Learn Linux 05: Working With Commands

Published Jan 6, 2022

Contents


Introduction

Up to this point, we have seen a series of mysterious commands, each with its own options and arguments. In this chapter, we will attempt to remove some of that mystery and even create our own commands. This chapter will introduce the following commands:

  • type - Indicate how a command name is interpreted
  • which - Display which executable program will be executed
  • help - Get help for shell builtins
  • man - Display a command's manual page
  • apropos - Display a list of appropriate commands
  • info - Display a command's info page
  • whatis - Display one-line manual page descriptions
  • alias - Create an alias for a command

What Exactly Are Commands?

A command can be one of four different things:

  • An executable program. It's like all those files we saw in /usr/bin. Within this category, programs can be compiled binaries such as programs written in C and C++, or programs written in scripting languages such as the shell, Perl, Python, Ruby, and so on.
  • A command built into the shell itself. bash supports a number of commands internally called shell builtins. The cd command, for example, is a shell builtin.
  • A shell function. Shell functions are miniature shell scripts incorporated into the environment.
  • An alias. Aliases are commands that we can define ourselves, built from other commands.

Identifying Commands

It is often useful to know exactly which of the four kinds of commands is being used, and Linux provides a couple of ways to find out.

type

The type command is a shell builtin that displays the kind of command the shell will execute, given a particular command name.

[user@linux ~]$ type type
type is a shell builtin
[user@linux ~]$ type ls
ls is aliased to `ls --color=tty'
[user@linux ~]$ type cp
cp is /bin/cp

which

To determine the exact location of a given executable, the which command is used.

[user@linux ~]$ which ls
/bin/ls

Getting A Command’s Documentation

With this knowledge of what a command is, we can now search for the documentation available for each kind of command.

help

bash has a built-in help facility available for each of the shell builtins. To use it, type help followed by the name of the shell builtin.

[user@linux ~]$ help cd
cd: cd [-L|[-P [-e]] [-@]] [dir]
                      Change the shell working directory.
                      Change the current directory to DIR. The default DIR is the value of the HOME shell variable.
                      ...

--help

Many executable programs support a --help option that displays a description of the command's supported syntax and options.

[user@linux ~]$ mkdir --help
Usage: mkdir [OPTION] DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.

  -Z, --context=CONTEXT (SELinux) set security context to CONTEXT Mandatory arguments to long options are mandatory for short options too.
  ...

man

Most executable programs intended for command line use provide a formal piece of documentation called a manual or man page. A special paging program called man is used to view them.

[user@linux ~]$ man ls

apropos

It is also possible to search the list of man pages for possible matches based on a search term. It’s crude but sometimes helpful. Note that the man command with the -k option performs the same function as apropos.

[user@linux ~]$ apropos partition
addpart (8)            - simple wrapper around the "add partition" ioctl
all-swaps (7)          - event signalling that all swap partitions have been ac...
cfdisk (8)             - display or manipulate disk partition table
cgdisk (8)             - Curses-based GUID partition table (GPT) manipulator
delpart (8)            - simple wrapper around the "del partition" ioctl

whatis

The whatis program displays the name and a one-line description of a man page matching a specified keyword.

[user@linux ~]$ whatis ls
ls                 (1)  - list directory contents

info

The GNU Project provides an alternative to man pages for their programs, called info. Info manuals are displayed with a reader program named, appropriately enough, info. Info pages are hyperlinked much like web pages. The info program reads info files, which are tree structured into individual nodes, each containing a single topic. Info files contain hyperlinks that can move you from node to node.

README And Other Program Documentation Files

Many software packages installed on your system have documentation files residing in the /usr/share/doc directory. Most of these are stored in ordinary text format and can be viewed with the less command. Some of the files are in HTML format and can be viewed with a web browser. We may encounter some files ending with a .gz extension. This indicates that they have been compressed with the gzip compression program. The gzip package includes a special version of less called zless that will display the contents of gzip-compressed text files.

Creating Our Own Commands With alias Command

Now let's create our own command. We will create a command of our own using the alias command. First we change the current working directory to /usr, then list the directory, and finally return to the original directory (by using cd -) so we end up where we started. Now let’s turn this sequence into a new command using alias.

[user@linux ~]$ alis boo='cd /usr; ls; cd -'

To remove an alias, the unalias command is used.

[user@linux ~]$ unalias boo

Note: To see all the aliases defined in the environment, use the alias command without arguments.

There is one tiny problem with defining aliases on the command line. They vanish when your shell session ends. In the next chapters, we will see how to add our own aliases to the files that establish the environment each time we log on, but for now, enjoy the fact that we have taken our first.

Summary

Now that we know how to find the documentation for commands, look up the documentation for all the commands we have encountered so far. Study their additional options and try them on your own.