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A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. [1] The various dialects of shell scripts are considered to be command languages . Typical operations performed by shell scripts include file manipulation, program execution, and printing text.
Two data structures are used. The first is a data table comprising a sequence of printing instructions that encode the bitmap for each character (in an encoding specific to the banner program). The second is an index into that table that indicates, for each character code, where the printing instructions for that character begin and end. [3]
In Unix and Unix-like operating systems, printf is a shell builtin (and utility program [2]) that formats and outputs text like the same-named C function. Originally named for outputting to a printer, it actually outputs to standard output. [3] The command accepts a format string, which specifies how to format values, and a list of values.
In Linux, if the script was executed by a regular user, the shell would attempt to execute the command rm -rf / as a regular user, and the command would fail. However, if the script was executed by the root user, then the command would likely succeed and the filesystem would be erased. It is recommended to use sudo on a per-command basis instead.
Multics had a pwd command (which was a short name of the print_wdir command) [11] from which the Unix pwd command originated. [12] The command is a shell builtin in most Unix shells such as Bourne shell, ash, bash, ksh, and zsh. It can be implemented easily with the POSIX C functions getcwd() or getwd().
Many early versions of Unix and Plan 9 did not have this command, and documentation and books used sed instead: sed 5q filename. The example prints every line (implicit) and quits after the fifth. Equivalently, awk may be used to print the first five lines in a file: awk 'NR < 6' filename
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echo began within Multics.After it was programmed in C by Doug McIlroy as a "finger exercise" and proved to be useful, it became part of Version 2 Unix. echo -n in Version 7 replaced prompt, (which behaved like echo but without terminating its output with a line delimiter).