% Startup Files: .bash\_profile and .bashrc
% Ian! D. Allen - idallen@idallen.ca - www.idallen.com
% Fall 2012 - September to December 2012 - Updated Thu Mar 14 02:46:10 EDT
  2013

Startup Files: .bash\_profile and .bashrc
=========================================

Loading shell defaults, options, aliases, the prompt, and functions is done
when the shell first starts. You can change things while the shell is
running, but the changes are *not* saved. Start-up files are places to save
things so that every new shell gets them.

All startup file references are to files in your home directory, which is
where **`cd`** takes you without arguments. Your home directory is also
usually available in the environment variable **`$HOME`**.

> Your start-up files are not re-read by your current shell after you edit
> them - you have to tell your current BASH shell to re-read them to make the
> new content take effect in the current shell, e.g.
> $ **`source ./.bashrc`**

$HOME/.bash\_profile
=====================

If you use a Linux system (e.g. RedHat, Knoppix, Ubuntu, Fedora, SuSE, etc.),
your login shell is already BASH and it reads (“sources”) the
**`.bash_profile`** file when you log in. The **`.bash_profile`** is only
read *once* by the login shell, not by any other non-login shells you might
start up after you log in.

> The **`.bashrc`** file is **not** read by a login shell, so most people put
> **`source ./.bashrc`** in their **`.bash_profile`**.

You can put things in **`.bash_profile`** that need to be done exactly *once*
when you log in and that are *inherited* by other shells:

-   set (and export) **`PATH`** and/or **`TERM`**
-   set **`umask`**
-   set your erase character (backspace character)
-   set your prompt **`PS1`** (and export it:  **`export PS1`**)

Any aliases you define in these login profile files will apply *only* to the
BASH shell at login, not to any other shells you start. It is usually better
to define aliases in your **`$HOME/.bashrc`** file and use `source ./.bashrc`
in the **`.bash_profile`**.

If you find that a nested shell is not inheriting PATH, TERM, or PS1, you
must “export” the variable, e.g. “export PS1”.

To define aliases and shell options that cannot be exported or inherited, use
the other start-up file **`.bashrc`**:

> Your start-up files are not re-read by your current shell after you edit
> them - you have to tell your current BASH shell to re-read them to make the
> new content take effect in the current shell, e.g.
> $ **`source ./.bash_profile`**

$HOME/.bashrc
==============

Put things in your **`.bashrc`** that don’t get exported or inherited from
parent shells and that must be set new for each new shell, such as aliases
and shell options.

Every non-login BASH shell reads (“sources”) **`.bashrc`** when it first
starts up. Every new non-login BASH shell reads this file, unless you use the
“`--norc`” option when you start it.

Any aliases and options you define in **`.bashrc`** will apply to all BASH
shells, except your login shell that sources **`.bash_profile`** instead.

> The **`.bashrc`** file is **not** read by a login shell, so most people put
> **`source ./.bashrc`** in the **`.bash_profile`**.

Set your personal BASH aliases and functions in the **`.bashrc`** file, e.g.
**`alias cd..='cd ..'`**

Set your own useful shell options in your **`.bashrc`**; I recommend these:

-   **`shopt -s failglob`**
-   **`set -o ignoreeof -o noclobber -o notify -o nounset`**
-   **`set -o physical -o interactive-comments`**

`shopt -s failglob`
:   Give an error message if a GLOB fails to match any pathnames, instead of
    silently returning the GLOB pattern unchanged.

`set -o ignoreeof`
:   The shell will ignore \^D (EOF) at the prompt; use “exit” to exit.

`set -o noclobber`
:   Protects files from being overwritten when **“`>`”** redirection is used.
    Use the “force” syntax **“`>|`”** (that’s a *pipe* character) to force
    overwriting, e.g. **`date >| file`**

`set -o notify`
:   Notifies user immediately when a background job (started with **“`&`”**
    or **`bg`**) completes. Without this, you won’t be told that the
    background job is done until you push ENTER.

`set -o nounset`
:   Displays an error when expanding a variable that has not been set.
    Essential for detecting typing errors in variable names.

`set -o physical`
:   Expand all symbolic links in output of pwd and when using cd so that you
    see the real directory names and paths. Essential for system
    administrators to know *exactly* where you are in the file system.

`set -o interactive-comments`
:   Allow **“`#`”** to comment out any following text when typed
    interactively, e.g. **`echo a b # this text is not seen`**

Refer to the BASH man page for details on these options.

> Your start-up files are not re-read by your current shell after you edit
> them - you have to tell your current BASH shell to re-read them to make the
> new content take effect in the current shell, e.g.
> $ **`source ./.bashrc`**

Recommended shell aliases
-------------------------

Don’t take aliases from other people unless you *know* what they do! Here are
some optional but recommended sysadmin aliases for your .bashrc:

    alias l='less'
    alias mv='mv -i'
    alias cp='cp -i -p'
    alias ls='ls -abp --color=auto'
    alias grep='grep --color=auto'

I *don’t* recommend aliasing “rm” to “rm -i”, since it requires you to
confirm *every* removal and quickly loses any protective value.

Saving lots of shell history
----------------------------

As system technicians, having a long history of previous commands available
is useful. The number of commands to keep in memory and number of commands to
save to disk (in the **`.bash_history`** file) are controlled by the BASH
shell variables **`HISTSIZE`** and **`HISTFILESIZE`**. Having all your shells
append their history, rather than overwrite the history file, is also useful.
Here are some suggested settings to preserve a long history:

    export HISTSIZE=2000
    export HISTFILESIZE=1000000
    export HISTCONTROL=ignoredups
    export HISTTIMEFORMAT=
    shopt -s cmdhist
    shopt -s histappend

See **`man bash`** for the details.

What happens when you Log In?
=============================

When you type in your user name (userid) and password to log in, the system
encrypts your password and compares it against the encrypted password in the
password files. If they match, you are allowed to log in.

At log-in time, the system sets your current directory to be the **`HOME`**
directory listed for your account in the password file. An initial
environment is prepared (e.g. containing a system default **`PATH`**).

For a text command-line (terminal or console) log in, the shell specified in
the password file is executed. (This shell is usually **`/bin/bash`**.)

For a graphical log-in, a window manager is started and your Desktop is
initialized for display.

If your login shell is **`/bin/bash`**, the BASH process reads (“sources”)
some system start-up files such as **`/etc/profile`** and
**`/etc/bash.bashrc`** that may set system defaults for various Environment
Variables.

Next, a login shell looks in your home directory for more start-up files such
as **`.bash_profile`** or **`.bash_login`** that you can use to alter the
system defaults. (See “`man bash`” for the full list.) You can create your
own initialization files to customize your environment and terminal settings.

(Some systems give default **`.bash_login`** and **`.bash_logout`** files
when you create a new account. The default files are copied from a system
directory such as **`/etc/skel`**.)

Non-interactive shells and PS1
==============================

> Do not set the **`PS1`** prompt string in any start-up file unless it is
> already set, indicating an interactive shell. Do not produce any output in
> any start-up file unless the shell is interactive (has **`PS1`** set
> non-empty).

Non-interactive shells do not set a prompt, and these shells must not
generate prompts or output when they run. Programs transferring files to/from
your account depend on non-interactive shells executing without generating
any unexpected output.

You can check to see if your shell is generating any unexpected output by
using **`ssh`** to execute the **`true`** command (or any command that
generates no output) and watching for any output after you enter your
password:

    $ ssh localhost true
    *** COURSE LINUX SERVER ***
    user@localhost's password: 
    Welcome back gracious Lord of All.
    $ 

The above “Welcome” line output from an **`echo`** command inside a
**`.bashrc`** will cause file transfer programs to fail.

To prevent the rest of a **`.bashrc`** from running if the shell is not
interactive, insert this line at the start of the file (make sure you type
this as shown, including the spaces):

    [ -z "$PS1" ] && return

The above line will skip the rest of the file if the shell is not being used
interactively. The result will be no unexpected output when a shell is used
non-interactively. You can test this using a command that produces no output,
such as **`true`** or **`cd`**, and making sure that no output appears:

    $ ssh localhost true
    *** COURSE LINUX SERVER ***
    user@localhost's password: 
    $ 

Above, there is no unexpected output from the non-interactive shell, but if
you fully log in and the shell is interactive, the **`.bashrc`** runs
normally and generates the expected output:

    $ ssh localhost
    *** COURSE LINUX SERVER ***
    user@localhost's password: 
    Welcome back gracious Lord of All.
    YesMaster$ exit
    exit
    $

    -- 
    | Ian! D. Allen  -  idallen@idallen.ca  -  Ottawa, Ontario, Canada
    | Home Page: http://idallen.com/   Contact Improv: http://contactimprov.ca/
    | College professor (Free/Libre GNU+Linux) at: http://teaching.idallen.com/
    | Defend digital freedom:  http://eff.org/  and have fun:  http://fools.ca/

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