Linux Admin - cat Command

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The cat command is used to concatenate files and print to standard output. Formerly, we have demonstrated both uses and abuses with the cat command. cat servers the following distinct purposes −

• Show files contents

• Write contents of one file to another file

• Combine multiple files into a single file

• Support special features: adding line numbers, showing special characters, eliminating blank lines

As noted previously, when using utilities such as grep, sort, and uniq we want to avoid piping output from cat if possible. We did this for simple demonstration of piping commands earlier. However, knowing when to perform an operation with a utility like grep is what separates Linux Administrators from Linux end-users.

Bad Habit

[root@centosLocal centos]# cat /etc/passwd | sort -t: -k1  | grep ":0" 
halt:x:7:0:halt:/sbin:/sbin/halt 
operator:x:11:0:operator:/root:/sbin/nologin 
root:x:0:0:root:/root:/bin/bash 
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown 
sync:x:5:0:sync:/sbin:/bin/sync 
[root@centosLocal centos]#

Good Habit

[root@centosLocal centos]# grep ":0" /etc/passwd | sort -t: -k 1  
halt:x:7:0:halt:/sbin:/sbin/halt 
operator:x:11:0:operator:/root:/sbin/nologin 
root:x:0:0:root:/root:/bin/bash 
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown 
sync:x:5:0:sync:/sbin:/bin/sync 
[root@centosLocal centos]#

Note − piping cat to secondary commands like sort or grep should only be done when it is needed.

One common use of cat is when dealing with Windows formatted line breaks. Both Linux and Windows by internal design, use a different control code to represent End Of Line (EOL) −

* Linux line break is always a Line Feed: LF or depicted as "\n". 
* Windows is Carriage Return followed by a Line Feed: CR LF or depicted as "\r\n".
* Macintosh, in all moderne releases of OS X and now macOS, has adopted the Linux/Unix 
standard of LF or "\n"

So, let's say we open our file in a GUI text-editor like gedit or are experiencing random issues while applying filtering commands. Text appears on a single line, or filtering commands do not operate as expected.

Especially, when the text file was downloaded off the Internet, we want to check line breaks. Following is a sample output from cat showing EOL characters.

[root@centosLocal centos]# cat -E  ./Desktop/WinNames.txt  
 $ed:Daniel:101 
 $enny:Colon:608 
 $ana:Maxwell:602 
 $arian:Little:903 
 $obbie:Chapman:403 
 $icolas:Singleton:203 
 $ale:Barton:901

Notice the preceding "$" on each line? Linux is reading the CR "\n", breaking the file. Then translating a Carriage Return over the first character of each file.

When viewed without the -E switch, the file looks fine −

[root@centosLocal centos]# cat  ./Desktop/WinNames.txt  
 Ted:Daniel:101 
 Jenny:Colon:608 
 Dana:Maxwell:602 
 Marian:Little:903 
 Bobbie:Chapman:403 
 Nicolas:Singleton:203 
 Dale:Barton:901

Luckily, with Linux filtering commands this is an easy fix −

[root@centosLocal centos]# sed -i 's/\r$//g' ./Desktop/WinNames.txt  
[root@centosLocal centos]# cat -E ./Desktop/WinNames.txt  
Ted:Daniel:101$ 
Jenny:Colon:608$ 
Dana:Maxwell:602$

Note − When viewed with the -E switch, all Linux line breaks will end in $.

cat can also be used to combine multiple files into a single file.

[root@centosLocal centos]# cat linux.txt  
CentOS 
Ubuntu 
Red Hat 
Suse 
Debian 
[root@centosLocal centos]# cat windwos.txt  
NT 3.5 
NT 4.0 
Server 2000 
Server 2003 
Server 2008 
Server 2012 
Server 2016 
[root@centosLocal centos]#

Let's now use cat to combine the two files.

[root@centosLocal centos]# cat windwos.txt linux.txt > server_class_operating_sytems.txt 
[root@centosLocal centos]# cat server_class_operating_sytems.txt  
NT 3.5 
NT 4.0 
Server 2000 
Server 2003 
Server 2008 
Server 2012 
Server 2016 
CentOS 
Ubuntu 
Red Hat 
Suse 
Debian 
[root@centosLocal centos]#

Finally, we can use the -n switch to number each output line. This will give us a total of lines.

[root@centosLocal centos]# cat -n ./server_class_operating_sytems.txt  
 1  NT 3.5 
 2  NT 4.0 
 3  Server 2000 
 4  Server 2003 
 5  Server 2008 
 6  Server 2012 
 7  Server 2016 
 8  CentOS 
 9  Ubuntu 
10  Red Hat 
11  Suse 
12  Debian [root@centosLocal centos]#