Using a CLI

Glossary

We first present a quick glossary of terms.

a) Terminal: a piece of software that emulates the physical telephone-typewriter. It runs a shell. Common terminals are "xfce Terminal", "urxvt", "xterm", "konsole", "GNOME Terminal", "(Apple) Terminal".

b) Shell: a call-response program that runs in the terminal. The shell is a program that loops, prompting you to enter commands, and displaying their output. Common shells are "bash", "zsh".

sample prompt:

nlhowell@mercury ~ $ _
                     ^ ---------- cursor, waiting for command

^^^^^^^^^^^^^^^^^^^^ ------------ prompt; has metadata

Prompts typically include username, hostname (the name of the computer the shell is running on), the current working directory, and the end-of-prompt delimiter $.

sample prompt-command-output:

nlhowell@mercury $ echo hi<RETURN>
hi
                          ^^^^^^^^ ---- <RETURN> executes command
                   ^^^^^^^ ------------ entered command
^^ ------------------------------------ output of the executed command

c) Command: a program, or pipeline of programs, together with their arguments.

command with program "echo" and argument "hi"

$ echo hi
       ^^ ---- first (and only) argument
  ^^^^ ------- command name
^ ------------ prompt

pipeline of two subcommands: program "echo" with argument "hi", and program "tr" with arguments "a-z" and "A-Z"; output of "echo" command is fed as input to "tr" command

$ echo hi | tr a-z A-Z
HI
            ^^^^^^^^^^ ---- second command
          ^ --------------- pipe, connecting commands
  ^^^^^^^ ----------------- first command
  ^^^^^^^^^^^^^^^^^^^^ ---- pipeline consisting of two commands

d) Pipeline: a chain of programs (with arguments) which are hooked together, so that the output of a program is fed to the next program in the chain.

e) Arguments: a small collection of configuration data provided to a program on the commandline.

Reading command specifications

There is a special syntax for describing how to run a command: things enclosed in [square brackets] are optional. An ascii ellipsis "..." means "repeat".

Example: the cat program takes zero or more values for OPTION and zero or more values for FILE. (The possible values for OPTION and FILE are described in the help output.)

cat [OPTION]... [FILE]...

"Options" are small features which adjust how the program performs its job.

Exercise: Compare the output of ls vs ls -l and ls -a; describe how they are similar, and how they are different.

Some options take values: compare seq 5 vs seq -s , 5

Some options have long and short names: seq --separator=, 5

Navigating with the command-line

Read the following sections from Using the Command Line of Linux Sea:

• Navigating
• Listing Content
• Basic File Manipulation

Read the --help output of the following commands:

• ls
• cd and pwd
• echo
• seq
• cat, head, and tail
• pushd and popd
• cp and mv and rm
• mkdir and rmdir
• date

Exercise: Explain pushd and popd; what data structure represents your directory history? Give an example of using them to organise a folder with music.

Exercise: Draw a partial tree of your filesystem, starting from the children of your home directory. Include ancestors of your home directory, and siblings of those ancestors. Exclude files, just show directories. Here is mine:

  (root)/
    |
    | - bin/
    | - boot/
    | - etc/
    | - dev/
    | - home/
    |     | - nlhowell/
    |     |      | - mail/
    |     |      | - media/
    |     |      | - passport/
    |     |      | - src/
    |     |      | - travel/
    |     |      | - usr/
    |     |      | - work/
    |     |
    |     | - spamd/
    |
    | - lib/
    | - lost+found/
    | - mnt/
    | - opt/
    | - proc/
    | - root/
    | - run/
    | - sbin/
    | - sys/
    | - tmp/
    | - usr/
    | - var/