What does “if [ -t 1 ]” do in shell scripting?

问题

I have code for setting the zsh as default shell:

if [ -t 1 ]; then 
exec zsh 
fi

What exactly does the command if [ -t 1 ] do here?

回答1:

if command; then other_command; fi runs command and then, if that command exits with a return code of 0 ("success"), runs other_command.

The command [...] is designed to take the place of the Boolean expressions that you find in traditional programming languages. It has a number of options for what goes between the brackets and exits with 0=success if those options evaluate to a true value.

The specific subcommand -t tests a file descriptor to see if it is attached to a terminal. The file descriptor 1 is where the script's output is going (aka "standard output" or "stdout" for short). So -t 1 is true and [ -t 1 ] returns success if and only if the output of the script is going to a terminal (instead of into a file or pipe or something).

In that case, the current shell is replaced (via exec) by a copy of zsh. Which will hopefully not run the same script, since zsh works the same way and will make the same decision and go into an infinite loop execing itself.

回答2:

I have code for setting the zsh as default shell:

No, you haven't. That's not what your code does, though it produces a similar effect.

if [ -t 1 ]; then 
exec zsh 
fi

What exactly does the command if [ -t 1 ] do here?

The command [ -t 1 ] is executed. If it exits with status 0 (indicating success, which for the [ command means the condition evaluates to true) then the commands in the body of the if statement are executed.

It may be a bit surprising that [ is a command, rather than part of shell syntax, but that's the case. Your if could be rewritten equivalently to use the test command instead:

if test -t 1; then
  # ...

The other key thing, then, is the -t 1 part. You can find out about that in the manual for the test or [ command, but to save you the trouble, it is a conditional expression that evaluates whether file descriptor 1 (the shell's standard output) is connected to a terminal. This is similar to, but not exactly the same thing as, evaluating whether the shell is an interactive one.

Overall, the code presented has the effect of replacing the current (presumably bash) shell with zsh if the standard output is connected to a terminal. This is indirect, and a bit tricky; it would probably be better to genuinely set your login shell to /bin/zsh (or wherever it is installed) via the chsh command.

来源：https://stackoverflow.com/questions/47417031/what-does-if-t-1-do-in-shell-scripting