Bash-Beginners-Guide

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Chapter 1. Bash and Bash scripts In this introduction module we ♦ Describe some common shells ♦ Point out GNU Bash advantages and features ♦ Describe the shell's building blocks ♦ Discuss Bash initialization files ♦ See how the shell executes commands ♦ Look into some simple script examples 1.1. Common shell programs 1.1.1. General shell functions The UNIX shell program interprets user commands, which are either directly entered by the user, or which can be read from a file called the shell script or shell program. Shell scripts are interpreted, not compiled. The shell reads commands from the script line per line and searches for those commands on the system (see Section 1.2), while a compiler converts a program into machine readable form, an executable file - which may then be used in a shell script. Apart from passing commands to the kernel, the main task of a shell is providing a user environment, which can be configured individually using shell resource configuration files. 1.1.2. Shell types Just like people know different languages and dialects, your UNIX system will usually offer a variety of shell types: • sh or Bourne Shell: the original shell still used on UNIX systems and in UNIX-related environments. This is the basic shell, a small program with few features. While this is not the standard shell, it is still available on every Linux system for compatibility with UNIX programs. • bash or Bourne Again shell: the standard GNU shell, intuitive and flexible. Probably most advisable for beginning users while being at the same time a powerful tool for the advanced and professional user. On Linux, bash is the standard shell for common users. This shell is a so-called superset of the Bourne shell, a set of add-ons and plug-ins. This means that the Bourne Again shell is compatible with the Bourne shell: commands that work in sh, also work in bash. However, the reverse is not always the case. All examples and exercises in this book use bash. • csh or C shell: the syntax of this shell resembles that of the C programming language. Sometimes asked for by programmers. • tcsh or TENEX C shell: a superset of the common C shell, enhancing user-friendliness and speed. That is why some also call it the Turbo C shell. • ksh or the Korn shell: sometimes appreciated by people with a UNIX background. A superset of the Bourne shell; with standard configuration a nightmare for beginning users. The file /etc/shells gives an overview of known shells on a Linux system: Chapter 1. Bash and Bash scripts 6
Bash Guide for Beginners mia:~> cat /etc/shells /bin/bash /bin/sh /bin/tcsh /bin/csh Your default shell is set in the /etc/passwd file, like this line for user mia: mia:L2NOfqdlPrHwE:504:504:Mia Maya:/home/mia:/bin/bash To switch from one shell to another, just enter the name of the new shell in the active terminal. The system finds the directory where the name occurs using the PATH settings, and since a shell is an executable file (program), the current shell activates it and it gets executed. A new prompt is usually shown, because each shell has its typical appearance: mia:~> tcsh [mia@post21 ~]$ 1.2. Advantages of the Bourne Again SHell 1.2.1. Bash is the GNU shell The GNU project (GNU's Not UNIX) provides tools for UNIX-like system administration which are free software and comply to UNIX standards. Bash is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh). It is intended to conform to the IEEE POSIX P1003.2/ISO 9945.2 Shell and Tools standard. It offers functional improvements over sh for both programming and interactive use; these include command line editing, unlimited size command history, job control, shell functions and aliases, indexed arrays of unlimited size, and integer arithmetic in any base from two to sixty-four. Bash can run most sh scripts without modification. Like the other GNU projects, the bash initiative was started to preserve, protect and promote the freedom to use, study, copy, modify and redistribute software. It is generally known that such conditions stimulate creativity. This was also the case with the bash program, which has a lot of extra features that other shells can't offer. 1.2.2. Features only found in bash 1.2.2.1. Invocation In addition to the single-character shell command line options which can generally be configured using the set shell built-in command, there are several multi-character options that you can use. We will come across a couple of the more popular options in this and the following chapters; the complete list can be found in the Bash info pages, Bash features->Invoking Bash. 1.2.2.2. Bash startup files Startup files are scripts that are read and executed by Bash when it starts. The following subsections describe different ways to start the shell, and the startup files that are read consequently. Chapter 1. Bash and Bash scripts 7
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Two regular expressions may be conc
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LABEL=/ / ext3 defaults 1 1 If this
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cathy ~> ls -ld [[:digit:]]* drwxrw
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Bash Guide for Beginners p Print te
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sandy ~> sed -n '/a text/,/This/p'
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Bash Guide for Beginners # This is
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Chapter 6. The GNU awk programming
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Bash Guide for Beginners 121script.
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Bash Guide for Beginners amd.conf a
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Bash Guide for Beginners record1 re
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Bash Guide for Beginners 6.3.6. The
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Chapter 7. Conditional statements I
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Bash Guide for Beginners Table 7-2.
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[root@elegance root]# We switch to
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Bash Guide for Beginners echo "Prop
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Bash Guide for Beginners We use the
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Bash Guide for Beginners case EXPRE
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Upon successful condition testing,
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Bash Guide for Beginners bit better
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8.2. Catching user input 8.2.1. Usi
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Bash Guide for Beginners michel ~/t
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Bash Guide for Beginners # DO NOT E
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File descriptor 5 Bash Guide for Be
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Bash Guide for Beginners The result
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Bash Guide for Beginners ♦ Compre
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Bash Guide for Beginners for i in `
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touch pic-`date +%s`.jpg sleep 300
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DISKFUL=$(df -h $WEBDIR | grep -v F
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;; 2) $FORTUNE startrek ;; 3) $FORT
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[carol@octarine ~/test] ls -a ./ ..
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Bash Guide for Beginners 9.7.2. Exa
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Chapter 10. More on variables In th
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Bash Guide for Beginners [bob in ~]
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if [ -z $1 ]; then echo "Usage: $0
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10.3.3. Transformations of variable
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Bash Guide for Beginners The opposi
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Chapter 11. Functions In this chapt
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The commands after && are only exec
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zip2 "$TAR" echo "...done." >> "$LO
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Chapter 12. Catching signals In thi
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Bash Guide for Beginners maud: ~> k
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Bash Guide for Beginners 2. Write a
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Bash Guide for Beginners jobs kill
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if [ $i -eq 5 ] ArrowUp+Enter or !!
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cd Change directory. cdp/cdplay An
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Bash Guide for Beginners gimp grep
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Send and receive mail. man Read man
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Bash Guide for Beginners rsh RPM Pa
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W Bash Guide for Beginners w Show w
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Index A aliases Section 3.5.1 ANSI-
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Bash Guide for Beginners Section 3.
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J Bash Guide for Beginners K kill k
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eadonly Section 10.1.3 regular expr
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Bash Guide for Beginners vi(m) Sect
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