grep -q command option and the result [duplicate]

echo red | grep -q red
echo $?

results in

0

grep -q turns off writing to standard output. The $? exit status is 0 if a match is found, otherwise not 0. This is pretty much the explanation for the code above. I want to know more about $?. What is it??

From the Special Parameters section of man bash:

   Special Parameters
       The  shell  treats  several parameters specially.  These parameters may
       only be referenced; assignment to them is not allowed.
       *      Expands to the positional parameters, starting from  one.   When
              the  expansion  is  not  within  double  quotes, each positional
              parameter expands to a separate word.  In contexts where  it  is
              performed, those words are subject to further word splitting and
              pathname expansion.  When the  expansion  occurs  within  double
              quotes,  it  expands  to  a  single  word with the value of each
              parameter separated by the first character of  the  IFS  special
              variable.   That  is, "$*" is equivalent to "$1c$2c...", where c
              is the first character of the value of the IFS variable.  If IFS
              is  unset,  the  parameters  are separated by spaces.  If IFS is
              null, the parameters are joined without intervening separators.
       @      Expands to the positional parameters, starting from  one.   When
              the  expansion  occurs  within  double  quotes,  each  parameter
              expands to a separate word.  That is, "$@" is equivalent to "$1"
              "$2"  ...   If the double-quoted expansion occurs within a word,
              the expansion of the first parameter is joined with  the  begin‐
              ning  part  of  the original word, and the expansion of the last
              parameter is joined with the last part  of  the  original  word.
              When  there  are no positional parameters, "$@" and $@ expand to
              nothing (i.e., they are removed).
       #      Expands to the number of positional parameters in decimal.
       ?      Expands to the exit status of the most recently  executed  fore‐
              ground pipeline.
       -      Expands  to  the  current option flags as specified upon invoca‐
              tion, by the set builtin command, or  those  set  by  the  shell
              itself (such as the -i option).
       $      Expands  to  the  process ID of the shell.  In a () subshell, it
              expands to the process ID of the current  shell,  not  the  sub‐
              shell.
       !      Expands  to  the process ID of the job most recently placed into
              the background, whether executed as an asynchronous  command  or
              using the bg builtin (see JOB CONTROL below).
       0      Expands  to  the name of the shell or shell script.  This is set
              at shell initialization.  If bash is invoked with a file of com‐
              mands,  $0  is set to the name of that file.  If bash is started
              with the -c option, then $0 is set to the first  argument  after
              the  string to be executed, if one is present.  Otherwise, it is
              set to the filename used to invoke bash, as  given  by  argument
              zero.
       _      At  shell  startup,  set to the absolute pathname used to invoke
              the shell or shell script being executed as passed in the  envi‐
              ronment  or  argument  list.   Subsequently, expands to the last
              argument to the previous command, after expansion.  Also set  to
              the  full  pathname  used  to  invoke  each command executed and
              placed in the environment exported to that command.  When check‐
              ing  mail,  this  parameter holds the name of the mail file cur‐
              rently being checked.

Note that "the most recently executed foreground pipeline" is not always the last command (if an intermediate command fails, for example); in bash, you can access the exit status of each command separately in a pipe via the PIPESTATUS array if you need finer-grained exit status information:

   PIPESTATUS
          An  array  variable (see Arrays below) containing a list of exit
          status values from the processes in  the  most-recently-executed
          foreground pipeline (which may contain only a single command).

Ex.

$ echo 'foo bar' | grep 'foo' | sed 's/f/g/'
goo bar
$ echo "${PIPESTATUS[*]}"
0 0 0
$ echo 'foo bar' | grep 'goo' | sed 's/g/f/'
$ echo "${PIPESTATUS[*]}"
0 1 0

The $? variable in bash stores the exit code of the last executed command. In your example, this would mean that echo red | grep -q red exited with code 0, which is almost always a sign of the command succeeding.

You can test this out with various other commands to see their return codes. For example,

commandthatdoesntexist; echo $?

will return error code 127, and so on.