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The test command in Unix evaluates the expression parameter. In most recent shell implementations, it is a shell builtin, even though the external version still exists.In the second form of the command, the [ ] (brackets) must be surrounded by blank spaces (this is because [is a program and POSIX compatible shells require a space between the program name and its arguments).
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 ...
In Unix-like operating systems, find is a command-line utility that locates files based on some user-specified criteria and either prints the pathname of each matched object or, if another action is requested, performs that action on each matched object.
A valid file URI must therefore begin with either file:/path (no hostname), file:///path (empty hostname), or file://hostname/path. file://path (i.e. two slashes, without a hostname) is never correct, but is often used. Further slashes in path separate directory names in a hierarchical system of directories and subdirectories. In this usage ...
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.) dircolors: Set up color for ls: install: Copies files and set attributes ln: Creates a link to a ...
O_APPEND data written will be appended to the end of the file. The file operations will always adjust the position pointer to the end of the file. 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.
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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.