What does “$?” give us exactly in a shell script?

问题:

This question already has an answer here:

• Meaning of $? in shell scripts 7 answers

I saw the code written somewhere online, and I wanted to know what exactly does "$?" do/give us. Googling did not help.

Here's the code I saw it in:

#!/bin/sh

ping -c 2 localhost
if [ $? != 0 ] ; then
    echo "Couldn't ping localhost, weird"
    fi

ping -c 2 veryweirdhostname.noend 
if [ $? != 0 ] ; then
    echo "Surprise, Couldn't ping a very weird hostname.."
    fi

echo "The pid of this process is $$"

Taken from: http://efod.se/writings/linuxbook/html/shell-scripts.html

回答1:

$? is a variable holding the return value of the last command you ran.

Example C program (example.c):

int main() { return 1; }

Example Bash:

gcc -o example example.c
./example
echo $? # prints 1

回答2:

Most of the answers are missing a bit of detail. A definitive answer is found in the POSIX standard for the shell, in the section on special parameters:

$? Expands to the decimal exit status of the most recent pipeline (see Pipelines ).

Don't be surprised by the word pipeline, because even a simple command such as ls is grammatically a pipeline consisting of a single command. But then, what is $? for a multi-command pipeline? It's the exit status of the last command in the pipeline.

And what about pipelines executing in the background, like grep foo bigfile|head -n 10 > result &?

Their exit status can be retrieved through wait once the pipeline's last command has finished. The background process pid is available as $!, and $? only reports whether the background command was correctly started.

Another detail worth mentioning is that the exit status is usually in the range 0 through 255, with 128 to 255 indicating the process exited due to a signal. Returning other values from a C program is likely to not be reflected accurately in $?.

回答3:

It's the return code from the most recently executed command.

By convention 0 is a successful exit and non-zero indicates some kind of error.

回答4:

This special variable shows the exit status of the last command that was run in a script or command-line. For example, in a command-line, the user could type

 who; echo $?

The output would then be

 user  tty7         2014-07-13 19:47
 0

This shows the output of who and the exit status of the command. A script would be the same.

 #!/bin/bash
 who
 echo $?

Output: 0

回答5:

the other answers cover bash pretty well, but you don't specify a shell in your question. In csh (and tcsh) $? can be used to query the existence of variables, e.g.

if $?my_var then
    echo my_var exists
endif