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The true command is sometimes substituted with the very similar null command, [1] written as a single colon (:). The null command is built into the shell, and may therefore be more efficient if true is an external program (true is usually a shell built in function). We can rewrite the upper example using : instead of true:
A shell script is a computer program ... the Bourne shell, and GNU Bash. While a Unix operating system ... but the reverse may not be true since BASH has extensions ...
The condition/expression is evaluated, and if the condition/expression is true, [1] the code within all of their following in the block is executed. This repeats until the condition/expression becomes false. Because the while loop checks the condition/expression before the block is executed, the control structure is often also known as a pre ...
Bash can execute the vast majority of Bourne shell scripts without modification, with the exception of Bourne shell scripts stumbling into fringe syntax behavior interpreted differently in Bash or attempting to run a system command matching a newer Bash builtin, etc. Bash command syntax includes ideas drawn from the Korn Shell (ksh) and the C ...
tcsh and sh shell windows on a Mac OS X Leopard [1] desktop. A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system to control the execution of the system using ...
If a script is invoked as an argument to the interpreting shell, it will be executed regardless of whether the user holds the execute permission for that script. Although Windows also specifies an execute permission, none of the Windows-specific shells block script execution if the permission has not been granted.
The steps the shell performs are: Create a new anonymous pipe. This pipe will be accessible with something like /dev/fd/63; you can see it with a command like echo <(true). Execute the substituted command in the background (sort file2 in this case), piping its output to the anonymous pipe.
If xxx1 is omitted, we get a loop with the test at the top (a traditional while loop). If xxx2 is omitted, we get a loop with the test at the bottom, equivalent to a do while loop in many languages. If while is omitted, we get an infinite loop. The construction here can be thought of as a do loop with the while check in the middle. Hence this ...