Shell script

Typical operations performed by shell scripts include file manipulation, program execution, and printing text.

The LC_COLLATE=C sets the default collation order to not fold upper and lower case together, not intermix dotfiles with normal filenames as a side effect of ignoring punctuation in the names (dotfiles are usually only shown if an option like -a is used), and the "$@" causes any parameters given to l to pass through as parameters to ls, so that all of the normal options and other syntax known to ls can still be used.

Shell scripts allow several commands that would be entered manually at a command-line interface to be executed automatically, and without having to wait for a user to trigger each stage of the sequence.

For example, in a directory with three C source code files, rather than manually running the four commands required to build the final program from them, one could instead create a script for POSIX-compliant shells, here named build and kept in the directory with them, which would compile them automatically: The script would allow a user to save the file being edited, pause the editor, and then just run ./build to create the updated program, test it, and then return to the editor.

Simple batch jobs are not unusual for isolated tasks, but using shell loops, tests, and variables provides much more flexibility to users.

[4] On the other hand, the various shells plus tools like awk, sed, grep, and BASIC, Lisp, C and so forth contributed to the Perl programming language.

Shell scripts often serve as an initial stage in software development, and are often subject to conversion later to a different underlying implementation, most commonly being converted to Perl, Python, or C. The interpreter directive allows the implementation detail to be fully hidden inside the script, rather than being exposed as a filename extension, and provides for seamless reimplementation in different languages with no impact on end users.

[7][8][9][10] Perhaps the biggest advantage of writing a shell script is that the commands and syntax are exactly the same as those directly entered at the command-line.

Interpretive running makes it easy to write debugging code into a script and re-run it to detect and fix bugs.

Inadvertent typing errors such as rm -rf * / (instead of the intended rm -rf */) are folklore in the Unix community; a single extra space converts the command from one that deletes all subdirectories contained in the current directory, to one which deletes everything from the file system's root directory.

Similar problems can transform cp and mv into dangerous weapons, and misuse of the > redirect can delete the contents of a file.

This is made more problematic by the fact that many UNIX commands differ in name by only one letter: cp, cd, dd, df, etc.

[16] The console alternatives 4DOS, 4OS2, FreeDOS, Peter Norton's NDOS and 4NT / Take Command which add functionality to the Windows NT-style cmd.exe, MS-DOS/Windows 95 batch files (run by Command.com), OS/2's cmd.exe, and 4NT respectively are similar to the shells that they enhance and are more integrated with the Windows Script Host, which comes with three pre-installed engines, VBScript, JScript, and VBA and to which numerous third-party engines can be added, with Rexx, Perl, Python, Ruby, and Tcl having pre-defined functions in 4NT and related programs.

On Unix and other POSIX-compliant systems, awk and sed are used to extend the string and numeric processing ability of shell scripts.

Tcl, Perl, Rexx, and Python have graphics toolkits and can be used to code functions and procedures for shell scripts which pose a speed bottleneck (C, Fortran, assembly language &c are much faster still) and to add functionality not available in the shell language such as sockets and other connectivity functions, heavy-duty text processing, working with numbers if the calling script does not have those abilities, self-writing and self-modifying code, techniques like recursion, direct memory access, various types of sorting and more, which are difficult or impossible in the main script, and so on.