cat command
The cat command concatenates and display files.
Netcat can be used instead of
cat in case networking capabilities are important. There is a version
of cat called
dog which
adds to cat some networking capabilities like ability to access URLs not
only plain files. Source package of dog can be retrieved from Cygwin and
compiled on any platform. It is also available on
Debian.
There are also
split and csplit
commands, The former split file(s) into equal chunks, the latter splits
file(s) based on context. Head and tail command can be used to
select certain area of the file. There is also a version of cat called
zcat which performs operation similar to uncompress -c
or gunzip -c on each input file
Even classic cat is a more versatile tool then it looks from the first site. For example
it can be used as a primitive editor for creation one liners or adding a
line to the config file.
Cat accepts multiple file as an arguments and reads each file in the order listed and writes the content
of merged stream to the standard output. For example
cat file prints file on your terminal
(Note: cat -n file will do the same
with line numbering)
cat file1 file2 > file3
concatenates file1 and file2, and writes the results in
file3.
If no input file is given, cat reads from the standard input
file. That capability is often used to create small files with cat
instead of editor like vi or ed. For example
cat > /etc/resolv.conf
> line
> line
> line
Ctrl-D
You can also add a line to the file if you use ">>" instead of ">":
cat >> /etc/group
If you're appending only a single line, it's better to use echo instead of
cat
Few Unix users know that cat can number lines so in a way it can serve as a preprocessor
to more to view files with line numbering. For example:
cat -n var/adm/messages | more
|
With option -n cat can number
lines
|
cat can show nonprinting characters, such as tabs and control
characters. For instance,
to see if any line has trailing blanks:
$ cat -A input.txt
This line has trailing blanks. $
This line does not.$
$
General syntax is
cat [ options ] [file...]
GNU Cut Options:
- -A, --show-all equivalent to -vET
- -b, --number-nonblank number nonblank output lines
- -e equivalent to -vE
- -E, --show-ends display $ at end of each
line
- -n, --number number all output lines
- -s, --squeeze-blank never more than one
single blank line
- -t equivalent to -vT
- -T, --show-tabs display TAB characters as ^I
- -u (ignored)
- -v, --show-nonprinting characters use ^ and M- notation,
except for LFD and TAB
- --help display this help and exit
- --version output version information and exit
Solaris Cut Options:
Solaris version is less feature rich then GNU version. Also in
Solaris version option -s has different semantic then in GNU version.
- -n Precede each line output with its line
number.
- -b Number the lines, as -n,
but omit the line numbers from blank lines.
- -u The output is not buffered. (The default
is buffered output.)
- -s cat is silent about non-existent
files.
- -v Non-printing characters (with the exception
of tabs, new-lines and form-feeds) are printed visibly. ASCII control
characters (octal 000 - 037) are printed as ^n,
where n is the corresponding ASCII character in the range
octal 100 - 137 (@, A, B, C, . . ., X, Y, Z, [, \, ], ^, and _); the
DEL character (octal 0177) is printed ^?. Other
non-printable characters are printed as M-x, where
x is the ASCII character specified by the low-order seven
bits.
When used with the -v option, the following options
may be used:
- -e A $ character will be printed
at the end of each line (prior to the new-line).
- -t Tabs will be printed as ^I's and
formfeeds to be printed as ^L's.
The -e and -t options are
ignored if the -v option is not specified.
Example 1 Concatenating a file
Example 2: Concatenating two files into one
cat doc1 doc2 > doc.all
concatenates the files doc1 and doc2 and writes
the result to doc.all.
Example 3: Concatenating several files with text entered on keyboard
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.
Example 4: Output f's contents, then standard input, then g's
contents.
cat f - g
Example 5: To concatenate several files:
cat section1.1 section1.2 section1.3 >section1
This command creates a file named section1 that is a copy of
section1.1 followed by section1.2 and
section1.3.
To suppress error messages about files that do not exist, enter:
cat
-q section2.1 section2.2 section2.3 >section2
If
section2.1 does not exist, this command concatenates
section2.2 and section2.3. The result is the same if
you do not use the -q flag, except that the cat command displays the error message:
cat: cannot open section2.1
You may want to suppress this message with
the -q flag when you use the cat
command in shell procedures.
Example 6: To append one file to the end of another:
cat section1.4 >> section1
The >> (two carets) appends a copy of section1.4 to the end of
section1. If you want to
replace the file, use the > (caret).
Example 7: To add text to the end of a file:
cat >> notes
Get milk on the way home
Ctrl-D
This command adds Get milk on the way
home to the end of the file called notes. The cat command does not prompt; it waits for you to enter
text. Press the Ctrl-D key sequence to indicate you are finished.
Environmental variables and exit status
See
environ(5) for descriptions of the following environment variables
that affect the execution of cat: LC_CTYPE, LC_MESSAGES,
and NLSPATH.
The following exit values are returned:
- 0 All input files were output successfully.
- >0 An error occurred.
#!/usr/local/bin/perl -wT
#
# $Id: cat,v 1.1.1.1 2001/06/06 08:54:05 sdague Exp $
#
# $Log: cat,v $
# Revision 1.1.1.1 2001/06/06 08:54:05 sdague
# initial import
#
# Revision 1.1.1.1 2001/05/13 19:55:38 sdague
# added initial import of PPT work
#
# Revision 1.1 1999/02/26 03:21:14 abigail
# Initial revision
#
#
use strict;
use Getopt::Std;
my ($VERSION) = '$Revision: 1.1.1.1 $' =~ /([.\d]+)/;
# Get the options.
# Print a usuage message on a unknown option.
# Requires my patch to Getopt::Std of 25 Feb 1999.
$SIG {__WARN__} = sub {
if (substr ($_ [0], 0, 14) eq "Unknown option") {
$0 =~ s{.*/}{};
print <) {
if ($squeeze_empty) {
my $is_empty = /^$/;
if ($is_empty && $was_empty)
{
next;
}
$was_empty = $is_empty;
}
$_ = sprintf "%6d $_", ++ $count if $number_lines
||
$number_non_blanks && /\S/;
$_ =~ s/$/\$/ if $ends;
if ($nonprinting) {
$_ =~ s/([\x80-\xFF])/"M-" .
("\x7F" & $1)/ge;
$_ =~ s/([\x00-\x08\x0B-\x1F])/"^"
. chr (0100 + ord $1)/ge;
$_ =~ s/\x7F/^?/g;
}
if ($tabs) {
$_ =~ s/\x09/^I/g;
}
print;
}
__END__
=pod
=head1 NAME
cat -- concatenate and print files.
=head1 SYNOPSIS
cat [-benstuv] [file ...]
=head1 DESCRIPTION
I reads and prints the files in order they are given. If no
files
are given, I is processed. A lone dash represents
I as well.
=head2 OPTIONS
I accepts the following options:
=over 4
=item -b
Number all the non blank lines, starting at 1.
=item -e
Print a dollar sign (B<$>) at the end of each lines. Implies I<-v>.
=item -n
Number all the lines, starting at 1.
=item -s
The I option. Sequential empty lines are squeezed into
a
single empty line.
=item -t
Display tabs as I<^I>. Implies I<-v>.
=item -u
Unbuffer output.
=item -v
Display non-printable characters in a printable way. Characters
in the
range I<\000> - I<\037>, with the exception of tabs and linefeeds,
are
printed as I<^X>, where I is the symbol I<\0100> higher. I is
printed as I<^?>. Characters whose highest bit is set are printed
as
I, followed by the representation of the character with the
high
bit stripped.
=back
=head1 ENVIRONMENT
The working of I is not influenced by any environment variables.
=head1 BUGS
I has no known bugs.
=head1 STANDARDS
This I implementation is compliant with the B
specification, also known as B.
This I implementation is compatible with the B implementation.
=head1 REVISION HISTORY
$Log: cat,v $
Revision 1.1.1.1 2001/06/06 08:54:05 sdague
initial import
Revision 1.1.1.1 2001/05/13 19:55:38 sdague
added initial import of PPT work
Revision 1.1 1999/02/26 03:21:14 abigail
Initial revision
=head1 AUTHOR
The Perl implementation of I was written by Abigail, I.
=head1 COPYRIGHT and LICENSE
This program is copyright by Abigail 1999.
This program is free and open software. You may use, copy, modify,
distribute
and sell this program (and any modified variants) in any way you
wish,
provided you do not restrict others to do the same.
=cut
Listing 3. Displaying non-printing characters
$ cat -t /etc/X11/XF86Config
...
# Multiple FontPath entries are allowed (they are concatenated together)
# By default, Red Hat 6.0 and later now use a font server independent of
# the X server to render fonts.
^IFontPath^I"/usr/X11R6/lib/X11/fonts/TrueType"
^IFontPath^I"unix/:7100"
EndSection
...
$ cat -E /etc/X11/XF86Config
...
# Multiple FontPath entries are allowed (they are concatenated together)$
# By default, Red Hat 6.0 and later now use a font server independent of$
# the X server to render fonts.$
$
FontPath "/usr/X11R6/lib/X11/fonts/TrueType"$
FontPath "unix/:7100"$
$
EndSection$
...
$ cat -v /etc/X11/XF86Config
...
^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@M-|M-8^X^@^@^@
P^@^O"M-X^O M-@^M^@^@^@M-^@^O"M-@M-k^@M-8*^@
@M-^H$M-@M-9|A(M-@)M-yM-|M-sM-*M-hW^A^@^@j^@
M-|M-sM-%1M-@M-9^@^B^@^@M-sM-+fM-^A= ^@ ^@
F^@^@ ^@M-9^@^H^@^@M-sM-$M-G^E(l!M-@M-^?
^IM-A5^@^@^D^@PM-^]M-^\X1M-H%^@^@^D^@tyM-G
...
Reversing files with tac
What if you wanted to reverse the order of lines in a file? That
is the job of the tac command. (Note that tac
is cat spelled backwards.) It reverses the order of
the lines or fields in a list of files.
It does not reverse the order of files -- this you must do yourself
by listing them in reverse order after the tac command.
For an example of how tac works, compare results of
ls -l | tail and ls -l | tail | tac on
some files in your home directory.
Questions or comments? I'd love to hear from you -- send mail
to jacek@artymiak.com.
Next time, we'll take a look at the sort and
tsort commands. See you then!
Example 1: View Compressed File and Uncompress with zcatCompressing a file using gzip creates a compressed file with *.gz
extension. You can view a compressed file with zcat with the following
way. Which would be as same as the uncompressed file operation ‘cat
filename’. zcat uncompresses the file and shows it in the stdout.
$ zcat filename.gz | more
$ ls -l big-file.*
-rw-r--r-- 1 ramesh ramesh 24853275 May 9 15:14 big-file.txt
$ gzip big-file.txt
[Note: Compress the file]
$ ls -l big-file.*
-rw-r--r-- 1 ramesh ramesh 9275204 May 9 15:14 big-file.txt.gz
$ zcat big-file.txt.gz
[Note: View the file without uncompressing it]
zcat big-file.txt.gz > big-file.txt
[Note: Uncompress the file]
Example 2: View a gzipped file which don’t have the gz suffix.You can uncompress a gzipped file which don’t have the gz suffix.
If you try to uncompress a gzipped file which don’t have the gz
suffix with “gunzip” or “gzip -d” command you will face the following
error.
gunzip: auth.log: unknown suffix -- ignored
But this zcat will uncompress the file and shows the content
as shown below.
$ cat > test-file.txt
This is a test file used for gunzip and zcat testing
zcat is awesome command.
$ gzip test-file.txt
$ mv test-file.txt.gz test-file-no-ext
$ gzip -d test-file-no-ext
gzip: test-file-no-ext: unknown suffix -- ignored
$ zcat test-file-no-ext
This is a test file used for gunzip and zcat testing
zcat is awesome command.
Example 3: Display the file content without worrying about
whether it is compressed or not
When you are not sure whether a file is compressed or not, you
can still view the file without worrying about it’s compression
status as shown below.
In this example, If the input-file is compressed zcat will display
the content by uncompressing it. If the input-file is not compressed
zcat will display the content as it is.
$ zcat -f input-file
Example 4: Paging the compressed file with zless / zmore.
You can paginate a compressed file with zless command or zmore
command as shown below.
$ zcat filename.gz | more
$ zcat filename.gz | less
(or)
$ zless filename.gz
$ zmore filename.gz
Note: To open any kind of file type, refer to
our previous article
Open & View 10 Different File Types with Linux Less Command – The
Ultimate Power of Less.
Expands a compressed file to standard output.
zcat [
-n ] [
-V ] [
File ... ]
The zcat command allows the user to expand and
view a compressed file without uncompressing that file. The
zcat command does not rename the expanded file
or remove the .Z extension. The zcat
command writes the expanded output to standard output.
Flags
| -n |
Omits the compressed file header from
the compressed file.
Note: Use the -n option if the file was compressed
using the -n option. |
| -V |
Writes the current version and compile options to
standard error. |
Parameters
| File
... |
Specifies the compressed files to expand. |
Return Values
If the zcat command exits with a status of 1
if any of the following events occur:
- The input file was not produced by the
compress command.
- An input file cannot be read or an output file
cannot be written.
If no error occurs, the exit status is 0.
Exit Status
| 0 |
Successful completion. |
| >0 |
An error occurred. |
Examples
To view the foo.Z file without uncompressing it, enter:
zcat foo.Z
The uncompressed contents of the foo.Z file are written
to standard output. The file is not renamed.
Related Information
The
compress command,
pack command,
uncompress command,
unpack command.
The venerable
Randal
L. Schwartz hands out Useless Use of Cat Awards from time to
time; you can see some recent examples in
Deja News. (The subject line really says "This Week's Useless
Use of Cat Award" although the postings are a lot less frequent
than that nowadays). The actual award text is basically the same
each time, and the ensuing discussion is usually just as uninteresting,
but there are some refreshing threads there among all the flogging
of this dead horse.
The oldest article Deja News finds is from 1995, but it's actually
a followup to an earlier article. By Internet standards, this is
thus an Ancient Tradition.
Exercise: Try to find statistically significant differences
between the followups from 1995 and the ones being posted today.
(See below for a reconstruction
of the Award text.)
Briefly, here's the collected wisdom on using cat:
The purpose of cat is to concatenate (or "catenate")
files. If it's only one file, concatenating it with nothing
at all is a waste of time, and costs you a process.
The fact that the same thread ("but but but, I think it's cleaner
/ nicer / not that much of a waste / my privelege to waste processes!")
springs up virtually every time the Award is posted is also Ancient
Usenet Tradition.
Of course, as Heiner points out, using cat on a
single file to view it from the command line is a valid use of
cat (but you might be better off if you get accustomed
to using less for this instead).
In a recent thread on comp.unix.shell, the following
example was posted by Andreas Schwab as another Useful Use of Cat
on a lone file:
{ foo; bar; cat mumble; baz } | whatever
Here, the contents of the file mumble are output to
stdout after the output from the programs foo and
bar, and before the output of baz. All
the generated output is piped to the program whatever.
(Read up on shell programming constructs if this was news to you
:-)
Edits with cat
Use cat, whose name stands for together, to concatenate
files and standard input streams, as in
Listing 1. The slackers
of the world also use it as a general pager (cat file)
and a complete text-editing environment (cat > file).
Its syntax is unrivaled in its simplicity and, for text editing
one-liners, it gives you quick ways to append or insert text without
an editor.
Listing 1. Using cat to concatenate files
and standard input streams$ (cat - input1 - input2 - input3 - input4) | mailx ted
Ted,
Take a look at these example files.
This is the first file ...
Ctrl-D
This is the second file ...
Ctrl-D
This is the third file -- note the fourth paragraph below ...
Ctrl-D
And here's the last file ...
Ctrl-D
$
Add text to the end of a file
The slackers are on to something, though. When you need to append
text to the end of a file, there's nothing quicker than
cat:
$ cat >> file
> line
> line
> line
Ctrl-D
$
While you're adding lines, pressing Ctrl-U erases the
current line, Ctrl-Z suspends the process, and Ctrl-C
aborts everything. When you're done, press Ctrl-D on a line
of its own. (These are some of the default Korn shell control keys,
but they work for most shells and editing modes.)
If the data you're entering is an X selection that you're pasting
from another window, this one-liner is generally quicker to use
than calling up an editor, opening the target file, moving to the
end of the file, pasting the selection, saving the file, and exiting
the editor. It can also be more useful when you're pasting formatted
or specially formatted text, and you want to keep the formatting
because some text editors and editing modes reformat the X selection
when you paste it.
Although this operation is a common, everyday practice, you always
have to be careful that you use the shell operator for appending
redirection (>>) and not the regular redirection operator
(>); if you mistakenly use the latter, you'll overwrite
the contents of the file with the text you mean to append.
To add the entire contents of one file to the end of another
file, give the filename:
$ cat footnotes.txt >> file
If you're appending only a single line instead of multiple lines
or an entire file, you can use echo instead of
cat:
$ echo "192.255.255.255 bigblue" >> /etc/hosts
To append lines of text that are itemized beginning with 1, use
cat's -n option; lines are preceded with
the line number (offset with up to five space characters) and a
tab character. Add the -b option to suppress the numbering
of blank lines:
$ cat -nb > file
This line is numbered
And so is this
Another numbered line
Ctrl-D
$ cat file
1 This line is numbered
2 And so is this
3 Another numbered line
$
Insert text at the beginning of a file
You can insert text at the beginning of a file with cat
by specifying the standard input with a hyphen (-)
and writing to a new file:
$ cat - file > newfile
This is the beginning of the file
And then the old file is inserted
Below this line:
Ctrl-D
$
Although it's simple, the disadvantage of this one-liner is that
it creates a new file. If you want to insert text into the original
file, the renaming shuffle you have to do makes this almost more
trouble than it's worth. Better ways are just ahead with
ed.
Show nonprinting characters
cat has several useful options. Some of them control
the way it outputs nonprinting characters, such as tabs and control
characters. To determine whether a file or a group of text files
has embedded control characters, use these options. For instance,
if a file has trailing blanks, you can see them:
$ cat -vet input.txt
This line has trailing blanks. $
This line does not.$
$
These options differ according to your UNIX implementation;
Table 1 gives the standard IBM
AIX® operating system options.
dog -- enhanced cat with networking capabilities
Gnu version options
-A, --show-all Equivalent to -vET
-b, --number-nonblank
number nonblank output lines
-e equivalent to -vE
-E, --show-ends
display $ at end of each line
-n, --number
number all output lines
-s, --squeeze-blank
never more than one single blank line
-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.
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rescheduled to a day after tomorrow : Classic Computer Humor :
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