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??

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2 Answers

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

2

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.