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File Handling

1. File viewing

In Linux, you are going to handle files with lots of different types, and want to see what’s in there. On this page, we will explore how to view the file contents if they are in text in the Linux system.

cat [options] [files]

The simplest viewer is cat, which just copies its files to standard output, concatenating them.

$ cat filename

This command dump all the contents on the screen, so large files will likely scroll off screen. If this is the case, you may consider using less command.

useful options

less [options] [files]

Use less to view text one page at a time (or one window or screenful at a time). It’s great for text files.

While running less, type h for a help message, but here are a few keystroke being used often.

Keystroke Meaning
h, H View a help page.
spacebar, f Move forward one screenful.
Enter Move forward one line.
b Move backward one screenful
/ Enter search model. n goes to the next match, while N goes to the previous match.
q quite less.

useful options

Other useful commands

head and tail are also useful in viewing files. To find out how to use them, you can refer to the manual after calling it.

$ man head
or
$ man tail

The manual of these command will be printed on the screen using less command. That means that you can move around between pages using the keystrokes in the table above.

2. File Creation and Editing

Creating files is easy and can be done in many different ways in Linux. Instead of printing out the output on the screen, you can create a file containing the output by redirecting it.

$ echo save text in the file > output
$ ls -l > listdir

You can also quickly create an empty file using touch command,

$ touch newfile

If you want to create and edit a file, you can try text editors. Two major text editors are emacs and vim in Linux, and new files can be created with the following commands.

$ emacs newfile
or
$ vim newfile

There is a steep learning curve for these editors, but once you get familiar with them, you can do so many things very efficiently. Throughout the course, you need to create many text files to analyze the atmospheric data, and these text editors may boost your productivity.

I use vim daily basis. So, I will introduce it briefly here with a few examples (There are tons of material about these text editors online. Of course, you do not have to use them.)

vim [options] [files]

vim operates in two modes, insert and normal. Insert mode is for entering text in the usual manner, while normal mode is for running commands like “delete a line” or copy/paste.

Here are basic keystrokes in normal mode.

Keystroke Meaning
$ vim run editor in current terminal.
i enter insert mode.
esc exit insert mode back to normal mode
:q quit vim when there is no modification
:q! quit vim without saving modification
:wq quit vim by overwriting the file with modification
:w save modification (overwrite)
:w filename save as filename
u undo
^R redo
l or right arrow move forward
h or left arrow move backward
k or up arrow move up
j or down arrow move down
w move to next word
b move to previous word
e move to the end of the current word
0 move to beginning of line
$ move to end of line
^f move down 1 screen
^b move up 1 screen
gg move to beginning of buffer
G move to end of buffer i
x delete next character
X delete previous character
dd delete current line
de delete all characters to the end of the current word
dw delete all characters before the beginning of the next word
de delete all characters from the beginning of the current word
d0 delete all characters from the beginning of the current line
d$ delete all characters to the end of the current line
yy copy current line
ye copy all characters to the end of the current word
yw copy all characters before the beginning of the next word
ye copy all characters from the beginning of the current word
y0 copy all characters from the beginning of the current line
y$ copy all characters to the end of the current line
p paste
r and character replace with the character
R replace mode
o enter insert mode at the one line below
O enter insert mode at the one line up
. repeat the previous action
: switch to command mode

If you are in the intert mode, editing the text is quite similar to other programs like MS words.