Working with the Linux Shell

# Shell is a program that functions as a user interface, command-line interpreter, and programing (scripting) language. Let’s take a brief look at each of these entities.

Linux provides two types of user interfaces : CLI & GUI

# Configuring the Shell

The shell configuration files set up the user environment (/etc/profile and ~/.bash_profile) and regulate how the shell operates (/etc/bashrc and ~/.bashrc).

• process is single instance of a program that operates in its own memory space. The operating system assigns a unique process ID (PID) each time a program is started. This PID is used to control and track resources assigned to the program.

• Parent and Child Processes When you log on a system, you are presented with a default login shell. This shell is a program (by default, bash) that allows you to execute additional programs.

  When you execute a command such as ls, a sub-shell or child process is created. This means that a copy of the current process (parent) is created and a new instruction set is loaded into the new process (child) Once the command has executed, the child is killed and control reverts to the parent.

• Variables A variable is a memory location that is assigned a name and is used to store data. Variables are used to configure operating environments or to temporarily store program data. When a user logs off or the system is turned off, data stored in these memory locations are lost.

  • Local Variables A local variable only exists in the memory space of the shell in which it is created. This means if you create a variable in a parent process and then create a child process, the variable created in the parent will not be present in the child.

    The syntax for creating a local variable is [variable_name]=[value].

    To view the contents of a variable, execute the command echo $[variable_name].

    The set command will display a list of all the variables and functions that exist in the current process.

  • Environmental or Global Variables In order to copy a shell variable from the parent process to the child process, you need to assign an export attribute to the variable. When a parent spawns a child process, all variables with this attribute are copied to the child

    An environmental (or global) variable is any variable assigned an export attribute. To create a variable and assign an export attribute, execute the command export [variable_name]=[value]

    The command env will display a list of all environmental variables in the current shell. To determine if a specific variable has an export attribute,

    execute the command env | grep [variable_name].

• Aliases is a command shortcut

  set alias alias [alias_name]='[command]'

  unset alias unalias [alias_name]='[command]'

  note : on logout aliases which define by above way are deleted to make alias permanent you must add it to ~/.bachrc

# Bash Configuration Files

/etc/profile & /etc/bashrc system (global) configuration files that are applied to all applicable users.

~/.bash_profile & ~/.bashrc & ~/.bash.logout Files located in the user’s home directory (~/<user_name>) contain user-specific configuration information. Settings in these files override the /etc configuration file settings.

• /etc/profile

  The file /etc/profile is a generic file only read when a user logs on to the system or executes a command that forces a logon (such as su -). When a login shell is run, the bash shell program searches for configuration files in the following order: ~/.bash_profile || ~/.bash_login || ~/.profile

  Once one of those configuration files is found the remaining files are ignored. This file is used to set up the environment for any user whose default shell is either the bourne, korn, or bash shell. The file /etc/profile is read when a user logs on.

  In /etc/profile you will find the following: HOSTNAME variable is set to the system’s HISTCONTROL History environmental variable that determines if bash will store command lines that begin with spaces or are duplicates. HISTSIZE History environmental variable that shows the number of commands stored by the history facility in memory. umask The value of umask determines what permissions will be denied when a file is created.

• /etc/bashrc

  contains commands that affect how the bash shell runs for all users whose default shell is bash. This file is read when called by a command found in ~/.bashrc. This configuration file contains system- wide configuration elements such as Aliases, Functions, Shell configuration options and PS1 prompt configuration

  The letters rc stand for “run command.” rc (run command) files contain startup configuration information that determines how an application will operate.

• ~/.bash_profile read after the file /etc/profile only when the user logs on to the system and if a user’s default shell is bash. The file .bash_profile is used to customize the bash environment for a user. The file contains variables used to configure the user’s working environment (such as time zone, locale, and editor).

• ~/.bashrc The file ~/.bashrc is found in the user’s home directory and is used to customize how the bash shell runs. The ~/.bashrc file contains aliases, functions or shell parameters. ~/.bashrc is executed each time a process is started. This file would contain user specific: Aliases, Functions, Shell configuration options and PS1 prompt configuration

• /etc/profile.d The files in the directory /etc/profile.d contain configuration information for specific programs and are read automatically during the login process. The file /etc/profile (for bourne, korn, and bash users) contains a statement that reads all the files in /etc/profile.d with a suffix of .sh (for example, vim.sh). The file csh.login (for C shell users) contains a statement that reads all the files in /etc/profile.d with the suffix .csh (for example, vim.csh).

The source Command Configuration changes are not active until they are stored in memory. To place changes in memory, you must execute the commands in the configuration file.

The command source [filename] or [filename] will execute the commands in a configuration file. The file ~/.bash_profile is only read when the user logs on. Changes to a user’s .bash_profile would not be read until the user’s next logon. The source command source ~/.bash_profile or . ~/.bash_profile would make those configuration changes available immediately.