Working with Commands
Up until now, we have seen a number of commands and their mysterious options and arguments. In this lesson, we will try to remove some of that mystery. We will introduce the following commands.
type- Display information about command typewhich- Locate a commandhelp- Display reference page for shell builtinman- Display an on-line command reference
What are "Commands?"
Commands can be one of 4 different kinds:
- An executable program 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, etc.
- A command built into the shell itself. bash provides a number of commands
internally called shell builtins. The
cdcommand, for example, is a shell builtin. - A shell function. These are miniature shell scripts incorporated into the environment. We will cover configuring the environment and writing shell functions in later lessons, but for now, just be aware that they exist.
- An alias. Commands that we can define ourselves, built from other commands. This will be covered in a later lesson.
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. It
works like this:
type command
where “command” is the name of the command we want to examine. Here are some examples:
type type
type is a shell builtin
[me@linuxbox me]$ type ls
s is aliased to `ls --color=auto'
[me@linuxbox me]$ type cp
cp is /bin/cpHere we see the results for three different commands. Notice that the one
for ls and how the ls command is actually an alias
for the ls command with the “-- color=auto” option
added. Now we know why the output from ls is
displayed in color!
which
Sometimes there is more than one version of an executable program installed
on a system. While this is not very common on desktop systems, it's not unusual
on large servers. To determine the exact location of a given executable, the
which command is used:
which ls
/bin/lswhich only works for executable programs, not
builtins nor aliases that are substitutes for actual executable programs.
Getting Command 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. Optionally, we can add the -m option to change the format of
the output. For example:
help -m cd
NAME
cd - Change the shell working directory.
SYNOPSIS
cd [-L|-P] [dir]
DESCRIPTION
Change the shell working directory.
Change the current directory to DIR. The default DIR is the value of the
HOME shell variable.
The variable CDPATH defines the search path for the directory containing
DIR. Alternative directory names in CDPATH are separated by a colon (:).
A null directory name is the same as the current directory. If DIR begins
with a slash (/), then CDPATH is not used.
If the directory is not found, and the shell option `cdable_vars' is set,
the word is assumed to be a variable name. If that variable has a value,
its value is used for DIR.
Options:
-L force symbolic links to be followed
-P use the physical directory structure without following symbolic
links
The default is to follow symbolic links, as if `-L' were specified.
Exit Status:
Returns 0 if the directory is changed; non-zero otherwise.
SEE ALSO
bash(1)
IMPLEMENTATION
GNU bash, version 4.1.5(1)-release (i486-pc-linux-gnu)
Copyright (C) 2009 Free Software Foundation, Inc.
A note on notation: When square brackets appear in the description of a command's
syntax, they indicate optional items. A vertical bar character indicates mutually exclusive
items. In the case of the cd command above:
cd [-L|-P] [dir]
This notation says that the command cd may be
followed optionally by either a “-L” or a “-P” and further, optionally followed
by the argument “dir”.
--help
Many executable programs support a “--help” option that displays a description of the command's supported syntax and options. For example:
mkdir --help
Usage: mkdir [OPTION] DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.
Mandatory arguments to long options are mandatory for short options
too.
-Z, --context=CONTEXT (SELinux) set security context to CONTEXT
-m, --mode=MODE set file mode (as in chmod), not a=rwx – umask
-p, --parents no error if existing, make parent directories as
needed
-v, --verbose print a message for each created directory
--help display this help and exit
--version output version information and exit
Some programs don't support the “--help” option, but try it anyway. Often it results in an error message that will reveal similar usage information.
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. It is
used like this:
man program
where “program” is the name of the command to view. Man pages vary somewhat
in format but generally contain a title, a synopsis of the command's syntax, a
description of the command's purpose, and a listing and description of each of
the command's options. Man pages, however, do not usually include examples, and
are intended as a reference, not a tutorial. Let's try viewing the man page for
the ls command:
man lsOn most Linux systems, man uses less to display the manual page, so all of the familiar
less commands work while displaying the page.
README and Other 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 plain text format and can be viewed with less. 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.