Meaning of dollar question mark in shell scripts

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What does   echo      mean in shell programming

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From http   www gnu org s bash manual bash html Special-Parameters    Expands to the exit status of the most recently executed foreground pipeline

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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

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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

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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

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Outputs the result of the last executed unix command  0 implies true 1 implies false

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It has the last status code  exit value  of a command

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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  quot   quot   Some similar special parameters in BASH are 1 2       Normally seen in echo command as  1   2             etc

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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      Update Based on the comment  Ideally  you should not use the above code block for comparison  refer to  tripleee comments and explanation

[shell] Meaning of $? (dollar question mark) in shell scripts

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