File Reading and Editing¶
This guide is only going to show an overview of the commands. To find out more about each command you can run the following:
man [command]
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.
cat fileone.txt
one
two
three
four
five
cat filetwo.txt
ONE
TWO
THREE
FOUR
FIVE
cat¶
cat is mostly used today to output a file to the screen.
[root@c7 ~]# cat fileone.txt
one
two
three
four
five
It was originally to combine the output of two files.
[root@c7 ~]# 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@c7 ~]# cat > filethree.txt
new file content
^C
[root@c7 ~]# 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.
Note
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@c7 ~]# 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@c7 ~]# 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@c7 ~]# 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]