Originální popis anglicky:
cat - concatenate and print files
Návod, kniha: POSIX Programmer's Manual
cat [-u][file ...]
The
cat utility shall read files in sequence and shall write their
contents to the standard output in the same sequence.
The
cat utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following option shall be supported:
- -u
- Write bytes from the input file to the standard output
without delay as each is read.
The following operand shall be supported:
- file
- A pathname of an input file. If no file operands are
specified, the standard input shall be used. If a file is
'-' , the cat utility shall read from the standard input at
that point in the sequence. The cat utility shall not close and
reopen standard input when it is referenced in this way, but shall accept
multiple occurrences of '-' as a file operand.
The standard input shall be used only if no
file operands are specified,
or if a
file operand is
'-' . See the INPUT FILES section.
The input files can be any file type.
The following environment variables shall affect the execution of
cat:
- LANG
- Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization
Variables for the precedence of internationalization variables used to
determine the values of locale categories.)
- LC_ALL
- If set to a non-empty string value, override the values of
all the other internationalization variables.
- LC_CTYPE
- Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments).
- LC_MESSAGES
- Determine the locale that should be used to affect the
format and contents of diagnostic messages written to standard error.
- NLSPATH
- Determine the location of message catalogs for the
processing of LC_MESSAGES .
Default.
The standard output shall contain the sequence of bytes read from the input
files. Nothing else shall be written to the standard output.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
- 0
- All input files were output successfully.
- >0
- An error occurred.
Default.
The following sections are informative.
The
-u option has value in prototyping non-blocking reads from FIFOs. The
intent is to support the following sequence:
mkfifo foo
cat -u foo > /dev/tty13 &
cat -u > foo
It is unspecified whether standard output is or is not buffered in the default
case. This is sometimes of interest when standard output is associated with a
terminal, since buffering may delay the output. The presence of the
-u
option guarantees that unbuffered I/O is available. It is
implementation-defined whether the
cat utility buffers output if the
-u option is not specified. Traditionally, the
-u option is
implemented using the equivalent of the
setvbuf() function defined in
the System Interfaces volume of IEEE Std 1003.1-2001.
The following command:
writes the contents of the file
myfile to standard output.
The following command:
concatenates the files
doc1 and
doc2 and writes the result to
doc.all.
Because of the shell language mechanism used to perform output redirection, a
command such as this:
causes the original data in
doc to be lost.
The command:
cat start - middle - end > file
when standard input is a terminal, gets two arbitrary pieces of input from the
terminal with a single invocation of
cat. Note, however, that if
standard input is a regular file, this would be equivalent to the command:
cat start - middle /dev/null end > file
because the entire contents of the file would be consumed by
cat the
first time
'-' was used as a
file operand and an end-of-file
condition would be detected immediately when
'-' was referenced the
second time.
Historical versions of the
cat utility include the options
-e,
-t, and
-v, which permit the ends of lines, <tab>s, and
invisible characters, respectively, to be rendered visible in the output. The
standard developers omitted these options because they provide too fine a
degree of control over what is made visible, and similar output can be
obtained using a command such as:
sed -n -e 's/$/$/' -e l pathname
The
-s option was omitted because it corresponds to different functions
in BSD and System V-based systems. The BSD
-s option to squeeze blank
lines can be accomplished by the shell script shown in the following example:
sed -n '
# Write non-empty lines.
/./ {
p
d
}
# Write a single empty line, then look for more empty lines.
/^$/ p
# Get next line, discard the held <newline> (empty line),
# and look for more empty lines.
:Empty
/^$/ {
N
s/.//
b Empty
}
# Write the non-empty line before going back to search
# for the first in a set of empty lines.
p
The System V
-s option to silence error messages can be accomplished by
redirecting the standard error. Note that the BSD documentation for
cat
uses the term "blank line" to mean the same as the POSIX "empty
line'': a line consisting only of a <newline>.
The BSD
-n option was omitted because similar functionality can be
obtained from the
-n option of the
pr utility.
None.
more , the System Interfaces volume of
IEEE Std 1003.1-2001,
setvbuf()
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
