04-25-2009
Output file redirection
Suppose I have a file named a
When I write
cat a>a
The following error message is displayed
cat: a: input file is output file
and my file a is truncated to zero size.
Also the exit status of the last command is 1
Can someone tell me what actually happens when I do so?
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CAT(1) User Commands CAT(1)
NAME
cat - concatenate files and print on the standard output
SYNOPSIS
cat [OPTION]... [FILE]...
DESCRIPTION
Concatenate FILE(s), or standard input, to standard output.
-A, --show-all
equivalent to -vET
-b, --number-nonblank
number nonempty output lines
-e equivalent to -vE
-E, --show-ends
display $ at end of each line
-n, --number
number all output lines
-s, --squeeze-blank
suppress repeated empty output lines
-t equivalent to -vT
-T, --show-tabs
display TAB characters as ^I
-u (ignored)
-v, --show-nonprinting
use ^ and M- notation, except for LFD and TAB
--help display this help and exit
--version
output version information and exit
With no FILE, or when FILE is -, read standard input.
EXAMPLES
cat f - g
Output f's contents, then standard input, then g's contents.
cat Copy standard input to standard output.
AUTHOR
Written by Torbjorn Granlund and Richard M. Stallman.
REPORTING BUGS
Report cat bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
Report cat translation bugs to <http://translationproject.org/team/>
COPYRIGHT
Copyright (C) 2010 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
SEE ALSO
The full documentation for cat is maintained as a Texinfo manual. If the info and cat programs are properly installed at your site, the
command
info coreutils 'cat invocation'
should give you access to the complete manual.
GNU coreutils 8.5 February 2011 CAT(1)