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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).
Change the file ownership PDP-7 UNIX cksum: Filesystem Mandatory Write file checksums and sizes 4.4BSD cmp: Filesystem Mandatory Compare two files; see also diff Version 1 AT&T UNIX comm: Text processing Mandatory Select or reject lines common to two files Version 4 AT&T UNIX command: Shell programming Mandatory Execute a simple command ...
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 ...
A rule of thumb in determining if a reply fits into the 4xx or the 5xx (Permanent Negative) category is that replies are 4xx if the commands can be repeated without any change in command form or in properties of the User or Server (e.g., the command is spelled the same with the same arguments used; the user does not change his file access or ...
The Single UNIX Specification (SUS) specifies that a series of tests are performed on the file specified on the command line: if the file cannot be read, or its Unix file type is undetermined, the file program will indicate that the file was processed but its type was undetermined. file must be able to determine the types directory, FIFO ...
TOCTOU race conditions are common in Unix between operations on the file system, [1] but can occur in other contexts, including local sockets and improper use of database transactions. In the early 1990s, the mail utility of BSD 4.3 UNIX had an exploitable race condition for temporary files because it used the mktemp() [2] function. [3]
In Unix-like operating systems, true and false are commands whose only function is to always return with a predetermined exit status.Programmers and scripts often use the exit status of a command to assess success (exit status zero) or failure (non-zero) of the command.
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 ...