Working with Linux file content
In this chapter we will look at the contents of linux files with cat, tac , head, tail, , more and less.We will also get a glimpse of the possibilities of tools like cat on the command line.
Note: Please check video at bottom of article
head
You can use head to display the first ten lines of a file.
[ali@rhel71 ~]$ head /etc/passwd root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/spool/mail:/sbin/nologin operator:x:11:0:operator:/root:/sbin/nologin
The head command can also display the first n lines of a file.
[ali@rhel71 ~]$ head -2 /etc/passwd root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin [ali@rhel71 ~]$
tail
The tail command will display the last ten lines of a file.
[ali@rhel71 ~]$ tail /etc/passwd qemu:x:107:107:qemu user:/:/sbin/nologin chrony:x:994:993::/var/lib/chrony:/sbin/nologin pulse:x:171:171:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin gdm:x:42:42::/var/lib/gdm:/sbin/nologin gnome-initial-setup:x:993:991::/run/gnome-initial-setup/:/sbin/nologin avahi:x:70:70:Avahi mDNS/DNS-SD Stack:/var/run/avahi-daemon:/sbin/nologin postfix:x:89:89::/var/spool/postfix:/sbin/nologin sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin tcpdump:x:72:72::/:/sbin/nologin ali:x:1000:1000:ali khan:/home/ali:/bin/bash [ali@rhel71 ~]$ You can give tail the number of lines you want to see. [ali@rhel71 ~]$ tail -6 /etc/passwd gnome-initial-setup:x:993:991::/run/gnome-initial-setup/:/sbin/nologin avahi:x:70:70:Avahi mDNS/DNS-SD Stack:/var/run/avahi-daemon:/sbin/nologin postfix:x:89:89::/var/spool/postfix:/sbin/nologin sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin tcpdump:x:72:72::/:/sbin/nologin ali:x:1000:1000:ali khan:/home/ali:/bin/bash [ali@rhel71 ~]$
cat
The cat (short for “concatenate“) command is one of the most universal tools. All it does is copy standard input to standard output. In combination with the shell this can be very powerful and diverse. Some examples will give a glimpse into the possibilities. The first example is simple, you can use cat to display a file on the screen. If the file is longer than the screen, it will scroll to the end.
[ali@rhel71 ~]$ cat /etc/resolv.conf # Generated by NetworkManager search example.com nameserver 192.168.0.1 [ali@rhel71 ~]$
concatenate
cat is short for concatenate. In this example We will concatenate contents of three files to standard output (terminal screen)
[ali@rhel71 ~]$ echo this is tutorial > file1 [ali@rhel71 ~]$ echo about linux > file2 [ali@rhel71 ~]$ echo enjoy > file3 [ali@rhel71 ~]$ cat file1 file2 file3 this is tutorial about linux enjoy [ali@rhel71 ~]$
Create files
You can use cat to create file. In below example we create file by cat > filename, once we are done with writing all content hit enter. Then press CTRL+d and you are done. To display content type cat filename
[ali@rhel71 ~]$ cat > health.txt Good health is blessing [ali@rhel71 ~]$ cat health.txt Good health is blessing [ali@rhel71 ~]$
The Ctrl d key combination will send an EOF (End of File) to the running process ending
the cat command.
tac
tac is reverse of cat. Tac will display lines in reverse order .Just one example will show you the purpose of tac (as the opposite of cat).
[ali@rhel71 ~]$ cat > tac.txt 1 2 3 4 this is line number 5 6 this is last line number 7 [ali@rhel71 ~]$ tac tac.txt this is last line number 7 6 this is line number 5 4 3 2 1 [ali@rhel71 ~]$
custom end marker
Instead of pressing CTRL d .You can choose an end marker for cat with << as is shown below. This construction is called a here directive and will end the cat command.
[ali@rhel71 ~]$ cat > ending_of_file.txt <<end > this file shows how to end file using here directive with marker end > end [ali@rhel71 ~]$ cat ending_of_file.txt this file shows how to end file using here directive with marker end [ali@rhel71 ~]$ copy files cat can also be used to copy files. [ali@rhel71 ~]$ cat > original.txt this text is in original file [ali@rhel71 ~]$ touch copy.txt [ali@rhel71 ~]$ cat copy.txt [ali@rhel71 ~]$ cat original.txt this text is in original file [ali@rhel71 ~]$ cat original.txt >copy.txt [ali@rhel71 ~]$ cat copy.txt this text is in original file [ali@rhel71 ~]$
more and less
If contents of file are taking space on display more than a page then you can use either more or less. More will allow you to see the contents of the file page by page. Use the space bar to see the next page, or q to quit.
And now you can watch video tutorial
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