What is the dollar question mark variable in shell scripting

Question

I m trying to learn shell scripting  and I need to understand someone else s code  What is the    variable hold  I can t Google search the answer because they block punctuation characters

User · Answer

That is the exit status of the last executed function program command  Refer to    exit   exit status   tldp org Special Shell Variables   tldp org Special Characters   tlpd org

User · Answer

is the exit status of a command  such that you can daisy-chain a series of commands   Example  command1  amp  amp  command2  amp  amp  command3   command2 will run if command1 s    yields a success  0  and command3 will execute if    of command2 will yield a success

User · Answer

The exit code of the last command ran

User · Answer

Minimal POSIX C exit status example To understand     you must first understand the concept of process exit status which is defined by POSIX  In Linux   when a process calls the exit system call  the kernel stores the value passed to the system call  an int  even after the process dies  The exit system call is called by the exit   ANSI C function  and indirectly when you do return from main   the process that called the exiting child process  Bash   often with fork   exec  can retrieve the exit status of the child with the wait system call   Consider the Bash code    false   echo    1  The C  quot equivalent quot  is  false c  include  lt stdlib h gt     exit     int main void        exit 1      bash c  include  lt unistd h gt     execl     include  lt stdlib h gt     fork     include  lt sys wait h gt     wait  WEXITSTATUS     include  lt stdio h gt     printf     int main void        if  fork      0               Call false             execl  quot   false quot    quot   false quot    char   NULL             int status         Wait for a child to finish         wait  amp status          Status encodes multiple fields         we need WEXITSTATUS to get the exit status         http   stackoverflow com questions 3659616 returning-exit-code-from-child              printf  quot       d n quot   WEXITSTATUS status       Compile and run  g   -ggdb3 -O0 -std c  11 -Wall -Wextra -pedantic -o bash bash c g   -ggdb3 -O0 -std c  11 -Wall -Wextra -pedantic -o false false c   bash  Output       1  In Bash  when you hit enter  a fork   exec   wait happens like above  and bash then sets    to the exit status of the forked process  Note  for built-in commands like echo  a process need not be spawned  and Bash just sets    to 0 to simulate an external process  Standards and documentation POSIX 7 2 5 2  quot Special Parameters quot  http   pubs opengroup org onlinepubs 9699919799 utilities V3 chap02 html tag 18 05 02      Expands to the decimal exit status of the most recent pipeline  see Pipelines    man bash  quot Special Parameters quot    The shell treats several parameters specially   These parameters may only be referenced  assignment to them is not allowed          Expands to the exit status of the most recently executed foreground pipeline   ANSI C and POSIX then recommend that   0 means the program was successful  other values  the program failed somehow  The exact value could indicate the type of failure  ANSI C does not define the meaning of any vaues  and POSIX specifies  values larger than 125   What is the meaning of  quot POSIX quot     Bash uses exit status for if In Bash  we often use the exit status    implicitly to control if statements as in  if true  then     fi  where true is a program that just returns 0  The above is equivalent to  true result    if    result   0    then     fi  And in  if   1   1    then     fi    is just an program with a weird name  and Bash built-in that behaves like it   and 1   1   its arguments  see also  Difference between single and double square brackets in Bash

User · Answer

It is the returned error code of the last executed command  0   success

User · Answer

It is well suited for debugging in case your script exit if set -e is used  For example  put  echo    after the command that cause it to exit and see the returned error value

User · Answer

A return value of the previously executed process      10 4 Getting the return value of a program      In bash  the return value of a program is stored in a special variable   called          This illustrates how to capture the return value of a program  I   assume that the directory dada does not exist   This was also   suggested by mike              bin bash         cd  dada  amp  gt   dev null         echo rv             cd   pwd   amp  gt   dev null         echo rv        See Bash Programming Manual for more details

User · Answer

is used to find the return value of the last executed command  Try the following in the shell   ls somefile echo      If somefile exists  regardless whether it is a file or directory   you will get the return value thrown by the ls command  which should be 0  default  success  return value   If it doesn t exist  you should get a number other then 0  The exact number depends on the program   For many programs you can find the numbers and their meaning in the corresponding man page  These will usually be described as  exit status  and may have their own section

User · Answer

is the result  exit code  of the last executed command

[shell] What is the $? (dollar question mark) variable in shell scripting?

Examples related to shell

Examples related to scripting