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expr is a command line utility on Unix and Unix-like operating systems which evaluates an expression and outputs the corresponding value. It first appeared in Unix v7.The command is available for Microsoft Windows as part of the UnxUtils collection [1] of native Win32 ports of common GNU Unix-like utilities. [2]
The less-than sign with the equals sign, <=, may be used for an approximation of the less-than-or-equal-to sign, ≤. ASCII does not have a less-than-or-equal-to sign, but Unicode defines it at code point U+2264. In BASIC, Lisp-family languages, and C-family languages (including Java and C++), operator <= means "less than
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).
The shells bash, zsh and PowerShell offer this as a specific feature. [72] [73] Shells which do not offer this as a specific feature may still be able to turn off echoing through some other means. Shells executing on a Unix/Linux operating system can use the stty external command to switch off/on echoing of input characters. [74]
While Bash was developed for UNIX and UNIX-like operating systems, such as GNU/Linux, it is also available on Android, macOS, Windows, and numerous other current and historical operating systems. [12] "Although there have been attempts to create specialized shells, the Bourne shell derivatives continue to be the primary shells in use."
In this way, a series of commands can be "piped" together, giving users the ability to quickly perform complex multi-stage processing from the command line or as part of a Unix shell script ("bash file"). In most Unix shells (command interpreters), this is represented by the vertical bar character. For example:
On these computers, users can access a Unix-like command-line interface by running the terminal emulator program called Terminal, which is found in the Utilities sub-folder of the Applications folder, or by remotely logging into the machine using ssh. Z shell is the default shell for macOS; Bash, tcsh, and the KornShell are also
In the lower right we can see a terminal emulator running a Unix shell, in which the user can type commands as if they were sitting at a terminal. In computing , a shell is a computer program that exposes an operating system 's services to a human user or other programs.