is permitted for such commands.

Errors detected by the shell cause it to return a non-zero exit status.