Linux / Unix: Cat Command Examples

How do I use cat command on Linux or Unix-like operating systems?

cat is one of the most frequently used flexible commands on Linux, Apple Mac OS X, Unix, *BSD (FreeBSD / OpenBSD / NetBSD) operating systems

It is a standard Unix program used to concatenate and display files. The cat command display file contents to a screen. Also, you can use cat command for quickly creating a file. The cat command can read and write data from standard input and output devices. Please note that some of the following option will only work with GNU version of the cat command.

Syntax:

The syntax is as follows

 
cat file1
cat > file2
cat file3 | command
cat file4 | grep something
 

Task: Display A File

To view a file, enter:

 
cat filename
cat /etc/passswd
 

Sample outputs:

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
....
...
..hplip:x:109:7:HPLIP system user,,,:/var/run/hplip:/bin/false
vivek:x:1000:1000:Vivek Gite,,,:/home/vivek:/bin/bash
bind:x:110:118::/var/cache/bind:/bin/false
haldaemon:x:111:119:Hardware abstraction layer,,,:/var/run/hald:/bin/false
sshd:x:112:65534::/var/run/sshd:/usr/sbin/nologin
mysql:x:113:121:MySQL Server,,,:/var/lib/mysql:/bin/false

Task: Create A File

To create a file called "foo.txt", enter:

 
cat >foo.txt
 

Type the following text:

This is a test.
Unix is the best.
<control-D>

You need to press [CTRL] + [D] i.e. hold the control key down, then tap d. The > symbol tells the Unix / Linux system that what is typed is to be stored into the file called foo.txt (see stdout for more information). To view a file you use cat command as follows:

 
cat foo.txt
 

Task: Viewing A Large File With cat Command And Shell Pipes

If the file is too large to fit on the computer scree, the text will scroll down at high speed. You will be not able to read. To solve this problem pass the cat command output to the more or less command as follows:

 
cat bigfile | more
cat bigfile | less
 

The more and less command acts as shell filters. However, you can skip the cat command and directly use the Linux / Unix more & less command like this:

 
more bigfile
less bigfile
 

Task: Combine Two Or More Files

You can combine two files and creates a new file called report.txt, enter:

 
cat score.txt names.txt > report.txt
cat report.txt
 

Task: Append Data To A Text File

To append (add data to existing) data to a file called foo.txt, enter:

 
cat >>foo.txt
 

Type the text:

A champion is someone who gets up, even when he can't
<control-D>

Task: Number All Output Lines

Type the following command:

 
cat -n filename
cat --number filename
 

Sample outputs:

Fig.01: Number all output lines with cat command

Task: View Non-printing Characters

To display TAB characters as ^I, enter:

 
cat -T filename
 

To display $ at end of each line, enter:

 
cat -E filename
cat --show-ends filename
 

Use ^ and M- notation, except for LFD and TAB and show all nonprinting:

 
cat -v filename
cat --show-nonprinting filename
 

To show all, enter:

 
cat -A fileName
 

OR

 
cat -vET fileName
 

Sample outputs:

Fig.02: Unix / Linux cat command: View Non-printing Characters

Viewing All Files

You can simply use the shell wildcard as follows:

 
cat *
 

To view only (c files) *.c files, enter:

 
cat *.c
 

Another option is bash for loop, or ksh for loop:

 
#!/bin/bash
for f in /source/project10/*.pl
do
   echo "***** [Start $f ] ****"
   cat -n "$f"
   echo "***** [End $f ] ****"
done
 

OR same using the ksh shell:

 
#!/bin/ksh
for f in $(ls /source/project10/*.pl)
do
        print "*** [Start $f ] ****"
        cat  "$f"
        print "*** [End $f ] ****"
done
 

Print Files

You can directly send a file to to the printing device such as /dev/lp

 
cat resume.txt > /dev/lp
 

On modern systems /dev/lp may not exists and you need to print a file using tool such as lpr:

 
cat resume.txt | lpr
 

OR

 
lpr resume.txt
 

Joining Binary Files

You can concatenate binary files. In old good days most ftp / http downloads were limited to 2GB. Sometime to save bandwidth files size were limited to 100MB. You can combine such files with cat easily:

 
cat file1.bin file2.bin file3.bin > large.tar.gz
### extract it 
tar -zxvf large.tar.gz
 

Another example with the rar command under Unix and Linux:

 
### First combine the files, and use the > shell redirection output to put the DVD image in a file ###
cat file.rar.001 file.rar.002 file.rar.003 file.rar.004 file.rar.005 > dvd.rar
 
## next unrar it ##
unrar e dvd.rar
 
## enjoy dvd ##
mplayer myfile.avi
 

Fooling Programs

You can use the cat command to fool many programs. In this example bc thinks that it is not running on terminals and it will not displays its copyright message. The default output:

 
bc -l
 

Samples session:

bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
4+5
9
quit

Now try with the cat command:

 
bc -l | cat
 

Samples session:

4+5
9
quit

Testing Audio Device

You can send files to sound devices such as /dev/dsp or /dev/audio to make sure sound output and input is working:

 
cat filename >/dev/dsp
cat recording.au >/dev/audio
 

You can simply use the following command for recording voice sample and play back with it cat command:

 
dd bs=8k count=4 </dev/audio >testing123.au
cat testing123.au >/dev/audio
 

Gathering Linux System Information

 
### To see CPU information ###
cat /proc/cpuinfo
 
### To see memory information ###
cat /proc/meminfo
 
### To see Linux kernel version ### 
cat /proc/version
 

Display Large Blocks of Textual Data In A Script

You can use a here document for displaying large blocks of textual data in a script such as help:

 
  cat <<HELPEOF
  Usage:
   opt1 : Do this
   opt2 : Do that
  HELPEOF
 

Another working example:

 
#!/bin/bash
# Author: Vivek Gite <http://www.cyberciti.biz/
# -----------------------------------------------
 
#Set default to my-dev-box
BASEDIR="/home/vivek/projects/bash/nginx-keepalived/chroot"
 
# Now switch to prod 
[[ $HOSTNAME == "lb2.nixcraft.net.in" ]] && BASEDIR="/etc/nixcraft/nginx/lb2"
[[ $HOSTNAME == "lb2.nixcraft.net.in" ]] && BASEDIR="/etc/nixcraft/nginx/lb1"
 
_profile="$BASEDIR/redhat.conf"
_etc_files="$BASEDIR/redhat.etc.files.conf"
_etc_dirs="$BASEDIR/redhat.etc.dirs.conf"
_hooks="$BASEDIR/hooks.sh"
 
usage(){
cat<<EOF
 Usage $0
  -e | --enable:        Enable the nginx-chroot environment
  -E | --upgrade:       Upgrade bind and libs in the nginx-chroot environment
  -p | --php:           Enable the php-cgi in the nginx-chroot environment
  -P | --phpupgrade:    Upgrade the php-cgi in the nginx-chroot environment
  -i | --info:          Display the php-cgi and nginx environment information such as version, users, connections etc
EOF
}
 
rootuser(){
        local uid=$(id -u)
        [[ $uid -ne 0 ]] && { echo "Only root may enable the nginx-chroot environment to the system."; exit 2; }
}
 
## function code removed to keep script short and sweet ##
enable_nginix_chroot(){
	:
}
upgrade_nginx_chroot(){
	:
}
enable_php_cgi_nginx_chroot(){
	:
}
upgrade_php_cgi_nginx_chroot(){
	:
}
 
get_nginx_chroot_info(){
	:
}
 
# Make sure only root run this script
rootuser
 
# Load local hooks
[ -f "${_hooks}" ] && . ${_hooks}
 
# Load os specifc paths
source ${_profile}
 
# Main logic 
case $1 in
    -e|--enable)  	enable_nginix_chroot ;;
    -E|--upgrade) 	upgrade_nginx_chroot;;
    -p|--php)     	enable_php_cgi_nginx_chroot;;
    -P|--phpupgrade) 	upgrade_php_cgi_nginx_chroot;;
    -i|--info)          get_nginx_chroot_info;;
    *)            usage; exit 9999;
esac
 

Print Files In Reverse

No cat cannot print in reverse order but tac command can concatenate and print files in reverse:

 
tac fileName
cat fileName | tac
tac <<<"$myFileName"