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:
The following operand shall be supported:
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:
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.
The following exit values shall be returned:
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:
cat myfile
writes the contents of the file myfile to standard output.
The following command:
cat doc1 doc2 > doc.all
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:
cat doc doc.end > doc
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.
more , the System Interfaces volume of IEEE Std 1003.1-2001, setvbuf()