Day 1 - First Steps in the Shell: How to Read a Single Command Line

📌 Key Takeaways

• The $ at the end of the prompt means a regular user, and # means root. That single character tells you your privilege level.
• Read a command in three chunks: command [options] [arguments].
• Options come in a short form (-a) and a long form (--all), and short forms can be combined, like -la.
• A path starting with / is absolute; otherwise it is relative to your current location. . = current, .. = parent, ~ = home.
• Look up unfamiliar commands with man command. Tab completion and the ↑ history cut down on typos.

Reading the Prompt

When you log in, a line like this appears. This is the prompt, the sign that the shell is waiting for a command.

   user@localhost:~$
    │      │      ││
    │      │      │└── $ = regular user,  # = root
    │      │      └─── current directory (~ is the home directory)
    │      └────────── hostname (the computer's name)
    └───────────────── name of the logged-in user

• Telling $ and # apart is the most important thing. In textbooks and docs, a command that starts with # means "run this with administrator privileges."
• The rule is not to stay logged in as root. Raise your privileges briefly with sudo only when you need to.

The Prompt Looks Different on Each Distribution

The prompt is drawn according to the format stored in a variable called PS1. The defaults differ by distribution, so the same situation looks different.

[user@localhost ~]$ cd /usr/share/doc
[user@localhost doc]$ su -
Password:
[root@localhost ~]#

• Exam passages use the RedHat-family style, such as [root@www ~]#. Even if the look differs, the way you read it is the same — user, host, location, and the final $ / #.
• In the example above, once we moved to /usr/share/doc the location slot changed to doc, and once we became root with su - the final character changed to #.
• You can also check each piece of the prompt with a command.

whoami       # user         ← name of the logged-in user
hostname     # localhost    ← hostname
pwd          # /home/user   ← full path of the current directory

Structure of a Command

   $ ls -l /home
     │   │    │
     │   │    └── argument — on what
     │   └─────── option   — how
     └─────────── command  — what to do

• The shell splits chunks at spaces. So if a file name contains a space, you have to wrap it in quotes or escape it with \ .
• Commands are case-sensitive. LS is not ls.
• Options and arguments can both be omitted. Typing just ls works, and so does giving only an argument with no option, like ls /home. The square brackets [ ] mean "optional."

Spaces Split Chunks — When You Need Quotes

touch my file.txt        # two files get created: my and file.txt
touch "my file.txt"      # one file containing a space
touch my\ file.txt       # same as above (\ protects the one space right after it)

echo "$HOME"     # /home/user   ← the variable's value is printed
echo '$HOME'     # $HOME        ← printed literally

Multiple Commands on One Line

Two Forms of Options

ls -l -a /home     # written separately
ls -la /home       # combined — same meaning
ls --all /home     # long form

Pitfall: a long form starts with -- (two hyphens). If you write it with one hyphen like -all, it is interpreted as -a -l -l.

Options That Take a Value

Some options are followed by a value. Short forms usually separate it with a space, and long forms usually join it with =.

head -n 5 a.txt          # short form + value
head --lines=5 a.txt     # long form + value — same meaning

• Here 5 is not an argument but the value of the -n option. The target the command processes (the argument) is just a.txt.
• Most GNU commands accept --help (usage) and --version (version) in common. If you can't remember an option, start by typing command --help.

Where the Shell Looks for Commands

The reason typing just ls works is that the shell searches the directories listed in the PATH variable from the front, one by one, looking for an executable file with the same name.

echo $PATH
# /usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/user/bin
which cp
# /usr/bin/cp

What we type as a "command" is really one of three kinds.

type cd      # cd is a shell builtin
type ls      # ls is aliased to `ls --color=auto'
type cp      # cp is /usr/bin/cp

• If you get command not found, it is one of three things — a typo, the program is not installed, or the executable is outside PATH.
• The current directory is not in PATH. So to run an executable in your current location, you add a path like ./script.sh.
• which finds the location of an executable in PATH, while type also tells you whether the name is a builtin, an alias, or external. A builtin may have no executable file of its own, so checking with type is more accurate.

Reading Paths

   Absolute path   /home/user/docs/a.txt     ← starts with /. Same place wherever you run it
   Relative path   docs/a.txt                ← relative to the current location
                   ../user2/b.txt            ← go up one level, then down

• Whether a path is absolute or relative depends only on whether it starts with /. This one line is an exam point.

How to Look Things Up When You Don't Know

Section Numbers of man Pages

When the same name is both a command and a configuration file, sections tell them apart.

man 1 passwd      # the passwd command
man 5 passwd      # the format of the /etc/passwd file

• Inside man, press q to quit, page with the arrow keys or Space, and search with /word.
• If you don't specify a section, you get only the first one found in a fixed search order. That is why man passwd shows the command (1), and you must specify 5 to see the file format.

There are originally nine sections. The exam mostly asks about 1, 5, and 8, but knowing the whole picture keeps you from getting confused.

What Makes Up One man Page

SYNOPSIS
       ls [OPTION]... [FILE]...

• [ ] means optional, and ... means you can chain several. The line above reads as "you can give no options and no files, and you can also give several."
• The number in parentheses in SEE ALSO, like passwd(5), is exactly the section number.

How the Help Commands Relate

whatis passwd
# passwd (1)   - update user's authentication tokens
# passwd (5)   - password file
apropos "copy files"
# cp (1)       - copy files and directories

• If you know the command name, use whatis; if you only know what you want to do, use apropos.
• For builtin commands, it is faster to use the shell's help, like help cd, than man.

Habits That Save Your Fingers

Tab completion not only cuts down typos but is also a way to check that a path is right. If it won't complete, that path doesn't exist.

Ctrl + C · Z · D Each Do a Different Thing

All three look like they "stop" something, but the results differ.

$ sleep 100
^Z
[1]+  Stopped                 sleep 100
$ fg
sleep 100
^C
$

• A command stopped with Ctrl + Z has not disappeared. If you meant to terminate it, you should have used Ctrl + C.
• When editing a line, Ctrl + A (to the start of the line) · Ctrl + E (to the end of the line) · Ctrl + U (erase before the cursor) are handy.

Tomorrow we will learn the commands that handle real files and directories using this syntax.

📖 Glossary

• Prompt : The sign that the shell is waiting for a command. It shows the user, host, current location, and privilege.
• Option : A marker that changes how a command behaves. There are short forms like -a and long forms like --all.
• Argument : The target a command processes. A file name or directory name goes here.
• Absolute path : A full path starting from /. It points to the same place regardless of the current location.
• Relative path : A path based on the current directory.
• Home directory : A personal directory assigned to a user. Abbreviated as ~.
• man page : A command's official manual. Section numbers distinguish identical names.
• Auto-completion : A feature that fills in the rest of a command or path with the Tab key.
• Builtin command : A command that lives inside the shell, like cd. Check it with type and see its help with help.
• PATH : The list of directories the shell searches for a command's executable. The current directory is not in it.