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Unix cat and zcat command

The cat utility concatenates and display files.  It also can be used as a primitive editor for creation one liners or adding a line to the config file. There is also a version of cat called zcat which performs operation similar to  uncompress -c or for GNU version gunzip -c on each input file, for example:

zcat file.tar.Z | tar xvf -

It reads each file in the order listed and writes the content of the file to the standard output.

Thus: cat file prints file on your terminal, and: 

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

Also it 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

Solaris version is less feature rich then GNU version. Also Solaris version and GNU version assign option -s different semantic.

General syntax is

cat [-nbsuvet] [file...]

Solaris Cut Options:

• -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.

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

Examples

Example 1 Concatenating a file

The following command:

cat -n myfile

writes the contents of the file myfile to standard output with each line numbered.

Example 2 Concatenating two files into one

The following command:

cat doc1 doc2 > doc.all

concatenates the files doc1 and doc2 and writes the result to doc.all.

Example 3 Concatenating two arbitrary pieces of input with a single invocation

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

ENVIRONMENT VARIABLES

See environ(5) for descriptions of the following environment variables that affect the execution of cat: LC_CTYPE, LC_MESSAGES, and NLSPATH.

EXIT STATUS

The following exit values are returned:

0

All input files were output successfully.

>0

An error occurred.

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

Old News ;-)

The Power of Z Commands – Zcat, Zless, Zgrep, Zdiff Examples

Example 1: View Compressed File and Uncompress with zcat

Compressing 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.

zcat Command

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.

Useless Use of Cat Award

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 :-)

[ Jan 16, 2007 ] Save time with text editing one-liners by Michael Stutz (stutz@dsl.org)

Take a quick look at some essential editing one-liners that can save you time and effort. Text-editing operations are normally done interactively, inside a text editor application. Some tasks, however, can be accomplished quickly and easily, right from the UNIX® command line. What's more, these one-liners can be used in scripts to automate various editing procedures.

Introduction

Most UNIX® developers settle on Emacs, vi, or one of the many variants, offshoots, and clones of these two text-editing applications. Files are normally opened in the editor of choice, and changes are interactively specified and applied to the file by the operator.

But you can often do an editing job at the command line more quickly than it takes to open the file in a text editor. A complex editing procedure can be programmed and specified from the command line and executed across multiple files, eliminating all unnecessary screen display, cursor motion, and manual interaction with the files. A good tactic is to keep a cache of relevant one-liners on hand to do common editing jobs. Not only do they save you time, especially in batch operations involving multiple files, but you can also use them in scripts.

One-liners for editing and processing text are a famous tradition in the Perl and AWK (and lately Ruby) languages and, of course, the shell. This article demonstrates basic text-editing techniques with three of the most primary command-line editing tools readily available on all systems: cat, ed, and sed. The editing examples that follow start with the simplest and most common constructs and work up to the more complex.

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.

Recommended Links

• cat (Unix) - Wikipedia, the free encyclopedia
• cat The Open Group Base Specifications Issue 7
• cat  GNU Coreutils reference
• cat(1)  OpenBSD General Commands Manual
• cat(1) FreeBSD General Commands Manual
• zcat(1)  FreeBSD General Commands Manual

Reference

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.