Bash Subshell – All Ways to Create

bashsubshell

There are 2 main ways that I know of so far:

Explicitly: wrapping parentheses around a list of commands
Implicitly: every command in a pipeline

Are there more ways, either explicitly or implicitly, in which one creates subshells in bash?

Best Answer

From man bash:

If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish, and the return status is 0.
A coprocess is a shell command preceded by the coproc reserved word.
A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the & control operator
Shell builtin complete command: when called with the -C command option, command is executed in a subshell environment, and its output is used as the possible completions.
Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment

Related Solutions

Bash – Rule for Invoking Subshell

The parentheses always start a subshell. What's happening is that bash detects that sleep 5 is the last command executed by that subshell, so it calls exec instead of fork+exec. The sleep command replaces the subshell in the same process.

In other words, the base case is:

( … ) create a subshell. The original process calls fork and wait. In the subprocess, which is a subshell:
1. sleep is an external command which requires a subprocess of the subprocess. The subshell calls fork and wait. In the subsubprocess:
  1. The subsubprocess executes the external command → exec.
  2. Eventually the command terminates → exit.
2. wait completes in the subshell.
wait completes in the original process.

The optimization is:

( … ) create a subshell. The original process calls fork and wait. In the subprocess, which is a subshell until it calls exec:
1. sleep is an external command, and it's the last thing this process needs to do.
2. The subprocess executes the external command → exec.
3. Eventually the command terminates → exit.
wait completes in the original process.

When you add something else after the call the sleep, the subshell needs to be kept around, so this optimization can't happen.

When you add something else before the call to sleep, the optimization could be made (and ksh does it), but bash doesn't do it (it's very conservative with this optimization).

Bash – How many different ways can you create an “alias” in Bash

Be careful when using the which command. Better to use the type ... command.

$ type ln
ln is /bin/ln

You can also use the whereis command:

$ whereis ln
ln: /bin/ln /usr/share/man/man1p/ln.1p.gz /usr/share/man/man1/ln.1.gz

Best Answer

Related Solutions

Bash – Rule for Invoking Subshell

Bash – How many different ways can you create an “alias” in Bash

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