time command1 | command2

Does time apply to command1 or command1 | command2? If your answer is one of the two, how would you specify the other?

Could you explain your answer based on the grammar of bash or how the shell interprets the command?

When the shell parses the command, does it recognize time as a reserved word before recognizing | as a control operator? Does the order between recognizing time as a reserved word and recognizing | as a control operator determines whether time applies to command1 or command1 | command2?

Thanks.

time command1 | command2

Does time apply to command1 or command1 | command2? If your answer is one of the two, how would you specify the other?

In Bash:

• 
  time is a reserved word


• It applies to pipelines

In POSIX:

• 
  time is an external utility


• It applies to simple commands

In Bash and other shells with a time reserved word, time applies to the entire pipeline (command1 | command2). In shells without a time reserved word, time applies to command1.

However, in Bash you can force time to apply to command1 running command time command1 | command2 (this one will run the external utility if it's available), time command1; | command2 (this one is POSIX-compliant) or time command1 | command2 (or any other variant where time is quoted, this will also run the external utility; thanks ilkkachu!).

Q: Could you explain your answer based on the grammar of bash or how the shell interprets the command?

Bash's manual explains this in the 3.2.2 Pipelines chapter:

The format for a pipeline is

[time [-p]] [!] command1 [ | or |& command2 ] …

The reserved word time causes timing statistics to be printed for the pipeline once it finishes.

And POSIX in time's manual:

time [-p] utility [argument...]

The time utility shall invoke the utility named by the utility operand with arguments supplied as the argument operands and write a message to standard error that lists timing statistics for the utility.

Q: When the shell parses the command, does it recognize time as a reserved word before recognizing | as a control operator?

Bash's manual says:

The following is a brief description of the shell’s operation when it reads and executes a command. Basically, the shell does the following:

1. Reads its input from a file (see Shell Scripts), from a string supplied as an argument to the -c invocation option (see Invoking Bash), or from the user’s terminal.




2. Breaks the input into words and operators, obeying the quoting rules described in Quoting. These tokens are separated by metacharacters. Alias expansion is performed by this step (see Aliases).




3. Parses the tokens into simple and compound commands (see Shell Commands).




4. Performs the various shell expansions (see Shell Expansions), breaking the expanded tokens into lists of filenames (see Filename Expansion) and commands and arguments.




5. Performs any necessary redirections (see Redirections) and removes the redirection operators and their operands from the argument list.




6. Executes the command (see Executing Commands).




7. Optionally waits for the command to complete and collects its exit status (see Exit Status).

So, as far as I understand, reserved words and operators are recognized as such basically at the same time (2nd step).

Q: Does the order between recognizing time as a reserved word and recognizing | as a control operator determines whether time applies to command1 or command1 | command2?

The Bash's time reserved word applies to pipelines and the POSIX's time external utility applies to simple commands, it's not a matter of recognition order.