Shell scripts are interpreted, not compiled. 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.
Probably most advisable for beginning users while being at the same time a powerful tool for the advanced and professional user. However, the reverse is not always the case. Sometimes asked for by programmers. That is why some also call it the Turbo C shell. A superset of the Bourne shell; with standard configuration a nightmare for beginning users.
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. 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.
The following subsections describe different ways to start the shell, and the startup files that are read consequently.
Bash Guide for Beginners
The shell is not running because a script has been activated. A login shell means that you got the shell after authenticating to the system, usually by giving your user name and password. If a file does not exist, bash searches for the next.
For instance, when you open a terminal using an icon, or a menu item, that is a non-login shell. They are programmed to do certain tasks and cannot be instructed to do other jobs than those for which they are programmed. They are intrinsically insecure because confidential data is sent over the network unencrypted. Different client programs are available for non-UNIX systems as well, see your local software mirror. What is an interactive shell? Is this shell interactive? This is also the prompt you get when the shell thinks you entered an unfinished command, for instance when you forget quotes, command structures that cannot be left out, etc.
In interactive mode this behavior is disabled. Unary expressions are often used to examine the status of a file. You only need one object, for instance a file, to do the operation on. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias. As a consequence, aliases defined in a function are not available until after that function is executed.
There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Arrays are zero-based.
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Normal programs are system commands that exist in compiled form on your system. When such a program is executed, a new process is created because Bash makes an exact copy of itself. This child process has the same environment as its parent, only the process ID number is different. This mechanism is used to create all UNIX processes, so it also applies to the Linux operating system. When the name of a built-in command is used as the first word of a simple command, the shell executes the command directly, without creating a new process.
Built-in commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities. For those commands for which this is not the case, we refer to the Info pages. This subshell reads the lines from the shell script one line at a time.
Commands on each line are read, interpreted and executed as if they would have come directly from the keyboard. When there are no more lines in the shell script to read, the subshell terminates. The parent shell awakes and displays a new prompt. Then these words and operators are translated into commands and other constructs, which return an exit status available for inspection or processing.
Alias expansion is performed. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. It can be a name, a number or a special value.
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For the shell's purpose, a variable is a parameter that stores a name. A variable has a value and zero or more attributes. Redirection may also be used to open and close files for the current shell execution environment. Words that are not variable assignments or redirections are expanded; the first remaining word after expansion is taken to be the name of the command and the rest are arguments to that command.
Then redirections are performed, then strings assigned to variables are expanded. If no command name results, variables will affect the current shell environment. If not, first check with the function list to see if it contains a command by the name we are looking for. Even though in Bash a lot of freedom is granted, you must ensure correct logic, flow control and efficiency so that users executing the script can do so easily and correctly. Where and with which permissions and ownerships?
When using the script on different systems, do all these systems have these commands in the required versions? When and why? Determines logical sequence of tasks so that the result is successful and controlled. This is your first step when developing a script. Itemizing the list of tasks involved in a program allows you to describe each process. Individual tasks can be referenced by their item number. Later, you can replace the everyday language statements with shell language words and constructs. It describes the rotation of log files.
Input from the user must be obtained and stored. The user should be notified that his crontab will change. An example Bash script: mysystem.
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Program starts now. After that, the shell that will execute the commands following the first line is defined. This script starts with clearing the screen on line 2. Line 3 makes it print a message, informing the user about what is going to happen. Line 5 greets the user. Lines 6, 9, 13, 16 and 20 are only there for orderly output display purposes. Line 8 prints the current date and the number of the week.
Line 11 is again an informative message, like lines 3, 18 and Line 19 gives the uptime and load information. The first always exits with a 0 status, and simply prints arguments followed by an end of line character on the standard output, while the latter allows for definition of a formatting string and gives a non-zero exit status code upon failure. Since the shell runs in non-interactive mode, any errors will cause the script to exit prematurely if you don't think about this.
The system log daemon, the power management daemon, the name and mail daemons are common examples. Init, the initial process, reads its configuration files and decides which services to start or stop in each run level. A run level is a configuration of processes; each system has a single user run level, for instance, for performing administrative tasks, for which the system has to be in an unused state as much as possible, such as recovering a critical file system from a backup.