There are several ways to output a file in Linux.
Here are two test files and their content, we’ll be using these in the upcoming examples.
[root@sitehub ~]# cat fileone.txt
one
two
three
four
five
[root@sitehub ~]# cat filetwo.txt
ONE
TWO
THREE
FOUR
FIVE
cat
cat is mostly used today to output a file to the screen.
[root@sitehub ~]# cat fileone.txt
one
two
three
four
five
It was originally to combine the output of two files.
[root@sitehub ~]# cat fileone.txt filetwo.txt
one
two
three
four
five
ONE
TWO
THREE
FOUR
FIVE
It can be used to add content to a file.
[root@sitehub ~]# cat > filethree.txt
new file content
[root@sitehub ~]# cat filethree.txt
new file content
After the cat > filethree.txt command, press enter and enter the text you want to add to your file. End this with a new line and ctrl + c.
This command will NOT tell you if the file already exists so it’s very easy to overwrite a file. Use with care.
less
For very large files less will allow you to scroll up and down a file. Search the file with the/searchtermkeys. Use then` key to keep searching.
head
By default head will show the top 10 lines of a file. This can be adjusted with the -n flag.
[root@sitehub ~]# head -n 2 fileone.txt
one
two
tail
By default tail will show the bottom 10 lines of a file. This can adjusted with the -n flag.
[root@sitehub ~]# tail -n 2 fileone.txt
four
five
If your file is changing, you can watch the output. This is best used on logs like mail logs.
[root@sitehub ~]# tail -f /var/log/maillog
Aug 10 00:00:21 c7 sSMTP[31684]: Unable to locate mail
Aug 10 00:00:21 c7 sSMTP[31684]: Cannot open mail:25
Aug 10 11:10:44 c7 postfix/postqueue[23404]: fatal: Cannot flush mail queue - mail system is down
Aug 10 11:10:54 c7 postfix/postfix-script[23546]: starting the Postfix mail system
Aug 10 11:10:54 c7 postfix/master[23548]: daemon started -- version 2.10.1, configuration /etc/postfix
Aug 11 00:00:07 c7 sSMTP[25365]: Unable to locate mail
Aug 11 00:00:07 c7 sSMTP[25365]: Cannot open mail:25
Aug 11 11:17:21 c7 postfix/smtpd[9292]: connect from localhost[::1]
Aug 11 11:18:32 c7 postfix/smtpd[9292]: lost connection after CONNECT from localhost[::1]
Aug 11 11:18:32 c7 postfix/smtpd[9292]: disconnect from localhost[::1]
