Meaning of $? in shell scripts

问题:

What does

echo $?

mean in bash programming?

回答1:

This is the exit status of the last executed command.

For example the command true always returns a status of 0 and false always returns a status of 1:

true
echo $? # echoes 0
false
echo $? # echoes 1

From the manual: (acessible by calling man bash in your shell)

$?       Expands to the exit status of the most recently executed foreground pipeline.

By convention an exit status of 0 means success, and non-zero return status means failure. Learn more about exit statuses on wikipedia.

There are other special variables like this, as you can see on this online manual: https://www.gnu.org/s/bash/manual/bash.html#Special-Parameters

回答2:

$? returns the exit value of the last executed command. echo $? prints that value on console. zero implies a successful execution while non-zero values are mapped to various reason for failure.

Hence when scripting; I tend to use the following syntax

if [ $? -eq 0 ]; then
 # do something
else
 # do something else
fi

The comparison is to be done on equals to 0 or not equals 0.

回答3:

It has the last status code (exit value) of a command.

回答4:

echo $? - Gives the EXIT STATUS of the most recently executed command . This EXIT STATUS would most probably be a number with ZERO implying Success and any NON-ZERO value indicating Failure

? - This is one special parameter/variable in bash.

$? - It gives the value stored in the variable "?".

Some similar special parameters in BASH are 1,2,*,# ( Normally seen in echo command as $1 ,$2 , $* , $# , etc., ) .

Hope this helps....

回答5:

From http://www.gnu.org/s/bash/manual/bash.html#Special-Parameters

?
Expands to the exit status of the most recently executed foreground pipeline. 

回答6:

Outputs the result of the last executed unix command

0 implies true
1 implies false

回答7:

See The Bash Manual under 3.4.2 Special Parameters:

? - Expands to the exit status of the most recently executed foreground pipeline.

It is a little hard to find because it is not listed as $? (the variable name is "just" ?). Also see the exit status section, of course ;-)

Happy coding.