The command#
Commands are our tool to tell the computer what to do. Most commands have options and arguments. Arguments are often essential for a command to operate properly; they are the pieces of information required by a command, such as a file name. Options are, of course, optional, and offer ways to modify the way the command works.
For instance, echo will take any text you give it as an argument and then send it back to you as output:
# My first command
echo 'Hello World!'
If you use the option -n, then it will not add a ‘new line’ to the end of the output:
# My second command
echo -n 'Hello World!'
Some commands end up with very complex structures, because they can have many options and arguments. In general, options will be of the format -a where a is a single letter or --word where word is a string (a series of letters, in computer terms).
Note: the command line is case-sensitive! So it does matter if you write -a or -A.
Getting help#
The man command will show a manual for most basic commands, providing the correct syntax to use it and the various options available.
# Read the manual
man ls
Other programs have different ways to provide help on how to use them. An online tutorial is best, or a comprehensive manual, but sometimes you only have the command line to help you.
# Help please!
python3 -h
python3 --help
Useful command line tricks#
You can use the up and down arrow keys to navigate through previously used commands (known as your history) and repeat or modify them.
Windows: To copy text from the terminal you will have to highlight it and right-click to use the in-browser menu and copy. Similarly you have to use the in-browser menu to paste into the terminal. The reason for this is that Ctrl + c and Ctrl + v have effects inside the terminal.
Mac: You can fortunately use Cmd + c and Cmd + v to copy and paste as normal. You can use Ctrl and various keys for in-terminal commands.
When typing a command or file name, you can press the ‘tab’ key to auto complete what you are typing. If there are multiple commands or files with similar names, auto completion will fill in as far as the first ambiguous character before you have to give it some more input. This method makes it much less likely that you make a spelling error. Also, if you double press the ‘tab’ key all the available options to complete will be shown.
Pressing Ctrl + c will send an interrupt signal that cancels the currently running command and brings you back to the command line.
Pressing Ctrl + r will allow you to search through your command history.
Pressing Ctrl + l will clear the screen.
See previous commands by typing history and pressing enter.
Double-click to select a word, triple-click to select a line
Using a # character allows you to make comments that have no effect when run.
Exercise 0.1
Connect to Cousteau
First, you have to open the command line interface on you computer For Windows: Type “command” into the taskbar menu For Mac: Click onto the launchpad icon in the dock and type “Terminal” into the search field
Second, connect to Cousteau with the ssh command. You need your user ID and your nethz password in order to connect to the serve. The command to connect to the Cousteau is:
ssh <your ETH-ID>@cousteau.ethz.ch
Try to echo “My first command”
#echoing "My first command"
echo 'My first command'
Use the arrow key to execute the same command again
# Press the up arrow once and the last command appears
echo 'My first command'
Try typing e then pressing tab twice, what do you see?
# You see all the possible commands that start with "e" when you press tab twice after entering “e”
e2freefrag edquota era_check eu-readelf
e2fsck efibootdump era_dump eu-size
e2image efibootmgr era_invalidate eu-stack
e2label efikeygen era_restore eu-strings
e2mmpstatus efisiglist esac eu-strip
e2undo efivar escputil eutp
e4crypt egrep espdiff eu-unstrip
e4defrag eject espeak-ng eval
eapol_test elfedit ether-wake evince
easy_install-2 elif ethtool evince-previewer
easy_install-2.7 else eu-addr2line evince-thumbnailer
easy_install-3 enable eu-ar evmctl
easy_install-3.6 encguess eu-elfclassify ex
ebtables enchant-2 eu-elfcmp exec
ebtables-restore enchant-lsmod-2 eu-elfcompress exempi
ebtables-save enscript eu-elflint exit
echo env eu-findtextrel exiv2
ed envsubst eu-make-debug-archive expand
edgepaint eog eu-nm export
edid-decode eps2eps eu-objdump exportfs
editdiff eqn eu-ranlib expr
Try adding c to make ec and pressing tab again. What happens?
# The command autocompletes after adding the “c” to the “e”
echo
Try to copy/paste your echo command “echo ‘My first command’”
# Note that ctrl + c to copy does not work in windows terminal or mobaxterm - instead highlighting text automatically copies it
# In mobaxterm ctrl + v to paste also does not work - instead right-click to paste
echo 'My first command'
Try to clear the screen, can you still paste your echo command?
# To clear the screen use ctrl + l or the command 'clear' and you can still paste the command
echo 'My first command'
Try to echo ‘My first command ‘once with the -n option and once with the -N option. What do you notice?
# echo -n does not add a new line to the output
echo -n 'My first command'
My first command[]$
# The -N option does not exist therefore “echo” will ill interpret '-N' as characters to display
echo -N 'My first command'
-N My first command
Try to look at the manuel of the echo command.
# man gives you the manual of the function
man echo