|, || and && has priority over &

[andry81]

(call;) && echo 123 & echo 456

Result:

123
456

(call;) || echo 123 & echo 456

Result:

456

(call) && echo 123 & echo 456

Result:

456

(call) || echo 123 & echo 456

Result:

123
456

Right expression consumes the output:

(echo 1) | (echo 2)

Result:

2

(echo 1) & call; | (echo 2)

Result:

1
2

( (echo 1) & call; ) | (echo 2)

Result:

2

[A-Mofi]

&& and || are conditional command operators while & is an unconditional command operator. That means the command or command block after && is executed only if the former executed command exits with exit code 0 while the command or command block after || is executed only if the former executed command exits with any other exit code than 0 whereby cmd.exe evaluates the current value of its dynamic variable ERRORLEVEL on evaluation of && and ||. The command or command block after & is always executed independent on the exit code of the former executed command.

In first use case on execution of the command line (call;) && echo 123 & echo 456 is executed first the command call successfully with exit code 0. That results in an execution of echo 123 which does not modify the value of the dynamic variable ERRORLEVEL of the Windows Command Processor. Independent on the executions of call and next echo with text 123 is executed next echo with text 456 because of the unconditional command operator &.

In second use case on execution of the command line (call;) || echo 123 & echo 456 is executed first the command call successfully with exit code 0. That results in skipping the execution of echo 123. Independent on the execution of call is executed next echo with text 456. That echo 123 was not executed at all here does not matter for cmd.exe.

The execution behavior for the second example would be different on using a command block with the command line (call;) || (echo 123 & echo 456). Both echo commands inside the command block beginning with ( and ending with matching ) are executed only if call would exit with an exit code not equal 0.

It should be clear now why echo 456 is always executed with the command lines 3 and 4.

| is a redirection operator and not a command operator instructing cmd.exe to pass the standard output stream (stdout) of one command to the standard input stream (stdin) of another command. The example (echo 1) | (echo 2) is a bit curious because of echo is an internal command of cmd.exe and not an executable which additionally does not support reading data from stdin. cmd.exe processing the command line runs in this very special case in background %ComSpec% /S /D /c" ( echo 1 )" and redirects the output 1 to its internal command echo which outputs 2 and ignores 1 from standard input stream.

The redirection operator | should be used only with executables or cmd internal commands supporting reading data from stdin handle. cmd.exe starts in case of | always first the executable (without or with arguments) which should get the standard output stream of another command via the standard input stream and then runs the cmd internal command or the executable which outputs something to its standard output stream.

The Windows Command Processor makes a fantastic job on a command line with multiple redirection operators of <, >, >>, | in finding out which file to open first and which next in which mode (read, write with creation new, write with append on existing) and which executable to run first and which next to get the input and output passed correct from one command/executable/file to the next command/executable/file.

See also on Stack Overflow topics:

• How does the Windows Command Interpreter (CMD.EXE) parse scripts?
• Single line with multiple commands using Windows batch file

PS: The Linux/Mac shell interpreters and PowerShell support also the conditional command operators && and || and an unconditional command operator which is ; for the shell interpreters instead of &. The operators && and || are called pipeline chain operators in the PowerShell documentation about_Operators.