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An existence check before reading a file can catch and/or prevent a fatal error, for instance. For that reason, most programming language libraries contain a means of checking whether a file exists. An existence check can sometimes involve a " brute force " approach of checking all records for a given identifier, as in this Microsoft Excel ...
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).
Changes file security context chgrp: Changes file group ownership chown: Changes file ownership chmod: Changes the permissions of a file or directory cp: Copies a file or directory dd: Copies and converts a file df: Shows disk free space on file systems dir: Is exactly like "ls -C -b". (Files are by default listed in columns and sorted vertically.)
(Hence the name, ECHO) Usually, this means directly to the screen, but the output of echo can be redirected, like any other command, to files or devices. Often used in batch files to print text out to the user. Another important use of the echo command is to toggle echoing of commands on and off in batch files.
O_CREAT Create the file if it does not exist; otherwise the open fails setting errno to ENOENT. O_EXCL Used with O_CREAT if the file already exists, then fail, setting errno to EEXIST. O_TRUNC If the file already exists then discard its previous contents, reducing it to an empty file. Not applicable for a device or named pipe.
File verification is the process of using an algorithm for verifying the integrity of a computer file, usually by checksum.This can be done by comparing two files bit-by-bit, but requires two copies of the same file, and may miss systematic corruptions which might occur to both files.
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>& file means both stdout and stderr will be written to file, overwriting it if it exists, and creating it if it doesn't. >> file means stdout will be appended at the end of file. >>& file means both stdout and stderr will be appended at the end of file. < file means stdin will be read from file. << string is a here document.