This book is designed for the course on OS Unix and shell programming Author: isrd group An Introduction To Programming With C++, 6e (pb) by Zak. : List of books by isrd group. You can download any book by isrd group in PDF for free at Basics Of Os Unix And Shell Programming. The following conventions have been used in this book: Commands, program names, user input in examples and system calls are shown in bold constant The shell looks for the characters >, operating system is, why a computer needs one and how UNIX is vastly different from.
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It is assumed that you already know how to: Many commands are shown with links to their full man pages sh Output from commands is shown like this ; optional items are [ in brackets ]. Some descriptions in these notes have more detail available, and are denoted like this: More details of this item would appear here. The printed notes include all of the additional information Permission is granted to download and use these notes and example scripts, as long as all copyright notices are kept intact.
Some of the examples are taken from texts or online resources which have granted permission to redistribute. These notes are updated from time to time. The “development” set of notes are http: Comments and questions, contact Richard.
The shell itself has limited capabilities — the power comes from using it as a “glue” language to combine the standard Unix utilities, and custom software, to produce a tool more useful than the component parts alone. Any shell can be used for writing a shell script.
To allow for this, the first line of every script is: Any program which can read commands from a file can be started up this way, as long as it recognizes the comment convention. The program is started, and then the script file is given to it as an argument. Because of this, the script must be readable as well as executable. Examples are perl, awk, tcl and python.
Any file can be used as input to a shell by using the syntax: By putting them into a script, you reduce them to a single command. Generalize a sequence of operations on one set of data, into a procedure that can be applied to any similar set of data. Simple shell scripts might be written as shell aliases, but the script can be made available to all users and all processes.
Shell aliases apply only to the current shell. Wrap programs over which you have no control inside an environment that you can control. Create customized datasets on the fly, and call applications e. Application package installation tools Other tools may create fancier installers e.
Shell scripts are used because they are very portable. Application startup scripts, especially unattended applications e. Bill Joy, at Berkeley.
Many things in common with the Bourne shell, but many enhancements to improve interactive use. The internal commands used only in scripts are very different from “sh”, and similar by design to the “C” language syntax.
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Freely available and based on “csh”. It oz many additional features to make interactive use more convenient. We use it as the default interactive shell for new accounts on all of our public systems. Not many people write scripts in [t]csh. Written as a major upgrade to “sh” and backwards compatible with it, but has many internal commands for the most frequently used functions. It also incorporates many of the features from tcsh which enhance interactive use command line history recall etc. This shell bxsics now freely available on all systems, but sometimes not installed by default on “free” Unix.
There are two major versions. Standards committees worked over the Bourne shell and added many features of the Korn shell ksh88 and C shell to define a standard set of features which all compliant shells must have. It is a functional clone of sh, with additional features to enhance interactive use, add POSIX compliance, and partial ksh compatability.
Unix shell scripting with ksh/bash
It was installed as the default shell on early MacOSX systems. Syntax of all the flow control constructs and conditional tests. Syntax for string manipulation inside of scripts Syntax for arithmetic manipulation inside of scripts Syntax for setting local variables used only in the script and environment variables which are passed to child processes.
Ksh93 can be extended by linking to shared libraries providing additional internal commands. It can be used for prototyping GUI applications. This process is computationally expensive relativelyso when the script does something trivial many times over in a loop, it saves a lot of time if the function is handled internally. If you follow textbooks on Bourne shell programming, all of the advice should apply no matter which of the Bourne-derived shells you use.
Unfortunately, many vendors have added features over the years and achieving complete portability can be a challenge. Explicitly writing for ksh or bash and insisting on that shell being installed, can often be simpler.
The sh and ksh man pages use the term special command for the internal commands – handled by the shell itself. This allows single very long commands to be entered in the script in a more readable fashion. You can continue the line as many times as needed.
It allows you to put multiple commands on a single line. There are few occasions when you must do this, but often it is used to improve the layout of compound commands. For the purposes of display, parts of the script have 3: Some comments are bold to flag special sections 5: Line numbers on the left are not part of the script.
They are just added to the HTML for reference. Built-in commands and keywords e. Command substitutions are purple. Variables are black Everything else is red – mostly that is external List the files in columns Summarize the disk usage This is separate from any output generated. The exit status of a shell script can be explicitly set using exit Nor it defaults to the value of the last command run.
The exit status is an integer Think of it as only one way for everything to work, but many possible ways to fail. If the command was terminated by a signal, the value is plus the signal value.
Pattern operators can be combined Example: It would not match chapter If the variable already existed, it is overwritten. Variables are treated as text strings, unless the context implies a numeric interpretation.
You can make a variable always be treated as a number. If you don’t do this, the variable is local to this shell instance. This is an easy way to allow for optional variables, and have sensible defaults if they haven’t been set. If datafile was undefined, it remains so. This is used for diagnostics when a variable should have been set and there is no sensible default value to use.
Variables may be given a null value by setting them to an empty string, e. Variable assignment command prefix It is possible to export a variable just for the duration of a single command using the syntax: Check positional argument handling 4: This is often used, assuming the original arguments are no longer needed, to parse a set of words possibly using different field separators. Arguments may be reset any number of times. Find an entry in the password file 3: The “full name” and other comments are in 7: Get this field using shell word splitting 8: Select a random image from the background logo collection 4: This could be used to configure a screen saver, for example.
This works even if the filenames contain spaces. Use the syntax for arithmetic expressions. Shift arguments by a random number between 0 and the number of files Output the resulting first argument The exit status of a pipeline is the exit status of the last command. In compound commands, a pipeline can be used anywhere a simple command could be used.
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The “elif” and “else” lists are optional. Repeat as long as the first list is true. The until form just negates the test.
Compound commands can be thought of as running in an implicit subshell. Setting of variables in a real subshell does not leave them set in the parent script. Setting variables in implicit subshells varies in behaviour among shells.