Unix: Final word
Other useful commands
This chapter is just an alphabetically sorted list of Unix commands that did not fit anywhere else in this ebook. The only criteria for including a command here is that it generally considered part of the Unix operating system, is widely distributed, and that I myself use it frequently.
clear
If your terminal is cluttered, it may not be easy to see what is going on. The following command will clear the screen:
$ clear
This will clear all text and leave you with the command line prompt at the top of the terminal window.
Managing disk space
The df (disk free) command reports on the space left
on the file system.
The df command without any arguments shows the entire
file system disk space using the default 1K-blocks.
$ df Filesystem 1K-blocks Used Available Use% Mounted on udev 32935544 0 32935544 0% /dev tmpfs 6597712 680740 5916972 11% /run /dev/mapper/vg01-root 34219840 26103864 6359740 81% / tmpfs 32988552 0 32988552 0% /dev/shm tmpfs 5120 0 5120 0% /run/lock tmpfs 32988552 0 32988552 0% /sys/fs/cgroup /dev/mapper/vg01-www 47926152 8146168 37322384 18% /www /dev/mapper/vg01-home 23897980 10187504 12473484 45% /home cgmfs 100 0 100 0% /run/cgmanager/fs tmpfs 6597712 4 6597708 1% /run/user/1003
To get the same overview in a human-readable format, use the -h flag:
$ df -h Filesystem Size Used Avail Use% Mounted on udev 32G 0 32G 0% /dev tmpfs 6,3G 665M 5,7G 11% /run /dev/mapper/vg01-root 33G 25G 6,1G 81% / tmpfs 32G 0 32G 0% /dev/shm tmpfs 5,0M 0 5,0M 0% /run/lock tmpfs 32G 0 32G 0% /sys/fs/cgroup /dev/mapper/vg01-www 46G 7,8G 36G 18% /www /dev/mapper/vg01-home 23G 9,8G 12G 45% /home cgmfs 100K 0 100K 0% /run/cgmanager/fs tmpfs 6,3G 4,0K 6,3G 1% /run/user/1003
There is also a -m flag to show 1M-blocks:
Use the -T flag to see the type of file system. To
see only a specific type of file system, use
the -t (lower case âtâ) flag followed by the type of file
system. For example, to see the free space on major partitions in a
human readable format, use:
$ df -h -t ext4 Filesystem Size Used Avail Use% Mounted on /dev/mapper/vg01-root 33G 25G 6,1G 81% / /dev/mapper/vg01-www 46G 7,8G 36G 18% /www /dev/mapper/vg01-home 23G 9,8G 12G 45% /home
A full stop shows the current file system. For example, to find out how much space is left on the partion holding the files for the web server, type:
$ cd /www $ df . /dev/mapper/vg01-www 47926152 8146168 37322384 18% /www
While the df command reports how much disk space we
have, the du (disk use) command reports how much disk
space is being consumed by the files and folders.
The du command outputs the number of blocks used by
each file or directory given as argument. The default block size
1K). This is useful if you are running short of space and you want to
find out which sub-directory uses up most space. Example of use:
$ du -s * …
The -s flag will display only a summary (total size)
and the * means check all files and directories.
Like the df command, there is a -h flag
to display the sizes in a human-readable format:
$ du -s -h * 112K bin 664K configfiles 203M db_backups 5.7G drush-backups 112K pareview 2.2G progressive 2.2M sink 4.0K z_tegn.txt
file
file classifies the named files according to the type
of data they contain, for example ASCII (text), pictures, compressed
data, etc.. To report on all files in your home directory, type:
$ file *
history
The shell keeps an ordered list of the most recent commands that you have entered. Each command is given a number according to the order it was entered. To see the list, type:
$ history
In most modern shells, including bash, you can use the
exclamation character (!) as a shortcut to recall
commands from the history
!!– recall last command!-2– recall second most recent command!810– recall 81th command in list!wc– recall last command starting with wc
You can set the size of the history buffer by assigning to the
shell variable HISTSIZE. Example:
$ HISTSIZE=200
Put this in you .profile and export it if you want the change to be permanent.
ln
The ln command let you create file system links. The -s flag
creates an symbolic (as opposed to a hard) link. Example:
$ ln -s /usr/share/phpmyadmin phpmyadmin
A symbolic link points to another file or directory, while a hard link is an additional directory entry for the same data. Deleting the target of a symbolic link renders the link useless. Deleting a hard linked file only removes the directory entry, and the data is not removed until all directory references to it is deleted..
who
If you're using Unix on a multi-user machine, you may want to know who, in addition to yourself, are logged in.
The command who will list the users that are currently
logged in. The list has the following four fields: user name,
tty/pts number, date and time of log-in, and the serial port or
the hostname of the machine used to connect.
$ who gisle tty3 2012-08-07 05:33 (:0) sally pts/0 2012-09-16 16:44 (hagbart.nvg.ntnu.no) ellen pts/1 2012-09-29 13:20 (slagelg.uio.no) john pts/2 2012-10-15 09:04 (login.roztr.com)
The letters “tty” refers to a teletype machine, which was used as a console for Unix systems back in the stone age. Teletype machines doesn't exist any more. Today, when a user is connected to a tty, it simply means that he or she is connected to the computer via a serial port (i.e. he or she is in the same building as the computer). The letters “pts” stands for pseudo-terminal slave. If a user is connected via a pts, he or she is using a network connection and may be using the computer from across the globe.
Obviously, if you're using Unix on your own personal computer,
there are no other users, and the output from the who
command is less interesting.
Summary
| Command | Meaning |
|---|---|
clear |
clear the terminal screen |
df |
report file system disk space usage |
du |
estimate file space usage |
file |
determine file type |
history |
show command history list |
who |
list users currently logged in |
