cat

 in
Here's a spiffy little command that can be used to combine files, look at the contents of a file and do limited text editing.

The Linux cat command at first seems so simple as to be unnecessary. In actuality, it is an excellent example of the Unix philosophy: create programs which do one thing and do it well. The thing cat does well is display the contents of a file or files. Many other utilities can handle this task, but none have all the options cat does. First, let's look at the simplest case:

cat /etc/motd
This command will display the contents of the motd (messages of the day) file to your screen. Unlike more (or less), cat will not stop when the screen is full. This is a feature, not a bug. You don't want pauses when redirecting (using the > operator) cat's output to a device, e.g., a printer or modem:
cat /etc/motd > /dev/modem
cat comes from the word conCATenate, which describes one of its best uses: to concatenate or “glue together” two or more files. If you have several individual files about animals you would like to collect together into one file, cat will do the work for you. For example:
cat tiger lion cougar > bigcats
would redirect the concatenation output, containing the three feline files in the specified order, into a new file named bigcats. If you find another file, panther, that needs to be added to the bigcats file, use cat with the append (>>) operator in the following way:
cat panther >> bigcats
Using >> ensures any prior content of bigcats is preserved. The content of panther is appended to bigcats. If you were to use the > operator here, you would replace the contents of bigcats with the contents of panther. Always use >> when you wish to add to the end of an existing file.

Be careful not to use the same file name when redirecting the output of the cat command, or you could lose one of the files. For example, don't do the following:

cat myfile yourfile > yourfile

In this case yourfile gets overwritten by myfile.

Another surprisingly handy use for cat is to redirect standard input like this:

cat > newfile
some notes I want to save in newfile.
CTRL-D

This creates a new file (named newfile). You type as much text as you wish, then type ctrl-D to save the file. You can backspace over mistakes, but you cannot go to a previous line after you press the enter key. I often teach this particular option of cat to novice Unix users, who occasionally need to create simple files, but don't want to learn vi or other simple editors. There may be Unix systems without vi or your favorite editor, but cat is always there.

The operator >> can also be used to append notes to the end of newfile:

cat >> newfile
Adding another note to newfile.
CTRL-D
cat Switches

Like most Unix commands, the behavior of cat can be modified by command line switches. If you use the diff command to compare files, it will show you the numbers of lines that differ between the files. However, most files don't have line numbers. Use cat with the -n switch to number each line of a file:

cat -n kittens > num_kittens

The file num_kittens is kittens with a number in front of each line, including blank lines. Use the -b switch to number only lines that are not blank.

One last cat trick: using the -v switch will show you “hidden” characters, such as control characters that may not show up in your editor. Try this experiment:

cat > catestv
CTRL-v testing CTRL-O Testing esc-b
CTRL-D

If we use cat to view the file, we see only the normal text:

cat catestv
Testing Testing
To see what's actually in the file, use the -v option:
cat -v catestv
^V Testing ^O Testing ^[b
Here, a ^ in front of a character signifies a control character. (CTRL-[ is the same as ESC).

Patrick Hill (apathos@bham.net) is a computer engineer at Alabama Power Company in Birmingham, Alabama. He is known around the office as the guy who uses cat for an editor.

______________________

Comments

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Good info

Pharmf521's picture

Hello! dbdegdd interesting dbdegdd site!

cat command for several grib files

Mubashar's picture

I have many grib files (starting from ecmwf19860101.grb to ecmwf20090101.grb). These are basically daily files and the total number of files is nearly 30000 and I want to cat them into one file. When I use the command:

cat ecmwf*>ecmwf.grb
-bash: /bin/cat: Argument list too long

Why its not working on large number of files?
Is there any limit of using "cat" command?

Although I used above command for some of the files and it works.

Please advise me if it is possible to cat large number of grib files?

Thanks in advance.

Regards,

Mubashar

Problem in unix scripting

Lakshmi's picture

Hi , i am facing the problem in unix shell scripting.

#!/bin/sh
if[$# -ne 2]
then
echo"Numbers not supplied"
echo"Usage $0 number1 number2"
exit 1
fi
echo"the sume is `expr $1 + $2`"

not able to execute.

giving the error

./test: line 2: if[0 != 2]: command not found
./test: line 3: syntax error near unexpected token `then'
./test: line 3: `then'

Please help ASAP

Regards,
Lakshmi

White space

Mitch Frazier's picture

You're missing some needed whitespace:

- Between the "if" keyword and the "[" command.
- After the "[" command.
- Before the "]" argument to the "[" command.
- After the echo commands and their arguments.

#!/bin/sh
if [ $# -ne 2 ]
then
echo "Numbers not supplied"
echo "Usage $0 number1 number2"
exit 1
fi
echo "the sume is `expr $1 + $2`"

It's also considered "good" practice, but not mandatory, to indent the commands within an "if" statement:

#!/bin/sh
if [ $# -ne 2 ]
then
    echo "Numbers not supplied"
    echo "Usage $0 number1 number2"
    exit 1
fi
echo "the sume is `expr $1 + $2`"

Mitch Frazier is an Associate Editor for Linux Journal.

1997...?

Anonymous's picture

1997? Is this correct?
The time stamp of the posting says: "October 1st, 1997 by Patrick Hill"

That's only... 12 years ago! :x

No, I just posted this in

Dif's picture

No, I just posted this in September of 2009

Using Cat

Dig's picture

Hi,

I have used the cat command to put the contents of a large number of files into one large file. So I have each files contents in this large file without a blank line between the files. How can I add the filename before the contents of the file and a blank line between the files in my large output file?

Using Cat

Dig's picture

OK, I have all the filenames showing before the contents and with a carriage return after the filename. I just need to know how to add a carriage return after each files contents. Here is the code.

#!/bin/sh

# no error checking
rm -f newfile
for i in *.spc;
do printf "%s\n" $i >> newfile
cat "$i" >> newfile
done

To print specific range of lines using cat

imtiyaz hussain's picture

Hello Patrick,

I am using AIX Unix. How is it possible to select lines to be printed using cat ? If the file size is very big and say it has more than thousand lines and i want to print lines 65 to 70 on standard output.

Regards,
Immi.

To print specific lines using cat and sed commands

Gibies George's picture

Hello Immi.

You can pipe the output of cat command to sed command as follows.

cat filename |sed -n '65,70p'

This will print lines 65 to 70 on standard output.

If you want to print only one line, you can use the following syntax.

cat filename |sed -n '5p'

This will print only line 50 of the file on standard output.

Sorry for the mistake.

Gibies George's picture

Sorry for the mistake.

please read the last line of the my commend corrected as "...line 5 of the file...."

Re: Take Command: cat

Anonymous's picture

Good examples. Something puzzles me though. I am taking a Unix class using Solaris 2.8 on Sun workstations, but I do my practicing on a PC with Linux (RedHat 8). In Unix you can create a new file with:

cat fileName
blah blah
Ctrl-D

But with Linux this produces a "no such file" error, so you must explicitely redirect the output with:

cat > fileName ...

I have tried it with all of the shells installed and get the same. I wonder why the difference?

Re: Take Command: cat

Anonymous's picture

i am very thankful for the detailed information
of the cat command. many of the linux sites
are not having the basic information of cat
command which is must. i pity the negligence
attributed by the linux site maintainers.

any how i am thankful for the detailed and
friendly information.

thanking you

White Paper
Linux Management with Red Hat Satellite: Measuring Business Impact and ROI

Linux has become a key foundation for supporting today's rapidly growing IT environments. Linux is being used to deploy business applications and databases, trading on its reputation as a low-cost operating environment. For many IT organizations, Linux is a mainstay for deploying Web servers and has evolved from handling basic file, print, and utility workloads to running mission-critical applications and databases, physically, virtually, and in the cloud. As Linux grows in importance in terms of value to the business, managing Linux environments to high standards of service quality — availability, security, and performance — becomes an essential requirement for business success.

Learn More

Sponsored by Red Hat

White Paper
Private PaaS for the Agile Enterprise

If you already use virtualized infrastructure, you are well on your way to leveraging the power of the cloud. Virtualization offers the promise of limitless resources, but how do you manage that scalability when your DevOps team doesn’t scale? In today’s hypercompetitive markets, fast results can make a difference between leading the pack vs. obsolescence. Organizations need more benefits from cloud computing than just raw resources. They need agility, flexibility, convenience, ROI, and control.

Stackato private Platform-as-a-Service technology from ActiveState extends your private cloud infrastructure by creating a private PaaS to provide on-demand availability, flexibility, control, and ultimately, faster time-to-market for your enterprise.

Learn More

Sponsored by ActiveState