Pipe files are a special kind of file shared between two programs.The file acts as a buffer for sharing information. One program writes to the pipe file and the other reads from the pipe.When the pipe file reaches a certain size, Linux halts the writing program until the reading program can “catch up.”
A similar kind of file is called a Unix domain socket file.A socket file acts like a pipe but works using network sockets.
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Introduction
A shell is a command line interpretor. It takes commands and executes them. As such, it implements a programming language. The Bourne shell is used to create shell scripts -- ie. programs that are interpreted/executed by the shell. You can write shell scripts with the C-shell; however, this is not covered here.
Necessity of Structured Shell Programming
Shell scripts remain a staple of the business world.With high development costs, it is not practical to develop everything in a high-level language. Many business processes are simply a matter of following a series of steps, after which the results are labeled and archived.This is the very type of activity scripts are made to handle. Over time, shells have collected new features, often by copying the capabilities of other shells. In his book, The Humane Interface, Apple Macintosh interface designer Jef Raskin once spoke with a pilot regarding the design of the plane’s autopilot.The device was designed with five ways to enter coordinates.The reason for this design was to reduce training costs by emulating other autopilots. However, a pilot pointed out that the operator of the aircraft is responsible for knowing how to operate every piece of equipment, and that meant he had to know how to adjust the autopilot in each of the five ways.
Many shells, including Bash, have developed a mixture of features with specialized or arcane applications. Like the autopilot, it is not always clear how these features should be
used nor which features are provided primarily for compatibility.Too often, poorly designed scripts are all-but illegible to another programmer, or even to the same programmer a few months after a script was hastily assembled.
This presents a problem for serious business scripts.The cost of developing a highlevel language solution is high, but a disorganized shell script can be expensive to maintain
over time. As the needs of the business change, and the shell script is modified by successive generations of programmers, the problem escalates.
As a rule of thumb, business programs never go away.At one place I recently worked,what started off as a simple shell script used to print reports gradually evolved into a Web-based reporting system complete with personal customizations and secure access.
The original programmer had no idea what his script would become. Because of the essential openness and complex syntax of Bash, it’s to the benefit of any serious project to adapt some kind of standard. Bash is very forgiving, but the costs are not. For example, there are many ways to add 2 + 2 in Bash. Like the autopilot anecdote, it’s not practical to expect the programmers who maintain the scripts to have to deal with this kind of redundancy. Likewise, shell word quoting is sometimes optional, sometimes required, depending on the context. A command that works with one form of quotation can suddenly stop working when different data appears in the quotes. Issues like shell word quoting, specialized capabilities, or portability features, when abused, can lead to increased maintenance and long-term development costs.
Many shells, including Bash, have developed a mixture of features with specialized or arcane applications. Like the autopilot, it is not always clear how these features should be
used nor which features are provided primarily for compatibility.Too often, poorly designed scripts are all-but illegible to another programmer, or even to the same programmer a few months after a script was hastily assembled.
This presents a problem for serious business scripts.The cost of developing a highlevel language solution is high, but a disorganized shell script can be expensive to maintain
over time. As the needs of the business change, and the shell script is modified by successive generations of programmers, the problem escalates.
As a rule of thumb, business programs never go away.At one place I recently worked,what started off as a simple shell script used to print reports gradually evolved into a Web-based reporting system complete with personal customizations and secure access.
The original programmer had no idea what his script would become. Because of the essential openness and complex syntax of Bash, it’s to the benefit of any serious project to adapt some kind of standard. Bash is very forgiving, but the costs are not. For example, there are many ways to add 2 + 2 in Bash. Like the autopilot anecdote, it’s not practical to expect the programmers who maintain the scripts to have to deal with this kind of redundancy. Likewise, shell word quoting is sometimes optional, sometimes required, depending on the context. A command that works with one form of quotation can suddenly stop working when different data appears in the quotes. Issues like shell word quoting, specialized capabilities, or portability features, when abused, can lead to increased maintenance and long-term development costs.
The Origin of Linux
The Linux operating system was created as a hobby by a young student, Linus Torvalds, at the University of Helsinki in Finland. Linus, interested in the Unix clone operating system Minix, wanted to create an expanded version of Minix with more capabilities. He began his work in 1991 when he released version 0.02 and invited programmers to participate in his project.Version 1.0 was released in 1994.
Linux uses GNU General Public License (GPL) and its source code is freely available to everyone. Linux distributions, CD-ROMs with the Linux kernel and other software ready for installation, do not have to be free, but the Linux source code must remain available. Making source code available is known as distributing open source. The word “Linux” is properly pronounced using a Swedish accent, making it difficult to say in North America. It is most often pronounced with a short “i” and with the firstsyllable stressed, as in LIH-nicks, but it is sometimes pronounced LYE-nicks (the anglicized “Linus’ Unix”) or LEE-nucks. Strictly speaking, Linux refers to the operating system kernel that starts and manages other programs and provides access to system resources.The various open source shells, compilers, standard libraries, and commands are a part of another project called GNU. The GNU project was started by the Free Software Foundation (FSF) as an attempt to create a free version of Unix.The main Linux C compiler, gcc, is a part of the GNU project. There is also a GNU kernel project, but the Linux kernel has largely superseded thiseffort.
X Windows is also not strictly a part of Linux. Xfree86, the open source version of X Windows, was adapted to the Linux operating system and was released under a different license.
Linux uses GNU General Public License (GPL) and its source code is freely available to everyone. Linux distributions, CD-ROMs with the Linux kernel and other software ready for installation, do not have to be free, but the Linux source code must remain available. Making source code available is known as distributing open source. The word “Linux” is properly pronounced using a Swedish accent, making it difficult to say in North America. It is most often pronounced with a short “i” and with the firstsyllable stressed, as in LIH-nicks, but it is sometimes pronounced LYE-nicks (the anglicized “Linus’ Unix”) or LEE-nucks. Strictly speaking, Linux refers to the operating system kernel that starts and manages other programs and provides access to system resources.The various open source shells, compilers, standard libraries, and commands are a part of another project called GNU. The GNU project was started by the Free Software Foundation (FSF) as an attempt to create a free version of Unix.The main Linux C compiler, gcc, is a part of the GNU project. There is also a GNU kernel project, but the Linux kernel has largely superseded thiseffort.
X Windows is also not strictly a part of Linux. Xfree86, the open source version of X Windows, was adapted to the Linux operating system and was released under a different license.
Files and File Systems
Each Linux disk (or other long-term block storage device) contains a collection of files organized according to a policy or set of rules called a file system. Each disk can be divided into partitions (or “slices”), whereby every partition has its own file system. Linux is not restricted to a single file system for all disks: the user can use disks created by other operating systems as if they were native Linux disks.
The standard file system is the second extended file system, or ext2.This is the second revision of the Minix file system with support for large disk sizes and filenames. ext2 permits partitions up to 4TB (terabytes), files up to 2GB (gigabytes), and 255-character filenames. Newer distributions use ext3, a version of ext2 with special features for error recovery.
Support for other file systems might be available depending on your distribution and
installation options.They might include Microsoft Windows NT, Apple HFS, or journalingfile systems.
The ext2 file system uses caching to increase performance. If an ext2 disk is not
properly shut down, files can be corrupted or lost. It is vitally important that a Linux computer is shut down properly or is protected by some kind of uninterruptible power supply.To save space, ext2 files that contain large amounts of zeros (called sparse files) are not actually stored on a disk. Certain shell commands provide options for creating and handling sparse files. Each file is identified by a name, and the allowed names are determined by the file system. For practicality, the names seldom exceed 32 characters and usually consist of lowercase characters, underscores, minus signs, and periods. Spaces and punctuation symbols, for example, are permitted, but can cause problems in shell scripts that do not expect them. Filenames do not require a suffix to identify their contents, but they are often used to avoid confusion about what data is contained in files. Some common suffix codes include:
> .sh—A Bash shell script
> .txt—A generic text file
> .log—A log file
> .html—A HTML Web page
> .tgz (or .tar.gz)—Compressed file archive
Commands usually have no suffix.
The standard file system is the second extended file system, or ext2.This is the second revision of the Minix file system with support for large disk sizes and filenames. ext2 permits partitions up to 4TB (terabytes), files up to 2GB (gigabytes), and 255-character filenames. Newer distributions use ext3, a version of ext2 with special features for error recovery.
Support for other file systems might be available depending on your distribution and
installation options.They might include Microsoft Windows NT, Apple HFS, or journalingfile systems.
The ext2 file system uses caching to increase performance. If an ext2 disk is not
properly shut down, files can be corrupted or lost. It is vitally important that a Linux computer is shut down properly or is protected by some kind of uninterruptible power supply.To save space, ext2 files that contain large amounts of zeros (called sparse files) are not actually stored on a disk. Certain shell commands provide options for creating and handling sparse files. Each file is identified by a name, and the allowed names are determined by the file system. For practicality, the names seldom exceed 32 characters and usually consist of lowercase characters, underscores, minus signs, and periods. Spaces and punctuation symbols, for example, are permitted, but can cause problems in shell scripts that do not expect them. Filenames do not require a suffix to identify their contents, but they are often used to avoid confusion about what data is contained in files. Some common suffix codes include:
> .sh—A Bash shell script
> .txt—A generic text file
> .log—A log file
> .html—A HTML Web page
> .tgz (or .tar.gz)—Compressed file archive
Commands usually have no suffix.
Suppose you often type the command
find . -name file -print
and you'd rather type a simple command, say
sfind file
Create a shell script
% cd ~/bin
% emacs sfind
% page sfind
find . -name $1 -print
% chmod a+x sfind
% rehash
% cd /usr/local/bin
% sfind tcsh
./shells/tcsh
Observations
This quick example is far from adequate but some observations:
1. Shell scripts are simple text files created with an editor.
2. Shell scripts are marked as executeable
3. %chmod a+x sfind
4. Should be located in your search path and ~/bin should be in your search path.
5. You likely need to rehash if you're a Csh (tcsh) user (but not again when you login).
6. Arguments are passed from the command line and referenced. For example, as $1.
#!/bin/sh
All Bourne Shell scripts should begin with the sequence
#!/bin/sh
"On the first line of an interpreter script, following the "#!", is the name of a program which should be used to interpret the contents of the file. For instance, if the first line contains "#! /bin/sh", then the con- tents of the file are executed as a shell script."
You can get away without this, but you shouldn't. All good scripts state the interpretor explicitly. Long ago there was just one (the Bourne Shell) but these days there are many interpretors -- Csh, Ksh, Bash, and others.
Comments
Comments are any text beginning with the pound (#) sign. A comment can start anywhere on a line and continue until the end of the line.
find . -name file -print
and you'd rather type a simple command, say
sfind file
Create a shell script
% cd ~/bin
% emacs sfind
% page sfind
find . -name $1 -print
% chmod a+x sfind
% rehash
% cd /usr/local/bin
% sfind tcsh
./shells/tcsh
Observations
This quick example is far from adequate but some observations:
1. Shell scripts are simple text files created with an editor.
2. Shell scripts are marked as executeable
3. %chmod a+x sfind
4. Should be located in your search path and ~/bin should be in your search path.
5. You likely need to rehash if you're a Csh (tcsh) user (but not again when you login).
6. Arguments are passed from the command line and referenced. For example, as $1.
#!/bin/sh
All Bourne Shell scripts should begin with the sequence
#!/bin/sh
"On the first line of an interpreter script, following the "#!", is the name of a program which should be used to interpret the contents of the file. For instance, if the first line contains "#! /bin/sh", then the con- tents of the file are executed as a shell script."
You can get away without this, but you shouldn't. All good scripts state the interpretor explicitly. Long ago there was just one (the Bourne Shell) but these days there are many interpretors -- Csh, Ksh, Bash, and others.
Comments
Comments are any text beginning with the pound (#) sign. A comment can start anywhere on a line and continue until the end of the line.
Creating a Script
Suppose you often type the command
find . -name file -print
and you'd rather type a simple command, say
sfind file
Create a shell script
% cd ~/bin
% emacs sfind
% page sfind
find . -name $1 -print
% chmod a+x sfind
% rehash
% cd /usr/local/bin
% sfind tcsh
./shells/tcsh
Observations
This quick example is far from adequate but some observations:
1. Shell scripts are simple text files created with an editor.
2. Shell scripts are marked as executeable
3. %chmod a+x sfind
4. Should be located in your search path and ~/bin should be in your search path.
5. You likely need to rehash if you're a Csh (tcsh) user (but not again when you login).
6. Arguments are passed from the command line and referenced. For example, as $1.
#!/bin/sh
All Bourne Shell scripts should begin with the sequence
#!/bin/sh
"On the first line of an interpreter script, following the "#!", is the name of a program which should be used to interpret the contents of the file. For instance, if the first line contains "#! /bin/sh", then the con- tents of the file are executed as a shell script."
You can get away without this, but you shouldn't. All good scripts state the interpretor explicitly. Long ago there was just one (the Bourne Shell) but these days there are many interpretors -- Csh, Ksh, Bash, and others.
Comments
Comments are any text beginning with the pound (#) sign. A comment can start anywhere on a line and continue until the end of the line.
find . -name file -print
and you'd rather type a simple command, say
sfind file
Create a shell script
% cd ~/bin
% emacs sfind
% page sfind
find . -name $1 -print
% chmod a+x sfind
% rehash
% cd /usr/local/bin
% sfind tcsh
./shells/tcsh
Observations
This quick example is far from adequate but some observations:
1. Shell scripts are simple text files created with an editor.
2. Shell scripts are marked as executeable
3. %chmod a+x sfind
4. Should be located in your search path and ~/bin should be in your search path.
5. You likely need to rehash if you're a Csh (tcsh) user (but not again when you login).
6. Arguments are passed from the command line and referenced. For example, as $1.
#!/bin/sh
All Bourne Shell scripts should begin with the sequence
#!/bin/sh
"On the first line of an interpreter script, following the "#!", is the name of a program which should be used to interpret the contents of the file. For instance, if the first line contains "#! /bin/sh", then the con- tents of the file are executed as a shell script."
You can get away without this, but you shouldn't. All good scripts state the interpretor explicitly. Long ago there was just one (the Bourne Shell) but these days there are many interpretors -- Csh, Ksh, Bash, and others.
Comments
Comments are any text beginning with the pound (#) sign. A comment can start anywhere on a line and continue until the end of the line.
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