3

Command Line Fundamentals

3.1 Introduction

In this day and age, it takes a certain level of skill to be alien to technology and as everyone has computers in their pockets, the interactions with them is almost uncountable. However, these interactions are done through what is called a GUI . Devices running on Windows, MacOS, iOS, and Android all use this interface to interact with the user.

It must be stressed as these visual components are all for the benefit of the user. While these greatly simplify tasks like photo editing or video creation, some applications just completely omit the use of GUI and instead use a simpler version of it called CLI . This is especially true for servers ^{1 1} A server is a software or hardware offering a service to a user, usually referred to as client. As an example, a hardware server is a shared computer on a network, usually powerful and housed in a data centre. , embedded applications and in many other areas where either memory is limited or efficiency of the computer (e.g., such as limiting the CPU load etc.) is highly desired. Server software, utilities, and other programs usually only need some text-based information to do what they do. Many of these programs run on a server in a data centre somewhere without a monitor so the overhead of a GUI is completely unnecessary .

One way we interact with these programs that don’t have a GUI is through the CLI . This is a text-based interface where the commands to execute are typed and all actions are shown as text on a terminal screen, whether it is updating a software or moving files around. The environment we use is called a shell, or command-line interpreter, and there are many shells out there.

A list of Shells that can be encountered in industry and academia can be seen in Table 3.1 .

The command-line interpreter was one of the earliest ways of interacting with the general-purpose computer, starting in 1971 with the Thompson shell for UNIX ^{2 2} A family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969. It is considered one of the most groundbreaking software ever designed. . As UNIX evolved and came to be replaced in many capacities by Linux, the shell environments evolved and improved as well.

Bash, or the Bourne-again shell is one of the most widely-used shells and odds are, it’s the one to be encountered in industry or in academic work. Bash is the shell that comes enabled by default with most of the popular Linux distributions. It’s also available on macOS ^{3 3} newer versions have zsh shell instead but they are designed to be compatible. The author of this work also uses zsh as his main driver. and in Windows with the Windows subsystem for Linux.

3.2 The Structure of Commands

There are a few concepts and principles which needs to be understood to be a productive member of the CLI family. Before jumping into using commands though, have a look at how command line statements are structured with the following:

command [-flag(s)] [-option(s) [value]] [argument(s)]

This is the general form. The pattern is command, options, and then arguments. Here’s a couple of common commands you’ll see with options and arguments that are used with them.

ls -l /tmp 
cd /usr/local 
cat /etc/passwd

The details of what the aforementioned commands do will be the focus in the future. I just want to show you the structure of what we’ll be working with before we get into what these actually do. Depending on the current action, you might just have a command or a command and one or more options or just a command with one or more arguments.

Command is the minimum thing which can be done with a CLI . Think of it as the atom of any action you can take. The command is the program you’re running or the action you’re taking. To give command to a UNIX system, type the name of the command, along with any associated information, such as a filename, and press the Return key.

The typed line is called the command line and UNIX uses a special program, called the shell or the command line interpreter, mentioned in the previous section, to interpret what you have typed into what you want to do.

The components of the command line are:

1.

the command,

2.

any options required by the command,

3.

the command’s arguments. ^{4 4} This is optional as some commands just don’t have any options.

3.2.1 Some Rules Regarding the Syntax

Since the introduction of UNIX System V, Release 3 (released 1983), any new commands must obey a particular syntax governed by the following rules:

• Command names must be between 2 and 9 characters in length,

• Command names must be comprised of lowercase characters and digits,

• Option names must be one character in length,

• All options are preceded by a hyphen (-),

• Options without arguments may be grouped after the hyphen,

• The first option argument, following an option, must be preceded by white space,

  • i.e., -o sfile is valid but -osfile is illegal .

• Option arguments are not optional ,

• If an option takes more than one argument then they must be separated by commas with no spaces, or if spaces are used the string must be included in double quotes (").,

  • i.e., both are acceptable: -f past,now,next and -f "past now next".

• All options must precede other arguments on the command line,

• A double hyphen -- may be used to indicate the end of the option list,

• The order of the options are order independent,

• The order of arguments may be important,

• A single hyphen - is used to mean standard input.

3.3 Helpful Keyboard Shortcuts for the Terminal

Before we moving on to more specific commands and get into CLI programming, there’s a few other helpful things to know about working at the Command Line. The first is Tab completion , a wonderful feature of the Bash shell, and is also included in many others. This feature let’s you skip typing out a whole file name or folder name when you’re working at the Command Line.

When you’re working in the command line it looks at all the information it has so far and makes a guess about what you mean. For example, I can type ls -l De and press Tab , and it completes the line with Desktop. Now type ls -l Do and nothing would happen when I press Tab . That’s because Tab doesn’t have one clear suggestion to return. As it can be either Documents of Downloads. However, pressing Tab again should give you a suggestion of which items can be completed to.

For reference, in the following page, there is a table for most useful keyboard shortcut for Linux Bash.

3.4 When you need help with Commands

If you ever see an experienced Linux user typing away at the command line in blazing speeds it can seem like memorising the ins and outs of commands and options is the only way to be productive and understand what’s going on. But everybody starts somewhere, and even experienced command-line users don’t memorize everything.

In the world of programming, it’s not practical to try to memorise all of the syntax and options of command-line tools. Of course, it’s important to remember the basics, but while you’re getting started, you only need to remember a few commands. The first one is man, which stands for the manual pages .

MAN(1)                        Manual pager utils                        MAN(1) 
 
NAME 
       man - an interface to the system reference manuals 
 
SYNOPSIS 
       man [man options] [[section] page ...] ... 
       man -k [apropos options] regexp ... 
       man -K [man options] [section] term ... 
       man -f [whatis options] page ...

A man page ^{5 5} Stands for short for manual page. is a form of software documentation found on UNIX and Unix-like operating systems. Topics covered include programs, system libraries, system calls, and sometimes local system details.

Think of the man pages as a technical reference book for your Linux distribution ^{6 6} i.e., Ubuntu, Mint . If you know the name of a command, you can find out a wealth of information about what it does, what options it provides or what arguments it takes. To look up something in the manual pages, type man, followed by a command you want to learn. Open up the Terminal by Ctrl + Alt + T . Earlier, you saw the command ls, so let’s look that up. Type man ls and press Return .

man ls | head -10

LS(1)                            User Commands                           LS(1) 
 
NAME 
       ls - list directory contents 
 
SYNOPSIS 
       ls [OPTION]... [FILE]... 
 
DESCRIPTION 
       List  information  about  the FILEs (the current directory by default).

Here, you can see some information about the ls command ^{7 7} Please ignore the head - 10, we will have a look at it later. . You can see that it’s for listing directory contents and in the synopsis section you get a quick overview of how to use the command. In this case it is ls [OPTION]... [FILE].... We write ls followed by any of the options we need, and the file or folder path we want to use.

The terms in square brackets ^{8 8} for example [OPTION] and [FILE] are optional. This basically means you don’t have to use these for the command to work. You can just use the ls command by itself to see the default output of listing the directory. Here, below the description header, there is a bit more detailed information about the command, including its default behaviour and usage notes, and below, is a listing of the options that the command takes.

Usage: ls [OPTION]... [FILE]... 
List information about the FILEs (the current directory by default). 
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified. 
 
Mandatory arguments to long options are mandatory for short options too. 
  -a, --all                  do not ignore entries starting with . 
  -A, --almost-all           do not list implied . and .. 
      --author               with -l, print the author of each file 
  -b, --escape               print C-style escapes for nongraphic characters 
      --block-size=SIZE      with -l, scale sizes by SIZE when printing them;

There are a lot of ways to use the man pages efficiently and is a powerful tool when you need to find what can a command do. There are other ways to learn about a command. Most of commands also have an option called help, which provides a brief amount of information about them. However, they usually refer you to the manual pages for more detailed documentation. Therefore, help will give you a brief information compared to the man command.

You can see if a command you’re using has this feature available by typing --help after the command.

ls --help | head -10

Usage: ls [OPTION]... [FILE]... 
List information about the FILEs (the current directory by default). 
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified. 
 
Mandatory arguments to long options are mandatory for short options too. 
  -a, --all                  do not ignore entries starting with . 
  -A, --almost-all           do not list implied . and .. 
     --author               with -l, print the author of each file 
  -b, --escape               print C-style escapes for nongraphic characters 
     --block-size=SIZE      with -l, scale sizes by SIZE when printing them;

Here you can scroll up and down to have a look at some of the information. There is another command that’s useful when you’re working in Bash, and that’s just help by itself.

help | head -10

GNU bash, version 5.2.21(1)-release (aarch64-unknown-linux-gnu) 
These shell commands are defined internally.  Type `help' to see this list. 
Type `help name' to find out more about the function `name'. 
Use `info bash' to find out more about the shell in general. 
Use `man -k' or `info' to find out more about commands not in this list. 
 
A star (
*) next to a name means that the command is disabled. 
 
 job_spec [&]                            history [-c] [-d offset] [n] or hist> 
 (( expression ))                        if COMMANDS; then COMMANDS; [ elif C>

As we get into working with the Bash shell, the help tool can act as a handy reminder for the syntax of some Bash specific commands.

In that case, you can use another program called apropos which searches a list of commands and their descriptions for text you provide as an argument.

So if you wanted to find out what can list things, I could type apropos list and see a number of results that match that word.

apropos list | head -10

port-contents(1)         - List the files installed by a given port 
port-dependents(1), port-rdependents(1) - List ports that depend on a given (installed) port 
port-deps(1), port-rdeps(1) - Display a dependency listing for the given port(s) 
port-distfiles(1)        - Print a list of distribution files for a port 
port-echo(1)             - Print the list of ports the argument expands to 
port-installed(1)        - List installed versions of a given port, or all installed ports 
port-list(1)             - List available ports 
port-outdated(1)         - List outdated ports 
port-variants(1)         - Print a list of variants with descriptions provided by a port 
AllPlanes(3), BlackPixel(3), WhitePixel(3), ConnectionNumber(3), DefaultColormap(3), DefaultDepth(3), XListDepths(3), DefaultGC(3), DefaultRootWindow(3), DefaultScreenOfDisplay(3), DefaultScreen(3), DefaultVisual(3), DisplayCells(3), DisplayPlanes(3), DisplayString(3), XMaxRequestSize(3), XExtendedMaxRequestSize(3), LastKnownRequestProcessed(3), NextRequest(3), ProtocolVersion(3), ProtocolRevision(3), QLength(3), RootWindow(3), ScreenCount(3), ScreenOfDisplay(3), ServerVendor(3), VendorRelease(3) - Display macros and functions

Here’s the command that can list directory contents we were looking for.

ls(1)                    - list directory contents

Searching for commands this way can be time-consuming, but if you know what you need to do but not the command to do it, apropos is very helpful and powerful.

3.5 Additional Information

3.5.1 Use Tab completion on the Shell

If you do not know the exact name of a command, then you can make use of tab completion. To use this action, launch the terminal by pressing Ctrl + Alt + T or just click on the terminal icon in the task bar. Just type the command name that you know in the terminal and then press Tab twice. For example, if we can’t remember man, we can write ma and can choose one of the option the Bash shell presents us.

~$ ls 

macptopbm                   make-ssl-cert 
mag                         mako-render 
mailmail3                   man 
make                        mandb 
make4ht                     manpath 
makeconv                    man-recode 
makedtx                     mapfile 
make-first-existing-target  mapscrn 
makeglossaries              match_parens 
makeglossaries-lite         mathspic 
makeindex                   mattrib 
makejvf                     mawk

3.5.2 The info command

Some commands do not have their manuals written or they are either incomplete . To get help with those commands, we use info. To use this command, launch the terminal by pressing Ctrl + Alt + T or just click on the terminal icon in the task bar. Just type info in the terminal and with a space, type the name of the command whose manual does not exist and press Return .

info ls | head -10

File: coreutils.info,  Node: ls invocation,  Next: dir invocation,  Up: Directory listing 
 
10.1 ls: List directory contents 
================================== 
 
The ls program lists information about files (of any type, including 
directories).  Options and file arguments can be intermixed arbitrarily, 
as usual.  Later options override earlier options that are incompatible. 
 
   For non-option command-line arguments that are directories, by

The info command reads documentation in the info format ^{9 9} A mostly a plain text transliteration of the Texinfo source, with the addition of a few control characters to separate nodes and provide navigational information, designed by the NU project. . It will give detailed information for a command when compared with the man page. The pages are made using the Texinfo tools which can link with other pages, create menus, and easy navigation.

3.5.3 The whatis command

This command is used with another command just to show a one liner usage of the latter command from its manual. It’s a quick way of knowing the usage of a command without going through the whole manual.

whatis command in Linux is used to get a one-line manual page description. In Linux, each manual page has some sort of description within it. So, this command search for the manual pages names and show the manual page description of the specified filename or argument.

To use this command, launch the terminal by pressing Ctrl + Alt + T or just click on the terminal icon in the task bar. Just type whatis in the terminal and after a space, type the name of the command whose one liner description you want (for example ls ) and then press Return .

whatis ls | head -10

dcmcjpls(1)              - Encode DICOM file to JPEG-LS transfer syntax 
dcmdjpls(1)              - Decode JPEG-LS compressed DICOM file 
gdircolors(1), dircolors(1) - color setup for ls 
gls(1), ls(1)            - list directory contents 
gdircolors(1), dircolors(1) - color setup for ls 
git-ls-files(1)          - Show information about files in the index and the working tree 
git-ls-remote(1)         - List references in a remote repository 
git-ls-tree(1)           - List the contents of a tree object 
git-mktree(1)            - Build a tree-object from ls-tree formatted text 
gls(1), ls(1)            - list directory contents