Linux - Bash Tips

What is the Bash Shell?

The GNU Bourne-Again SHell (BASH) incorporates features from the C Shell (csh) and the Korn Shell (ksh) and conforms to the POSTIX 2 shell specifications. It provides a Command Line Interface (CLI) for working on *nix systems and is the most common shell used on Linux systems. Useful bash features will be the subject of the rest of this document.

Bash's Configuration Files

Because what I want to say here has already been written I will quote the section entitled "Files used by Bash" from freeunix.dyndns.org's "Customizing your Bash environment"

In your home directory, 3 files have a special meaning to Bash, allowing you to set up your environment automatically when you log in and when you invoke another Bash shell, and allow you to execute commands when you log out. These files may exist in your home directory, but that depends largely on the Linux distro you're using and how your sysadmin (if not you) has set up your account. If they're missing, Bash defaults to /etc/profile. You can easily create these files yourself using your favorite texteditor. They are: .bash_profile : read and the commands in it executed by Bash every time you log in to the system .bashrc : read and executed by Bash every time you start a subshell .bash_logout : read and executed by Bash every time a login shell exits Bash allows 2 synonyms for .bash_profile : .bash_login and .profile. These are derived from the C shell's file named .login and from the Bourne shell and Korn shell files named .profile. Only one of these files is read when you log in. If .bash_profile isn't there, Bash will look for .bash_login. If that is missing too, it will look for .profile. .bash_profile is read and executed only when you start a login shell (that is, when you log in to the system). If you start a subshell (a new shell) by typing bash at the command prompt, it will read commands from .bashrc. This allows you to separate commands needed at login from those needed when invoking a subshell. However, most people want to have the same commands run regardless of whether it is a login shell or a subshell. This can be done by using the source command from within .bash_profile to execute.bashrc. You would then simply place all the commands in .bashrc.

These files are useful for automatically executing commands like: set, alias, unalias, and setting the PS(1-4) variables, which can all be used to modify your bash environment.
Also use the source command to apply the changes that you have just made in a configuration file. For example if you add an alias to /etc/profile to apply the changes to your current session execute:

$ source /etc/profile

Modifying the Bash Shell with the set Command

Two options that can be set using the set command that will be of some interest to the common user are "-o vi" and "-o emacs". As with all of the environment modifying commands these can be typed at the command prompt or inserted into the appropriate file mentioned above.

Set Emacs Mode in Bash

$ set -o emacs

This is usually the default editing mode when in the bash environment and means that you are able to use commands like those in Emacs (defined in the Readline library) to move the cursor, cut and paste text, or undo editing.
Commands to take advantage of bash's Emacs Mode:

ctrl-a	Move cursor to beginning of line
ctrl-e	Move cursor to end of line
meta-b	Move cursor back one word
meta-f	Move cursor forward one word
ctrl-w	Cut the last word
ctrl-u	Cut everything before the cursor
ctrl-k	Cut everything after the cursor
ctrl-y	Paste the last thing to be cut
ctrl-_	Undo

NOTE: ctrl- = hold control, meta- = hold meta (where meta is usually the alt or escape key).
A combination of ctrl-u to cut the line combined with ctrl-y can be very helpful. If you are in middle of typing a command and need to return to the prompt to retrieve more information you can use ctrl-u to save what you have typed in and after you retrieve the needed information ctrl-y will recover what was cut.

Set Vi Mode in Bash

$ set -o vi

Vi mode allows for the use of vi like commands when at the bash prompt. When set to this mode initially you will be in insert mode (be able to type at the prompt unlike when you enter vi). Hitting the escape key takes you into command mode.
Commands to take advantage of bash's Vi Mode:

h	Move cursor left
l	Move cursor right
A	Move cursor to end of line and put in insert mode
0	(zero) Move cursor to beginning of line (doesn't put in insert mode)
i	Put into insert mode at current position
a	Put into insert mode after current position
dd	Delete line (saved for pasting)
D	Delete text after current cursor position (saved for pasting)
p	Paste text that was deleted
j	Move up through history commands
k	Move down through history commands
u	Undo

Useful Commands and Features

The commands in this section are non-mode specific, unlike the ones listed above.

Flip the Last Two Characters

If you type like me your fingers spit characters out in the wrong order on occasion. ctrl-t swaps the order that the last two character appear in.

Searching Bash History

As you enter commands at the CLI they are saved in a file ~./.bash_history. From the bash prompt you can browse the most recently used commands through the least recently used commands by pressing the up arrow. Pressing the down arrow does the opposite. If you have entered a command a long time ago and need to execute it again you can search for it. Type the command 'ctrl-r' and enter the text you want to search for.

Dealing with Spaces

First, I will mention a few ways to deal with spaces in directory names, file names, and everywhere else.

Using the Backslash Escape Sequence

One option is to use bash's escape character \. Any space following the backslash is treated as being part of the same string. These commands create a directory called "foo bar" and then remove it.

$ mkdir foo\ bar $ rm -r foo\ bar

The backslash escape sequence can also be used to decode commands embedded in strings which can be very useful for scripting or modifying the command prompt as discussed later.

Using Single/Double Quotes with Spaces and Variables

Single and double quotes can also be used for dealing with spaces.

$ touch 'dog poo' $ rm "dog poo"

The difference between single and double quotes being that in double quotes the $, \, and ' characters still preserve their special meanings. Single quotes will take the $ and \ literally and regard the ' as the end of the string. Here's an example:

$ MY_VAR='This is my text' $ echo $MY_VAR This is my text $ echo "$MY_VAR" This is my text $ echo '$MY_VAR' $MY_VAR

The string following the $ character is interpreted as being a variable except when enclosed in single quotes as shown above.

Lists Using { and }

The characters { and } allow for list creation. In other words you can have a command be executed on each item in the list. This is perhaps best explained with examples:

$ touch {temp1,temp2,temp3,temp4}

This will create/modify the files temp1, temp2, temp3, and temp4 and as in the example above when the files share common parts of the name you can do:

$ mv temp{1,2,3,4} ./foo\ bar/

This will move all four of the files into a directory 'foo bar'.

Executing Multiple Commands in Sequence

This is a hefty title for a simple task. Consider that you want to run three commands, one right after the other, and you do not want to wait for each to finish before typing the next. You can type all three commands on a line and then start the process:

$ ./configure; make; make install OR $ ./configure && make && make install

With the first if the ./configure fails the other two commands will continue to execute. With the second the commands following the && will only execute if the command previous finishes without error. Thus, the second would be most useful for this example because there is no reason to run 'make' or 'make install' if the configuration fails.

Piping Output from One Command to Another

Piping allows the user to do several fantastic thing by combining utilities. I will cover only very basic uses for piping. I most commonly use the pipe command, |, to pipe text that is output from one command through the grep command to search for text.
Examples:

See if a program, centericq, is running: $ ps ax | grep centericq 25824 pts/2 S 0:18 centericq Count the number of files in a directory (nl counts things): $ ls | nl 1 #.emacs# 2 BitchX 3 Outcast double cd.lst 4 bm.shader 5 bmtexturesbase.pk3 If my memory serves using RPM to check if a package is installed: $ rpm -qa | grep package_name A more advance example: $ cat /etc/passwd | awk -F: '{print $1 "\t" $6}' | sort > ./users This sequence takes the information if the file passwd, pipes it to awk, which takes the first and sixth fields (the user name and home directory respectively), pipes these fields separated by a tab ("\t") to sort, which sorts the list alphabetically, and puts it into a file called users.

Aliasing Commands

Once again I like how this topic is covered on freeunix.dyndns.org:8088 in "Customizing your Bash environment" I will quote the section entitled "Aliasses":

If you have used UNIX for a while, you will know that there are many commands available and that some of them have very cryptic names and/or can be invoked with a truckload of options and arguments. So, it would be nice to have a feature allowing you to rename these commands or type something simple instead of a list of options. Bash provides such a feature : the alias . Aliasses can be defined on the command line, in .bash_profile, or in .bashrc, using this form : alias name=command This means that name is an alias for command. Whenever name is typed as a command, Bash will substitute command in its place. Note that there are no spaces on either side of the equal sign. Quotes around command are necessary if the string being aliassed consists of more than one word. A few examples : alias ls='ls -aF --color=always' alias ll='ls -l' alias search=grep alias mcd='mount /mnt/cdrom' alias ucd='umount /mnt/cdrom' alias mc='mc -c' alias ..='cd ..' alias ...='cd ../..' The first example ensures that ls always uses color if available, that dotfiles are listed as well,that directories are marked with a / and executables with a *. To make ls do the same on FreeBSD, the alias would become : alias ls='/bin/ls -aFG' To see what aliasses are currently active, simply type alias at the command prompt and all active aliasses will be listed. To "disable" an alias type unalias followed by the alias name.

Altering the Command Prompt Look and Information

Bash has the ability to change how the command prompt is displayed in information as well as colour. This is done by setting the PS1 variable. There is also a PS2 variable. It controls what is displayed after a second line of prompt is added and is usually by default '> '. The PS1 variable is usually set to show some useful information by the Linux distribution you are running but you may want to earn style points by doing your own modifications.
Here are the backslash-escape special characters that have meaning to bash:

\a an ASCII bell character (07) \d the date in "Weekday Month Date" format (e.g., "Tue May 26") \e an ASCII escape character (033) \h the hostname up to the first `.' \H the hostname \j the number of jobs currently managed by the shell \l the basename of the shell's terminal device name \n newline \r carriage return \s the name of the shell, the basename of $0 (the portion following the final slash) \t the current time in 24-hour HH:MM:SS format \T the current time in 12-hour HH:MM:SS format \@ the current time in 12-hour am/pm format \u the username of the current user \v the version of bash (e.g., 2.00) \V the release of bash, version + patchlevel (e.g., 2.00.0) \w the current working directory \W the basename of the current working direcory \! the history number of this command \# the command number of this command \$ if the effective UID is 0, a #, otherwise a $ \nnn the character corresponding to the octal number nnn \\ a backslash \[ begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt \] end a sequence of non-printing characters

Colours In Bash:

Black 0;30 Dark Gray 1;30 Blue 0;34 Light Blue 1;34 Green 0;32 Light Green 1;32 Cyan 0;36 Light Cyan 1;36 Red 0;31 Light Red 1;31 Purple 0;35 Light Purple 1;35 Brown 0;33 Yellow 1;33 Light Gray 0;37 White 1;37

Here is an example borrowed from the Bash-Prompt-HOWTO:

PS1="\[\033[1;34m\][\$(date +%H%M)][\u@\h:\w]$\[\033[0m\] "

This turns the text blue, displays the time in brackets (very useful for not losing track of time while working), and displays the user name, host, and current directory enclosed in brackets. The "\[\033[0m\]" following the $ returns the colour to the previous foreground colour.
How about command prompt modification thats a bit more "pretty":

PS1="\[\033[1;30m\][\[\033[1;34m\]\u\[\033[1;30m\]@\[\033[0;35m\]\h\[\033[1;30m\]] \[\033[0;37m\]\W \[\033[1;30m\]\$\[\033[0m\] "

This one sets up a prompt like this: [user@host] directory $
Break down:

\[\033[1;30m\] - Sets the color for the characters that follow it. Here 1;30 will set them to Dark Gray. \u \h \W \$ - Look to the table above \[\033[0m\] - Sets the colours back to how they were originally.

Each user on a system can have their own customized prompt by setting the PS1 variable in either the .bashrc or .profile files located in their home directories.

FUN STUFF!

A quick note about bashish. It allows for adding themes to a terminal running under a GUI. Check out the site for some screen-shots of what it can do. Also, the program fortune is a must [At least I have considered it so every since my Slackware days (it is default)]. It doesn't have anything to do with bash and is a program that outputs a quote to the screen. Several add-ons are available to make it say stuff about programming, the xfiles, futurama, starwars, and more. Just add a line in your /etc/profile like this to brighten your day when you log into your computer:

echo;fortune;echo

CDargs - Shell Bookmarks

Impress your friends and colleagues with lightening fast directory switching using the CDargs bookmarking tool. CDargs is not exclusive to BASH, but is a great addition and works on *nix based systems, including OS X. Download CDargs here in source or rpm.
CDargs allow for setting named marks in directories and moving to them quickly using the cdb command or a ncurses view.

Install

• Compile / install source
• Move cdargs-bash.sh to /etc
• Add this line to your users .bashrc file

source /etc/cdargs-bash.sh

• Relogin or run source ~/.bashrc

Usage

mark Mark a directory that you want to get to quickly in the future. Move to the desired directory and type mark  or simply mark to have it take the name of the current directory. You can also mark a directory using the ncurses tool. Run cdargs or cdb to start the ncurses tool. Add a new mark by pressing a. cdb Now you have a bunch of marked directories. Simply type cdb  to move to the marked directory. Alternatively use cdb and navigate with arrows or number to the desired mark. manage Start the ncurses tool cdb. Some useful keys to thump:

a add new mark d delete mark e edit mark right left arrows move in and out of directories l list the files in the highlighted directory c make a copy of a mark enter go to selected directory / mark

You can also edit the ~/.cdargs text file directly to manage marks

Basic and Extended Bash Completion

Basic Bash Completion will work in any bash shell. It allows for completion of:

1. File Names
2. Directory Names
3. Executable Names
4. User Names (when they are prefixed with a ~)
5. Host Names (when they are prefixed with a @)
6. Variable Names (when they are prefixed with a $)

This is done simply by pressing the tab key after enough of the word you are trying to complete has been typed in. If when hitting tab the word is not completed there are probably multiple possibilities for the completion. Press tab again and it will list the possibilities. Sometimes on my machine I have to hit it a third time.
Extended Programmable Bash Completion is a program that you can install to complete much more than the names of the things listed above. With extended bash completion you can, for example, complete the name of a computer you are trying to connect to withssh or scp. It achieves this by looking through the known_hosts file and using the hosts listed there for the completion. This is greatly customizable and the package and more information can be found here.
Configuration of Programmable Bash Completion is done in /etc/bash_completion. Here is a list of completions that are in my bash_completion file by default.

completes on signal names completes on network interfaces expands tildes in pathnames completes on process IDs completes on process group IDs completes on user IDs completes on group IDs ifconfig(8) and iwconfig(8) helper function bash alias completion bash export completion bash shell function completion bash complete completion service completion chown(1) completion chgrp(1) completion umount(8) completion mount(8) completion Linux rmmod(8) completion Linux insmod(8), modprobe(8) and modinfo(8) completion man(1) completion renice(8) completion kill(1) completion Linux and FreeBSD killall(1) completion GNU find(1) completion Linux ifconfig(8) completion Linux iwconfig(8) completion RedHat & Debian GNU/Linux if{up,down} completion Linux ipsec(8) completion (for FreeS/WAN) Postfix completion cvs(1) completion rpm completion apt-get(8) completion chsh(1) completion chkconfig(8) completion user@host completion host completion based on ssh's known_hosts ssh(1) completion scp(1) completion rsync(1) completion Linux route(8) completion GNU make(1) completion GNU tar(1) completion jar(1) completion Linux iptables(8) completion tcpdump(8) completion autorpm(8) completion ant(1) completion

mysqladmin(1) completion gzip(1) completion bzip2(1) completion openssl(1) completion screen(1) completion lftp(1) bookmark completion ncftp(1) bookmark completion gdb(1) completion Postgresql completion psql(1) completion createdb(1) completion dropdb(1) completion gcc(1) completion Linux cardctl(8) completion Debian dpkg(8) completion Debian GNU dpkg-reconfigure(8) completion Debian Linux dselect(8) completion Java completion PINE address-book completion mutt completion Debian reportbug(1) completion Debian querybts(1) completion update-alternatives completion Python completion Perl completion rcs(1) completion lilo(8) completion links completion FreeBSD package management tool completion FreeBSD kernel module commands FreeBSD portupgrade completion FreeBSD portinstall completion Slackware Linux removepkg completion look(1) completion ypcat(1) and ypmatch(1) completion mplayer(1) completion KDE dcop completion wvdial(1) completion gpg(1) completion iconv(1) completion dict(1) completion cdrecord(1) completion mkisofs(8) completion mc(1) completion yum(8) completion yum-arch(8) completion ImageMagick completion