cat is one of the most frequently used flexible commands on Linux, Apple Mac OS X, Unix, *BSD (FreeBSD / OpenBSD / NetBSD) operating systems.
- View A File
- Create A File
- Cat and Pipes
- Combine Files
- Append To A File
- Number All Lines
- Non-printing Characters
- View Large Number Of Files
- Print Files
- Joining Binary Files
- Fooling Programs
- Testing Audio Device
- Gathering Linux System Information
- Display Large Blocks of Textual Data In A Script
- Print Files In Reverse
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:Sample outputs:
cat filename
cat /etc/passswd
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:Type the following text:
cat >foo.txt
This is a test.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:
Unix is the best.
<control-D>
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: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:
cat bigfile | more
cat bigfile | less
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:Type the text:
cat >>foo.txt
A champion is someone who gets up, even when he can't
<control-D>
Task: Number All Output Lines
Type the following command:Sample outputs:
cat -n filename
cat --number filename
Task: View Non-printing Characters
To display TAB characters as ^I, enter:To display $ at end of each line, enter:
cat -T filename
Use ^ and M- notation, except for LFD and TAB and show all nonprinting:
cat -E filename
cat --show-ends filename
To show all, enter:
cat -v filename
cat --show-nonprinting filename
OR
cat -A fileName
Sample outputs:
cat -vET fileName
Viewing All Files
You can simply use the shell wildcard as follows:To view only (c files) *.c files, enter:
cat *
Another option is bash for loop, or ksh for loop:
cat *.c
OR same using the ksh shell:
#!/bin/bash
for f in /source/project10/*.pl
do
echo "***** [Start $f ] ****"
cat -n "$f"
echo "***** [End $f ] ****"
done
#!/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/lpOn modern systems /dev/lp may not exists and you need to print a file using tool such as lpr:
cat resume.txt > /dev/lp
OR
cat resume.txt | lpr
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:Another example with the rar command under Unix and Linux:
cat file1.bin file2.bin file3.bin > large.tar.gz
### extract it
tar -zxvf large.tar.gz
### 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:Samples session:
bc -l
bc 1.06.95Now try with the cat command:
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
Samples session:
bc -l | cat
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:You can simply use the following command for recording voice sample and play back with it cat command:
cat filename >/dev/dsp
cat recording.au >/dev/audio
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:Another working example:
cat <<HELPEOF
Usage:
opt1 : Do this
opt2 : Do that
HELPEOF
#!/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"
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