nop

Pretty-print one or more graphs in canonical format

$ nop [path/to/input1.gv path/to/input2.gv ...] > [path/to/output.gv]

$ nop -p [path/to/input1.gv path/to/input2.gv ...]

$ nop -?

true [options]
: [arguments]

None
    The true command and the : (colon) command (conceptual equivalents of nop) take no operational parameters or arguments that alter their behavior. They always succeed and perform no actions.
Note: Standard options like --help or --version might be supported by the specific shell built-in implementation, but are not unique to its 'no-operation' function.

The term "nop" (No OPeration) refers to an instruction or command that does nothing, literally performing a null operation.
Unlike many Linux commands, there is typically no standalone executable file named `/bin/nop`. Instead, the functionality of a no-operation is embodied by highly optimized shell built-in commands or specific CPU instructions.
In Unix-like shell environments, the most common commands that serve as "nop" equivalents are `true` and `:` (the colon command).

The `true` command always exits with a status of 0 (indicating success) and performs no action. It is extremely lightweight and fast.
The `:` command is also a null command, a shell built-in that takes arguments but ignores them, always exiting with a status of 0. It is often used as a placeholder or a very efficient way to ensure a successful exit status.

The primary utility of a no-operation command lies in scripting, where it can be used as a placeholder, to create infinite loops (`while true; do ...; done`), or to ensure a successful command chain without performing any side effects.

There is no standard standalone executable program named `nop` on most Linux systems. Its functionality is provided by shell built-ins (`true`, `:`) or is a concept from CPU instruction sets.
While `true` and `:` do 'nothing', they are still commands that the shell must parse and execute, consuming a negligible amount of CPU time and memory. They are highly optimized for this purpose and are almost instantaneous.

No-operation commands like `true` or `:` are extensively used in shell scripting for various purposes:

1. Placeholders: To reserve a spot for future code without causing syntax errors.
Example: `if condition; then
:
else
# do something
fi`

2. Infinite Loops: `true` is commonly used to create loops that run indefinitely.
Example: `while true; do
echo 'Looping forever... Press Ctrl+C to exit.'
sleep 1
done`

3. Consuming Piped Input: While not their primary role, they can be used to consume input from a pipe, effectively discarding it.
Example: `cat /etc/passwd | :`

4. Ensuring Success: To ensure a command chain always exits successfully, especially when a command might fail but its failure shouldn't stop the script.
Example: `unzip archive.zip || true` (attempts unzip, but if it fails, `true` ensures the command exits with 0).

5. Conditional Expressions: Used in `if` statements or `&&` / `||` constructs.
Example: `[ "$DEBUG" = "true" ] && : || echo "Not in debug mode"` (if DEBUG is true, do nothing, otherwise print message).

At the lowest level of computing, a CPU NOP instruction is a single machine language instruction that performs no action other than advancing the program counter. It's crucial for:

1. Padding: Aligning code for optimal performance or patching programs without re-compilation.
2. Timing: Introducing delays in real-time systems (though less common now with dedicated timers).
3. Stubs: Temporarily replacing or disabling specific code sections during development or debugging.
4. Security: In some exploits (e.g., NOP sleds), a sequence of NOPs can be used to reliably jump to malicious code.

The concept of a 'No OPeration' instruction originates from low-level computer architecture and assembly language programming (e.g., x86 `NOP` instruction). It's a CPU instruction that tells the processor to do nothing for one clock cycle, often used for timing, memory alignment, or as a placeholder.

In the context of Unix-like operating systems and shell scripting, this concept evolved into commands like `true` and `:`. These commands serve the same high-level purpose of performing no meaningful action while still providing a successful exit status, making them invaluable for flow control and scripting logic without side effects. They have been fundamental built-ins in various shells (like sh, bash, zsh) since early Unix versions.

true(1), false(1), bash(1), sh(1)