csh.1

早期freebsd实现

If it had existed previously these previous contents would have been discarded!
A shell option
.I noclobber
exists to prevent this from happening accidentally;
it is discussed in section 2.2.
.PP
The system normally keeps files which you create with `>' and all other files.
Thus the default is for files to be permanent.  If you wish to create a file
which will be removed automatically, you can begin its name with a `#'
character, this `scratch' character denotes the fact that the file will
be a scratch file.*
.FS
*Note that if your erase character is a `#', you will have to precede the
`#' with a `\e'.  The fact that the `#' character is the old (pre-\s-2CRT\s0)
standard erase character means that it seldom appears in a file name, and
allows this convention to be used for scratch files.  If you are using a
\s-2CRT\s0, your erase character should be a ^H, as we demonstrated
in section 1.1 how this could be set up.
.FE
The system will remove such files after a couple of days,
or sooner if file space becomes very tight.
Thus, in running the
.I date
command above, we don't really want to save the output forever, so we
would more likely do
.DS
date > #now
.DE
.NH 2
Metacharacters in the shell
.PP
The shell has a large number of
special characters (like `>')
which indicate special functions.
We say that these notations have
.I syntactic
and
.I semantic
meaning to the shell.
In general, most characters which are neither letters nor digits
have special meaning to the shell.
We shall shortly learn a means of
.I quotation
which allows us to use
.I metacharacters
without the shell treating them in any special way.
.PP
Metacharacters normally have effect only when the shell is reading
our input.
We need not worry about placing shell metacharacters in a letter
we are sending via
.I mail,
or when we are typing in text or data to some other program.
Note that the shell is only reading input when it has prompted with
`% ' (although we can type our input even before it prompts).
.NH 2
Input from files; pipelines
.PP
We learned above how to
.I redirect
the
.I "standard output"
of a command
to a file.
It is also possible to redirect the
.I "standard input"
of a command from a file.
This is not often necessary since most commands will read from
a file whose name is given as an argument.
We can give the command
.DS
sort < data
.DE
to run the
.I sort
command with standard input, where the command normally
reads its input, from the file
`data'.
We would more likely say
.DS
sort data
.DE
letting the
.I sort
command open the file
`data'
for input itself since this is less to type.
.PP
We should note that if we just typed
.DS
sort
.DE
then the sort program would sort lines from its
.I "standard input."
Since we did not
.I redirect
the standard input, it would sort lines as we typed them on the terminal
until we typed a ^D to indicate an end-of-file.
.PP
A most useful capability is the ability to combine the standard output
of one command with the standard input of another, i.e. to run the
commands in a sequence known as a
.I pipeline.
For instance the command
.DS
ls \-s
.DE
normally produces a list of the files in our directory with the size
of each in blocks of 512 characters.
If we are interested in learning which of our files is largest we
may wish to have this sorted by size rather than by name, which is
the default way in which
.I ls
sorts.
We could look at the many options of
.I ls
to see if there was an option to do this but would eventually discover
that there is not.
Instead we can use a couple of simple options of the
.I sort
command, combining it with
.I ls
to get what we want.
.PP
The
.I \-n
option of sort specifies a numeric sort rather than an alphabetic sort.
Thus
.DS
ls \-s | sort \-n
.DE
specifies that the output of the
.I ls
command run with the option
.I \-s
is to be
.I piped
to the command
.I sort
run with the numeric sort option.
This would give us a sorted list of our files by size, but with the
smallest first.
We could then use the
.I \-r
reverse sort option and the
.I head
command in combination with the previous command doing
.DS
ls \-s | sort \-n \-r | head \-5
.DE
Here we have taken a list of our files sorted alphabetically,
each with the size in blocks.
We have run this to the standard input of the
.I sort
command asking it to sort numerically in reverse order (largest first).
This output has then been run into the command
.I head
which gives us the first few lines.
In this case we have asked
.I head
for the first 5 lines.
Thus this command gives us the names and sizes of our 5 largest files.
.PP
The notation introduced above is called the
.I pipe
mechanism.
Commands separated by `\||\|' characters are connected together by the
shell and the standard output of each is run into the standard input of the
next.
The leftmost command in a pipeline will normally take its standard
input from the terminal and the rightmost will place its standard
output on the terminal.
Other examples of pipelines will be given later when we discuss the
history mechanism;
one important use of pipes which is illustrated there is in the
routing of information to the line printer.
.NH 2
Filenames
.PP
Many commands to be executed will need the names of files as arguments.
\s-2UNIX\s0
.I pathnames
consist of a number of
.I components
separated by `/'.
Each component except the last names a directory in which the next
component resides, in effect specifying the
.I path
of directories to follow to reach the file.
Thus the pathname
.DS
/etc/motd
.DE
specifies a file in the directory
`etc'
which is a subdirectory of the
.I root
directory `/'.
Within this directory the file named is `motd' which stands
for `message of the day'.
A
.I pathname
that begins with a slash is said to be an
.I absolute
pathname since it is specified from the absolute top of the entire
directory hierarchy of the system (the
.I root ).
.I Pathnames
which do not begin with `/' are interpreted as starting in the current
.I "working directory" ,
which is, by default, your
.I home
directory and can be changed dynamically by the
.I cd
change directory command.
Such pathnames are said to be
.I relative
to the working directory since they are found by starting
in the working directory and descending to lower levels of directories
for each
.I component
of the pathname.  If the pathname contains no slashes at all then the
file is contained in the working directory itself and the pathname is merely
the name of the file in this directory.
Absolute pathnames have no relation
to the working directory.
.PP
Most filenames consist of a number of alphanumeric characters and
`.'s (periods).
In fact, all printing characters except `/' (slash) may appear in filenames.
It is inconvenient to have most non-alphabetic characters in filenames
because many of these have special meaning to the shell.
The character `.' (period) is not a shell-metacharacter and is often used
to separate the
.I extension
of a file name from the base of the name.
Thus
.DS
prog.c prog.o prog.errs prog.output
.DE
are four related files.
They share a
.I base
portion of a name
(a base portion being that part of the name that is left when a trailing
`.' and following characters which are not `.' are stripped off).
The file
`prog.c'
might be the source for a C program,
the file `prog.o' the corresponding object file,
the file
`prog.errs' the errors resulting from a compilation of the program
and the file
`prog.output' the output of a run of the program.
.PP
If we wished to refer to all four of these files in a command, we could
use the notation
.DS
prog.*
.DE
This expression is expanded by the shell, before the command to which it is
an argument is executed, into a list of names which begin with `prog.'.
The character `*' here matches any sequence (including the empty sequence)
of characters in a file name.
The names which match are alphabetically sorted and placed in the
.I "argument list"
of the command.
Thus the command
.DS
echo prog.*
.DE
will echo the names
.DS
prog.c prog.errs prog.o prog.output
.DE
Note that the names are in sorted order here, and a different
order than we listed them above.
The
.I echo
command receives four words as arguments, even though we only typed
one word as as argument directly.
The four words were generated by
.I "filename expansion"
of the one input word.
.PP
Other notations for
.I "filename expansion"
are also available.
The character `?' matches any single character in a filename.
Thus
.DS
echo ? \|?? \|???
.DE
will echo a line of filenames; first those with one character names,
then those with two character names, and finally those with three
character names.
The names of each length will be independently sorted.
.PP
Another mechanism consists of a sequence of characters between `[' and `]'.
This metasequence matches any single character from the enclosed set.
Thus
.DS
prog.[co]
.DE
will match
.DS
prog.c prog.o
.DE
in the example above.
We can also place two characters around a `\-' in this notation to denote
a range.
Thus
.DS
chap.[1\-5]
.DE
might match files
.DS
chap.1 chap.2 chap.3 chap.4 chap.5
.DE
if they existed.
This is shorthand for
.DS
chap.[12345]
.DE
and otherwise equivalent.
.PP
An important point to note is that if a list of argument words to
a command (an
.I "argument list)"
contains filename expansion syntax, and if this filename expansion syntax
fails to match any existing file names, then the shell considers this
to be an error and prints a diagnostic
.DS
No match.
.DE
and does not execute the command.
.PP
Another very important point is that files with the character `.' at the