z/OS V1R9.0 UNIX System Services Command ... - Christian Grothoff

interpret - (hyphen) as the old working directory (equivalent to the tcsh shell
variable owd). This is not a substitution at all, but an abbreviation recognized only
by those commands. Nonetheless, it too can be prevented by quoting.
Command execution
The next three topics describe how the shell executes commands and deals with
their input and output.
Simple commands, pipelines, and sequences
A simple command is a sequence of words, the first of which specifies the
command to be executed. A series of simple commands joined by ’|’ characters
forms a pipeline. The output of each command in a pipeline is connected to the
input of the next.
Simple commands and pipelines may be joined into sequences with ’;’, and will be
executed sequentially. Commands and pipelines can also be joined into sequences
with ’||’ or ’&&’, indicating, as in the C language, that the second is to be executed
only if the first fails or succeeds respectively.
A simple command, pipeline or sequence may be placed in parentheses, ’()’, to
form a simple command, which may in turn be a component of a pipeline or
sequence. A command, pipeline or sequence can be executed without waiting for it
to terminate by following it with an ’&’.
Built-in and non-built-in command execution
tcsh Built-in commands are executed within the shell. If any component of a
pipeline except the last is a built-in command, the pipeline is executed in a subshell.
Parenthesized commands are always executed in a subshell:
(cd; pwd); pwd
which prints the home directory, leaving you where you were (printing this after the
home directory), while
cd; pwd
leaves you in the home directory. Parenthesized commands are most often used to
prevent cd from affecting the current shell.
tcsh
When a command to be executed is found not to be a built-in command the tcsh
shell attempts to execute the command via execve. Each word in the variable path
names a directory in which the tcsh shell will look for the command. If it is given
neither a -c nor a -t option, the shell hashes the names in these directories into an
internal table so that it will only try an execve in a directory if there is a possibility
that the command resides there. This greatly speeds command location when a
large number of directories are present in the search path. If this mechanism has
been turned off (via unhash), if the shell was given a -c or -t argument or in any
case for each directory component of path which does not begin with a /, the shell
concatenates the current working directory with the given command name to form a
pathname of a file which it then attempts to execute.
If the file has execute permissions but is not an executable to the system (that is, it
is neither an executable binary nor a script which specifies its interpreter), then it is
assumed to be a file containing shell commands and a new shell is spawned to
read it. The shell special alias may be set to specify an interpreter other than the
shell itself.
Chapter 2. Shell command descriptions 645