A shell script is a series of computer operating systems commands that are stored in a plain text file for simultaneous execution. These commands are typically performed individually by manually entering them into a command-line interface. Shell scripts expedite and streamline this process by combining the commands into an automated executable program. A typical script consists of keywords, commands, functions and control loops, and it serves to complete a task or perform a specific purpose.
One convenient use of a shell script is to automate scheduled tasks and processes. Examples of this include file conversions, regular file backups, checking for software updates and mass uploading of files. This can be achieved relatively quickly compared to writing a typical computer program to perform the same function, because a shell script does not require compilation. Additionally, a shell script can be used to link together other programs in a certain order and have them function in a specific way. Existing shell scripts can be edited and parted out to create new scripts, allowing them to serve as a template for efficient script creation.
The main advantage of a shell script is that it operates as a core system level feature, meaning that it is not restricted to a certain type of programming language, nor does it require being run directly within the shell itself. This flexibility allows the script to function as needed, instead of being restricted by what programs it can interact with or processes it can perform. Shell scripts also allow for ease of debugging within the program itself, providing additional utility and use.
Shell scripts are limited in size and complexity by the nature of available commands within the shell language itself. As such, a shell script is often the first stage of a larger program's development. The scripts can then be converted to other computer languages, such as Python, Perl or C. Beyond the natural limitation of the shell language, shell scripts are also restricted in size and purpose by slow execution and potential cross-platform compatibility issues.
Within a shell script, there will be keywords, including "if," "else," "do," and "while." There are commands issued such as "echo" and "test," and there can be text processing utilities such as the "grep" function. The commands and keywords are linked together by specific shell script functions and "if," "then" and "else" command loops. These commands are saved into a plain text file, which is then executed by the shell itself, depending on the operating system of the computer. This can be done by a command-line interface such as with Unix-based systems or in a graphical interface.