Bash as interactive command interpreter

Bash has a long history of development and is now in version 4. As this is the dominant version we will cover it. While it is mostly compatible with version 3.x it contain several useful enhancements such as autocd, option -p in declare statement, &>> and |& redirects, etc.

Bash as a login shell is defined in /etc/passwd file. If other shell is defined by your sysadmin you usually ask him/her to change. If not you can stull run bash envoking it yourself. To make bash as a default login shell, run the following command:

At the same time, no matter what version of bash is used typical user account in Linux is still in "stone age" often without customized prompts, useful aliases and functions. Instead of using environment modules many still are redefining PATH and LD_LIBRARY_PATH directly in .bashrc

Listing Files

The ls (list) command shows the contents of the current directory. Although ls is a familiar command available on all Unix-like operating system, Linux uses the ls command from the GNU fileutils project and has many special switches and features.

ls has switches that affect how the files are listed, including the level of detail, the sorting order, and the number of columns.

Most Linux distributions set up certain defaults for ls command. Red Hat, for example, has the -q and -F switches on by default. From the point of view of script writing, it's not safe to use the ls command in a script without specifying the appropriate switches because you can't be sure which defaults a particular distribution uses.

ls hides files that begin with a period. This is the Linux convention for configuration files, history files, and other files that a user isn't normally interested in. To see these files, use the -A (all) switch. Use -a (absolutely all) to show the implicit . and .. files as well.

The filenames can be printed in color to show the kind of file they are. The colors are defined in a file /etc/DIR_COLORS. You can customize the colors using a .dir_colors file in your own directory. The format of the file is described in the /etc/DIR_COLORS file.

To display the files without color and with symbols instead, use the --color and --classify (or -F) switches. (On most Linux distributions, this feature is turned on using aliases.)

The classify symbols are directories (/), programs (*), symbolic links (@), pipes (|), and Unix domain socket files (=). These symbols are not a part of the name: They are hints as to the type of file. In this example, archive is a directory and check-orders.sh is a program.

Another very important switch is --hide-control-chars (or -q). Linux filenames can contain any character, even control characters. It is possible to create a filename with hidden characters in the name. In these cases, you can't rename or delete the file unless you know what the hidden characters are. Contrary to what the name implies, the --hide-control-chars switch displays any unprintable characters in the filename as question marks, making their locations visible.

Build-in macros for directories

Born-shell family has special macros that expand to the home directory. They can be used only outside strings:

Getting Help

!!! bash help system is hopelessly outdated and not very informative. The same is true about man pages. you are better off browsing them from Internet. For example

The Bash shell comes with a built-in help command to describe the various built-in Bash commands. The -s switch displays a summary for the command you specify.

Help only describes Bash commands. To get help on Linux commands, you need to use the man (manual) command.

Linux divides its manual pages into a number of logical volumes. man displays any matching entries from any volume. Volume 1 contains the commands you can execute from the shell. To restrict your search to shell commands, use man 1.

If there are pages in more than one manual volume, only the page from the first volume that matches your search is displayed. To find matching pages across all manual volumes, use the -a (all) switch.

The -k switch searches the Linux manual for a particular keyword and lists all man pages referring to that keyword.

The command help type gives you different information than man 1 type. The help type command tells you about Bash's built-in type command, whereas the man 1 type command tells you about the Linux type command. If you are not sure whether a command is a Bash command, always try the help command before using the man command.

Fixing the Display

Certain character sequences can lock your display, hide what you type, or change the characters being shown into strange symbols. This can happen, for example, when you're trying to display a binary file.

If reset fails, you might also need to use stty sane to restore the session to a normal state.

The clear command clears the display and returns the cursor to the upper-left corner.

Working with Files

mkdir (make directory) creates a new directory. Use mkdir to organize your files.

The values for mode are discussed with the chmod command in Chapter 15, “Shell Security.” However, when --parents is used, the --mode affects only the final directory in the list.

RM and RMDIR commands

rmdir (remove directory) deletes a directory. The directory must be empty before it can be removed. There are two switches:

The rm (remove) command permanently deletes files. If the file is a symbolic or hard link, it removes the link but leaves the file intact.

Using the --recursive and --force switches simultaneously removes all the specified files, including all subdirectories, without warning. Make sure you are deleting the correct files.

Some Linux distributions have the --interactive switch on by default so that rm requires that you confirm when you want to delete a particular file.

Normally rm won't delete a directory, but the --recursive switch deletes any directories encountered.

CP command

The cp (copy) command copies files from any location to another. If the final file listed is a directory, copy copies the other files into that directory.

There are many switches for cp—the complete list is in the reference section at the end of this chapter. Some common switches are as follows:

Like rm, some Linux distributions have --interactive on by default, warning when a file will be overwritten.

MV Command

The mv (move) command moves and renames files. This is the same as making a copy of the file and deleting the original. Move also effectively renames a file by moving it to a new name in the same directory.

There is no --recursive switch. When move moves a directory, it moves the directory and all its contents automatically.

The namei (name inode) command lists all the components in a path, including any symbolic links.

In this case, the file named files is a symbolic link. Each of the files in the link path is a directory, marked with a d. Other file designations include l for symbolic link, s for socket, b for block device, c for character device, - for regular file, and ? for an error accessing a file in the path.

Complete file permissions, such as seen with ls -l, can be shown with the -m (mode) switch.

Working with people

The who command shows who is on the computer, which connection they are using, and when they signed on.

The w command provides even more information, including system statistics and what the users are currently running.

Shell Aliases

Shell aliases are parameteless positional macros. They are recognized only if they are the first in the command string and only in interactive sessions. General form

The built-in alias command creates simple abbreviations for the current Bash session. To create an alias, use the alias command to assign a command and its switches a name.

alias command without parameters lists all active aliases. You should generally avoid self-aliased commands. But in some cases they are useful. For example, the command rm -i is often aliased as rm. The effect is that the -i option appears whenever you issue the rm command, whether or not you type the option. The -i option specifies that the shell will prompt for confirmation before deleting files. This helps avoid accidental deletion of files, which can be particularly hazardous when you're logged in as root. The alias ensures that you're prompted for confirmation even if you don't ask to be prompted. If you don't want to be prompted, you can issue a command like:

Bash interprets an alias only once, allowing the aliasing of a command with its own name.

Normally, only the first word of a command is checked for an alias. As a special exception, if the last character in the alias string is a blank, Bash checks the next word in the command to see whether it is also an alias.

There is no method for giving arguments to an alias. If arguments are needed, define a more powerful shell function instead.

The built-in unalias command removes an alias. Use the -a switch to remove them all.

Most Linux distributions have aliases defined for common commands. dir, for example, is often an alias for ls. Some distributions define an alias for commands such as rm -i to force user prompting, which is not required by default. This can be a problem for some users such as experienced Unix programmers who are used to working with these features disabled. Use unalias to remove any aliases that you don't want to use.

Aliases mixed with shell functions can be confusing because aliases are expanded only when a line from a script is read. If aliases are used in a shell function, they are expanded when the shell function is defined, not when it is executed. For this reason, it is safer to avoid aliases altogether in shell scripts. However, they can be turned on in scripts using the shopt -s expand_aliases command.

where files specifies the files to be deleted. The -f option has an effect opposite that of the -i option; it forces deletion of files without prompting for confirmation. Because the command is aliased, the command actually executed is:

Here the -f option takes precedence over the -i option, because it occurs later in the command line.

where alias specifies the alias you want to remove. Aliases last only for the duration of a log in session, so you needn't bother to remove them before logging off. If you want an alias to be effective each time you log in, you can use a shell script. The next subsection shows you how to do so.

Your Session Profile

When you log into a computer and start a new Bash session, you might need to type several commands to customize your session. Perhaps you want to change the editing mode from emacs to vi, create new key bindings, or change your command-line prompt. Rather than typing these commands, Bash will run these commands for you each time you log on if they are saved in a special file. This file is called a profile file because it contains the commands to tailor the session to your particular requirements.

The original Bourne shell ran two profile files whenever a user logged on. First, a file called /etc/profile was the general profile file executed for all users. Second, if a file named .profile appeared in the user's home directory, this contained additional commands for each user. Bash mimics this behavior when it is started as sh instead of bash.

Bash extended the principle to run several profile files depending on the circumstances. In addition, Linux distributions often customize the general profile files to run additional commands stored in other scripts.

Bash differentiates between a login session and other instances. Bash runs as a login shell when a user or program first logs in or when a login is simulated with Bash's --login (or -l) switch. A login shell is not necessarily one that presents a user with a prompt. It only indicates that Bash is the top program in the current session, and when Bash completes its run, the login session will be over.

The login_shell shell option is turned on when you are in a login shell. This option can be used to verify that you are in a login shell.

Bash runs as an interactive shell when it is started without a script or when the -i switch is used. An interactive shell presents the user with a command prompt. An interactive shell is not necessarily a login shell. A user can start a non-login interactive shell by typing bash at a Bash prompt, thus starting a new copy of Bash.

Whether the Bash session is interactive or a login shell determines which profile files are used. This way, Bash separates commands specifically for customizing an interactive session from the more general-purpose commands.

The /etc/profile file contains the setup commands and environment changes common to all users. A general profile file might look something like this:

When Bash is used as a login shell, it executes the first file it finds named ~/.bash_profile, ~/.bash_login, or ~/.profile. When a session completes, Bash runs ~/.bash_logout, if it exists.

For example, both RHEL Linux uses ~/.bash_profile for the user's profile file. (You can check this by listing the files in your home directory.) By editing this file, you can add commands that will always execute when you log in.

You can test your changes by simulating a login using the command bash --login.

Running a new Bash interactive session without logging in will not run the profile file.

Scripts will not normally execute the login profile scripts. Bash will load a profile for scripts if the BASH_ENV environment variable contains a pathname to a file to execute. However, you should avoid using BASH_ENV.

Setting the common environment for a set of scripts is usually done with the source command, which is discussed in Chapter 14, “Functions and Script Execution.”

You can block running of the login profile files by using the --noprofile switch with bash

Bash also runs a different set of files for interactive sessions that are not also login sessions. Bash looks for a customization script called ~/.bashrc (Bash resources) and executes it instead of the login profile files. Aliased functions are only allowed in the resource files. A different resource file can be specified with the --init-file or --rcfile switch. A typical resource file might contain the following:

Some distributions add lines to your login profile to source .bashrc -- to run the commands contained ~/.bashrc as well. This is not a feature of Bash, but a change made by your Linux distribution. You can add the following lines to the ~/.bashrc file to test its behavior:

Test it from the command prompt by starting new sessions. In this case, SuSE Linux always runs the resource file.

As a result, you cannot be certain of the behavior of the resource file without checking your distribution.

Customizing Your Prompt

The default bash prompt is usually something uninformative that ends with $ and doesn’t tell you much, so you would like to customize it to show information you find useful.

The default prompt varies depending on your system. bash itself will show its major and minor version (\s-\v\$), for example, bash-3.00$. However, your operating system may have its own default, such as [user@host~]$ ([\u@\h\W]\$) .

Here are some examples of useful prompts that will work with any bash version. The trailing \$ displays # if the effective UID is zero (i.e., you are root) and $ otherwise:

Username@hostname, the date and time, and the current working directory:

$ export PS1='[\u@\h \d \A] \w \$ '
[userj@mybox Wed Dec 28 19:32] ~ $ cd /usr/local/bin/
[userj@mybox Wed Dec 28 19:32] /usr/local/bin $

Username@hostname, and the current working directory (\w):

$ export PS1='[\u@\h \w] \$ '
[userj@mybox] ~ $ cd /usr/local/bin/
[userj@mybox] /usr/local/bin $

Username@hostname, the exit status of the last command, and the current working directory. Note the exit status will be reset (and thus useless) if you execute any commands from within the prompt:

$ export PS1='[\u@\h $? \w \$ '
[userj@mybox 0 ~ $ cd /usr/local/bin/
[userj@mybox 0 /usr/local/bin $ true
[userj@mybox 0 /usr/local/bin $ false
[userj@mybox 1 /usr/local/bin $ true
[userj@mybox 0 /usr/local/bin $

See Shell Prompts Customarization for more information

Additional Dot files

there are additional dpt-scriptts that are run at the begining of the user session. /etc/profile Executed each time when the user logs in

BASH can also use bash-specific shells:

~/.bash_logout Executed when the user logs out

See Dot files for more information.

More about shell variables

There are multiple system variable in shell, the variables that are set by the system.

For example, PATH variable. Of cause you cal also set it yourself:

PATH=/usr/bin:/usr/sbin/:usr/local/bin

By default, shell variables are typeless and can have both athithmentic and non-numeric values.

You can see a list of system variables in you env by issuing the env command. Usually, the command produces more than a single screen of output. So, you can use a pipe redirector and the more command to view the output one screen at a time:

env | more

Press the Space bar to see each successive page of output. You'll probably see several of the shell variables described below:

DISPLAY -- the X display to be used; for example, localhost:0
HOME -- the absolute path of the user's home directory
HOSTNAME -- the Internet name of the host
LOGNAME -- the user's login name
MAIL -- the absolute path of the user's mail file
PATH -- the search path (see next subsection)
SHELL -- the current shell (with the absolute path)
TERM -- the terminal type
USER -- the user's current username; may differ from the login name if the user executes the su command

You can use the value of a shell variable in a command by preceding the name of the shell variable by a dollar sign ($). To avoid confusion with surrounding text, you can enclose the name of the shell variable within curly braces ({}); For example, you can change the current working directory to your home directory by issuing the command:

cd $HOME

An easy way to see the value of a shell variable is to specify the variable as the argument of the echo command. For example, to see the value of the PATH shell variable, issue the command:

echo $PATH

To make the value of a shell variable available not just to the shell, but to programs invoked by using the shell, you must export the shell variable. To do so, use the export command, which has the form:

export variable

where variable specifies the name of the variable to be exported. A shorthand form of the command lets you assign a value to a shell variable and export the variable in a single command:

export variable=value

You can remove the value associated with shell variable by giving the variable an empty value:

variable=

However, a shell variable with an empty value remains a shell variable and appears in the output of the set command. To dispense with a shell variable, you can issue the unset command:

unset variable

Once you unset the value of a variable, the variable no longer appears in the output of the set command.

The Search Path

The special shell variable PATH holds a series of paths known collectively as the search path. This is a very important varibale and that's why we will study it separately. Whenever you issue an external command, the shell searches paths that comprise the search path, seeking the program file that corresponds to the command. The startup scripts establish the initial value of the PATH shell variable, but you can modify its value to include any desired series of paths. You must use a colon (:) to separate each path of the search path.

For example, suppose that PATH has the following value:

/usr/bin:/bin:/usr/local/bin:/usr/bin/X11:/usr/X11R6/bin

You can add a new search directory, say /opt/bin, with the following command:

PATH=$PATH:/opt/bin/

Now, the shell will look for external programs in /opt/bin/ as well as the default directories. However, it will look there last. If you prefer to check /opt/bin first, issue the following command instead:

PATH=/opt/bin:$PATH

The which command helps you work with the PATH shell variable. It checks the search path for the file specified as its argument and prints the name of the matching path, if any. For example, suppose you want to know where the program file for the wc command resides. Issuing the command:

which wc

will tell you that the program file is /usr/bin/wc, or whatever other path is correct for your system.

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