Is opening a file faster than reading variable content?












28















In a bash script I need various values from /proc/ files. Until now I have dozens of lines grepping the files directly like that:



grep -oP '^MemFree: *K[0-9]+' /proc/meminfo


In an effort to make that more efficient I saved the file content in a variable and grepped that:



a=$(</proc/meminfo)
echo "$a" | grep -oP '^MemFree: *K[0-9]+'


Instead of opening the file multiple times this should just open it once and grep the variable content, which I assumed would be faster – but in fact it is slower:





bash 4.4.19 $ time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null
real 0m0.803s
user 0m0.619s
sys 0m0.232s
bash 4.4.19 $ a=$(</proc/meminfo)
bash 4.4.19 $ time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null
real 0m1.182s
user 0m1.425s
sys 0m0.506s


The same is true for dash and zsh. I suspected the special state of /proc/ files as a reason, but when I copy the content of /proc/meminfo to a regular file and use that the results are the same:



bash 4.4.19 $ cat </proc/meminfo >meminfo
bash 4.4.19 $ time for i in $(seq 1 1000);do grep ^MemFree meminfo; done >/dev/null
real 0m0.790s
user 0m0.608s
sys 0m0.227s


Using a here string to save the pipe makes it slightly faster, but still not as fast as with the files:



bash 4.4.19 $ time for i in $(seq 1 1000);do <<<"$a" grep ^MemFree; done >/dev/null
real 0m0.977s
user 0m0.758s
sys 0m0.268s


Why is opening a file faster than reading the same content from a variable?










share|improve this question





























    28















    In a bash script I need various values from /proc/ files. Until now I have dozens of lines grepping the files directly like that:



    grep -oP '^MemFree: *K[0-9]+' /proc/meminfo


    In an effort to make that more efficient I saved the file content in a variable and grepped that:



    a=$(</proc/meminfo)
    echo "$a" | grep -oP '^MemFree: *K[0-9]+'


    Instead of opening the file multiple times this should just open it once and grep the variable content, which I assumed would be faster – but in fact it is slower:





    bash 4.4.19 $ time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null
    real 0m0.803s
    user 0m0.619s
    sys 0m0.232s
    bash 4.4.19 $ a=$(</proc/meminfo)
    bash 4.4.19 $ time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null
    real 0m1.182s
    user 0m1.425s
    sys 0m0.506s


    The same is true for dash and zsh. I suspected the special state of /proc/ files as a reason, but when I copy the content of /proc/meminfo to a regular file and use that the results are the same:



    bash 4.4.19 $ cat </proc/meminfo >meminfo
    bash 4.4.19 $ time for i in $(seq 1 1000);do grep ^MemFree meminfo; done >/dev/null
    real 0m0.790s
    user 0m0.608s
    sys 0m0.227s


    Using a here string to save the pipe makes it slightly faster, but still not as fast as with the files:



    bash 4.4.19 $ time for i in $(seq 1 1000);do <<<"$a" grep ^MemFree; done >/dev/null
    real 0m0.977s
    user 0m0.758s
    sys 0m0.268s


    Why is opening a file faster than reading the same content from a variable?










    share|improve this question



























      28












      28








      28


      2






      In a bash script I need various values from /proc/ files. Until now I have dozens of lines grepping the files directly like that:



      grep -oP '^MemFree: *K[0-9]+' /proc/meminfo


      In an effort to make that more efficient I saved the file content in a variable and grepped that:



      a=$(</proc/meminfo)
      echo "$a" | grep -oP '^MemFree: *K[0-9]+'


      Instead of opening the file multiple times this should just open it once and grep the variable content, which I assumed would be faster – but in fact it is slower:





      bash 4.4.19 $ time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null
      real 0m0.803s
      user 0m0.619s
      sys 0m0.232s
      bash 4.4.19 $ a=$(</proc/meminfo)
      bash 4.4.19 $ time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null
      real 0m1.182s
      user 0m1.425s
      sys 0m0.506s


      The same is true for dash and zsh. I suspected the special state of /proc/ files as a reason, but when I copy the content of /proc/meminfo to a regular file and use that the results are the same:



      bash 4.4.19 $ cat </proc/meminfo >meminfo
      bash 4.4.19 $ time for i in $(seq 1 1000);do grep ^MemFree meminfo; done >/dev/null
      real 0m0.790s
      user 0m0.608s
      sys 0m0.227s


      Using a here string to save the pipe makes it slightly faster, but still not as fast as with the files:



      bash 4.4.19 $ time for i in $(seq 1 1000);do <<<"$a" grep ^MemFree; done >/dev/null
      real 0m0.977s
      user 0m0.758s
      sys 0m0.268s


      Why is opening a file faster than reading the same content from a variable?










      share|improve this question
















      In a bash script I need various values from /proc/ files. Until now I have dozens of lines grepping the files directly like that:



      grep -oP '^MemFree: *K[0-9]+' /proc/meminfo


      In an effort to make that more efficient I saved the file content in a variable and grepped that:



      a=$(</proc/meminfo)
      echo "$a" | grep -oP '^MemFree: *K[0-9]+'


      Instead of opening the file multiple times this should just open it once and grep the variable content, which I assumed would be faster – but in fact it is slower:





      bash 4.4.19 $ time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null
      real 0m0.803s
      user 0m0.619s
      sys 0m0.232s
      bash 4.4.19 $ a=$(</proc/meminfo)
      bash 4.4.19 $ time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null
      real 0m1.182s
      user 0m1.425s
      sys 0m0.506s


      The same is true for dash and zsh. I suspected the special state of /proc/ files as a reason, but when I copy the content of /proc/meminfo to a regular file and use that the results are the same:



      bash 4.4.19 $ cat </proc/meminfo >meminfo
      bash 4.4.19 $ time for i in $(seq 1 1000);do grep ^MemFree meminfo; done >/dev/null
      real 0m0.790s
      user 0m0.608s
      sys 0m0.227s


      Using a here string to save the pipe makes it slightly faster, but still not as fast as with the files:



      bash 4.4.19 $ time for i in $(seq 1 1000);do <<<"$a" grep ^MemFree; done >/dev/null
      real 0m0.977s
      user 0m0.758s
      sys 0m0.268s


      Why is opening a file faster than reading the same content from a variable?







      bash shell-script shell zsh variable






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 28 mins ago









      l0b0

      28.2k18119246




      28.2k18119246










      asked 17 hours ago









      dessertdessert

      1,218523




      1,218523






















          2 Answers
          2






          active

          oldest

          votes


















          32














          grep -oP '^MemFree: *K[0-9]+' /proc/meminfo forks a process that executes grep that opens /proc/meminfo (a virtual file, in memory, no disk I/O involved) reads it and matches the regexp.



          The most expensive part in that is forking the process and loading the grep utility and its library dependencies, doing the dynamic linking, open the locale database, dozens of files that are on disk (but likely cached in memory).



          The part about reading /proc/meminfo is insignificant in comparison, the kernel needs little time to generate the information in there and grep needs little time to read it.



          In:



          a=$(</proc/meminfo)


          In most shells that support that $(<...) ksh operator, the shell just opens the file and read its content (and strips the trailing newline characters). bash is different and much less efficient in that it forks a process to do that reading and passes the data to the parent via a pipe. But here, it's done once so it doesn't matter.



          In:



          printf '%sn' "$a" | grep '^MemFree'


          The shell needs to spawn two processes, which are running concurrently but interact between each other via a pipe. That pipe creation, tearing down, and writing and reading from it has some little cost. The much greater cost is the spawning of an extra process. The scheduling of the processes has some impact as well.



          You may find that using the zsh <<< operator makes it slightly quicker:



          grep '^MemFree' <<< "$a"


          In zsh and bash, that's done by writing the content of $a in a temporary file, that is less expensive than spawning an extra process, but will probably not give you any gain compared to getting the data straight off /proc/meminfo. That's still less efficient than your approach that copies /proc/meminfo on disk, as the writing of the temp file is done at each iteration.



          dash doesn't support here-strings, but its heredocs are implemented with a pipe that doesn't involve spawning an extra process. In:



           grep '^MemFree' << EOF
          $a
          EOF


          The shell creates a pipe, forks a process. The child executes grep with its stdin as the reading end of the pipe, and the parent writes the content at the other end of the pipe.



          But that pipe handling and process synchronisation is still likely to be more expensive than just getting the data straight off /proc/meminfo.



          The content of /proc/meminfo is short and takes not much time to produce. If you want to save some CPU cycles, you want to remove the expensive parts: forking processes and running external commands.



          Like:



          IFS= read -rd '' meminfo < /proc/meminfo
          memfree=${meminfo#*MemFree:}
          memfree=${memfree%%$'n'*}
          memfree=${memfree#"${memfree%%[! ]*}"}


          Avoid bash though whose pattern matching is very ineficient. With zsh -o extendedglob, you can shorten it to:



          memfree=${${"$(</proc/meminfo)"##*MemFree: #}%%$'n'*}


          Note that ^ is special in many shells (Bourne, fish, rc, es and zsh with the extendedglob option at least), I'd recommend quoting it. Also note that echo can't be used to output arbitrary data (hence my use of printf above).






          share|improve this answer





















          • 4





            In the case with printf you say the shell needs to spawn two processes, but isn't printf a shell builtin?

            – David Conrad
            11 hours ago






          • 5





            @DavidConrad It is, but most shells don't try to analyze the pipeline for which parts it could run in the current process. It just forks itself and lets the children figure it out. In this case, the parent process forks twice; the child for the left side then sees a built-in and executes it; the child for the right side sees grep and execs.

            – chepner
            10 hours ago



















          6














          In your first case you are just using grep utility and finding something from file /proc/meminfo, /proc is a virtual file system so /proc/meminfo file is in the memory, and it requires very little time to fetch its content.



          But in the second case, you are creating a pipe, then passing the first command's output to the second command using this pipe, which is costly.



          The difference is because of /proc (because it is in memory) and pipe, see the example below:



          time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null

          real 0m0.914s
          user 0m0.032s
          sys 0m0.148s


          cat /proc/meminfo > file
          time for i in {1..1000};do grep ^MemFree file;done >/dev/null

          real 0m0.938s
          user 0m0.032s
          sys 0m0.152s


          time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null

          real 0m1.016s
          user 0m0.040s
          sys 0m0.232s





          share|improve this answer

























            Your Answer








            StackExchange.ready(function() {
            var channelOptions = {
            tags: "".split(" "),
            id: "106"
            };
            initTagRenderer("".split(" "), "".split(" "), channelOptions);

            StackExchange.using("externalEditor", function() {
            // Have to fire editor after snippets, if snippets enabled
            if (StackExchange.settings.snippets.snippetsEnabled) {
            StackExchange.using("snippets", function() {
            createEditor();
            });
            }
            else {
            createEditor();
            }
            });

            function createEditor() {
            StackExchange.prepareEditor({
            heartbeatType: 'answer',
            autoActivateHeartbeat: false,
            convertImagesToLinks: false,
            noModals: true,
            showLowRepImageUploadWarning: true,
            reputationToPostImages: null,
            bindNavPrevention: true,
            postfix: "",
            imageUploader: {
            brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
            contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
            allowUrls: true
            },
            onDemand: true,
            discardSelector: ".discard-answer"
            ,immediatelyShowMarkdownHelp:true
            });


            }
            });














            draft saved

            draft discarded


















            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f501828%2fis-opening-a-file-faster-than-reading-variable-content%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown

























            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            32














            grep -oP '^MemFree: *K[0-9]+' /proc/meminfo forks a process that executes grep that opens /proc/meminfo (a virtual file, in memory, no disk I/O involved) reads it and matches the regexp.



            The most expensive part in that is forking the process and loading the grep utility and its library dependencies, doing the dynamic linking, open the locale database, dozens of files that are on disk (but likely cached in memory).



            The part about reading /proc/meminfo is insignificant in comparison, the kernel needs little time to generate the information in there and grep needs little time to read it.



            In:



            a=$(</proc/meminfo)


            In most shells that support that $(<...) ksh operator, the shell just opens the file and read its content (and strips the trailing newline characters). bash is different and much less efficient in that it forks a process to do that reading and passes the data to the parent via a pipe. But here, it's done once so it doesn't matter.



            In:



            printf '%sn' "$a" | grep '^MemFree'


            The shell needs to spawn two processes, which are running concurrently but interact between each other via a pipe. That pipe creation, tearing down, and writing and reading from it has some little cost. The much greater cost is the spawning of an extra process. The scheduling of the processes has some impact as well.



            You may find that using the zsh <<< operator makes it slightly quicker:



            grep '^MemFree' <<< "$a"


            In zsh and bash, that's done by writing the content of $a in a temporary file, that is less expensive than spawning an extra process, but will probably not give you any gain compared to getting the data straight off /proc/meminfo. That's still less efficient than your approach that copies /proc/meminfo on disk, as the writing of the temp file is done at each iteration.



            dash doesn't support here-strings, but its heredocs are implemented with a pipe that doesn't involve spawning an extra process. In:



             grep '^MemFree' << EOF
            $a
            EOF


            The shell creates a pipe, forks a process. The child executes grep with its stdin as the reading end of the pipe, and the parent writes the content at the other end of the pipe.



            But that pipe handling and process synchronisation is still likely to be more expensive than just getting the data straight off /proc/meminfo.



            The content of /proc/meminfo is short and takes not much time to produce. If you want to save some CPU cycles, you want to remove the expensive parts: forking processes and running external commands.



            Like:



            IFS= read -rd '' meminfo < /proc/meminfo
            memfree=${meminfo#*MemFree:}
            memfree=${memfree%%$'n'*}
            memfree=${memfree#"${memfree%%[! ]*}"}


            Avoid bash though whose pattern matching is very ineficient. With zsh -o extendedglob, you can shorten it to:



            memfree=${${"$(</proc/meminfo)"##*MemFree: #}%%$'n'*}


            Note that ^ is special in many shells (Bourne, fish, rc, es and zsh with the extendedglob option at least), I'd recommend quoting it. Also note that echo can't be used to output arbitrary data (hence my use of printf above).






            share|improve this answer





















            • 4





              In the case with printf you say the shell needs to spawn two processes, but isn't printf a shell builtin?

              – David Conrad
              11 hours ago






            • 5





              @DavidConrad It is, but most shells don't try to analyze the pipeline for which parts it could run in the current process. It just forks itself and lets the children figure it out. In this case, the parent process forks twice; the child for the left side then sees a built-in and executes it; the child for the right side sees grep and execs.

              – chepner
              10 hours ago
















            32














            grep -oP '^MemFree: *K[0-9]+' /proc/meminfo forks a process that executes grep that opens /proc/meminfo (a virtual file, in memory, no disk I/O involved) reads it and matches the regexp.



            The most expensive part in that is forking the process and loading the grep utility and its library dependencies, doing the dynamic linking, open the locale database, dozens of files that are on disk (but likely cached in memory).



            The part about reading /proc/meminfo is insignificant in comparison, the kernel needs little time to generate the information in there and grep needs little time to read it.



            In:



            a=$(</proc/meminfo)


            In most shells that support that $(<...) ksh operator, the shell just opens the file and read its content (and strips the trailing newline characters). bash is different and much less efficient in that it forks a process to do that reading and passes the data to the parent via a pipe. But here, it's done once so it doesn't matter.



            In:



            printf '%sn' "$a" | grep '^MemFree'


            The shell needs to spawn two processes, which are running concurrently but interact between each other via a pipe. That pipe creation, tearing down, and writing and reading from it has some little cost. The much greater cost is the spawning of an extra process. The scheduling of the processes has some impact as well.



            You may find that using the zsh <<< operator makes it slightly quicker:



            grep '^MemFree' <<< "$a"


            In zsh and bash, that's done by writing the content of $a in a temporary file, that is less expensive than spawning an extra process, but will probably not give you any gain compared to getting the data straight off /proc/meminfo. That's still less efficient than your approach that copies /proc/meminfo on disk, as the writing of the temp file is done at each iteration.



            dash doesn't support here-strings, but its heredocs are implemented with a pipe that doesn't involve spawning an extra process. In:



             grep '^MemFree' << EOF
            $a
            EOF


            The shell creates a pipe, forks a process. The child executes grep with its stdin as the reading end of the pipe, and the parent writes the content at the other end of the pipe.



            But that pipe handling and process synchronisation is still likely to be more expensive than just getting the data straight off /proc/meminfo.



            The content of /proc/meminfo is short and takes not much time to produce. If you want to save some CPU cycles, you want to remove the expensive parts: forking processes and running external commands.



            Like:



            IFS= read -rd '' meminfo < /proc/meminfo
            memfree=${meminfo#*MemFree:}
            memfree=${memfree%%$'n'*}
            memfree=${memfree#"${memfree%%[! ]*}"}


            Avoid bash though whose pattern matching is very ineficient. With zsh -o extendedglob, you can shorten it to:



            memfree=${${"$(</proc/meminfo)"##*MemFree: #}%%$'n'*}


            Note that ^ is special in many shells (Bourne, fish, rc, es and zsh with the extendedglob option at least), I'd recommend quoting it. Also note that echo can't be used to output arbitrary data (hence my use of printf above).






            share|improve this answer





















            • 4





              In the case with printf you say the shell needs to spawn two processes, but isn't printf a shell builtin?

              – David Conrad
              11 hours ago






            • 5





              @DavidConrad It is, but most shells don't try to analyze the pipeline for which parts it could run in the current process. It just forks itself and lets the children figure it out. In this case, the parent process forks twice; the child for the left side then sees a built-in and executes it; the child for the right side sees grep and execs.

              – chepner
              10 hours ago














            32












            32








            32







            grep -oP '^MemFree: *K[0-9]+' /proc/meminfo forks a process that executes grep that opens /proc/meminfo (a virtual file, in memory, no disk I/O involved) reads it and matches the regexp.



            The most expensive part in that is forking the process and loading the grep utility and its library dependencies, doing the dynamic linking, open the locale database, dozens of files that are on disk (but likely cached in memory).



            The part about reading /proc/meminfo is insignificant in comparison, the kernel needs little time to generate the information in there and grep needs little time to read it.



            In:



            a=$(</proc/meminfo)


            In most shells that support that $(<...) ksh operator, the shell just opens the file and read its content (and strips the trailing newline characters). bash is different and much less efficient in that it forks a process to do that reading and passes the data to the parent via a pipe. But here, it's done once so it doesn't matter.



            In:



            printf '%sn' "$a" | grep '^MemFree'


            The shell needs to spawn two processes, which are running concurrently but interact between each other via a pipe. That pipe creation, tearing down, and writing and reading from it has some little cost. The much greater cost is the spawning of an extra process. The scheduling of the processes has some impact as well.



            You may find that using the zsh <<< operator makes it slightly quicker:



            grep '^MemFree' <<< "$a"


            In zsh and bash, that's done by writing the content of $a in a temporary file, that is less expensive than spawning an extra process, but will probably not give you any gain compared to getting the data straight off /proc/meminfo. That's still less efficient than your approach that copies /proc/meminfo on disk, as the writing of the temp file is done at each iteration.



            dash doesn't support here-strings, but its heredocs are implemented with a pipe that doesn't involve spawning an extra process. In:



             grep '^MemFree' << EOF
            $a
            EOF


            The shell creates a pipe, forks a process. The child executes grep with its stdin as the reading end of the pipe, and the parent writes the content at the other end of the pipe.



            But that pipe handling and process synchronisation is still likely to be more expensive than just getting the data straight off /proc/meminfo.



            The content of /proc/meminfo is short and takes not much time to produce. If you want to save some CPU cycles, you want to remove the expensive parts: forking processes and running external commands.



            Like:



            IFS= read -rd '' meminfo < /proc/meminfo
            memfree=${meminfo#*MemFree:}
            memfree=${memfree%%$'n'*}
            memfree=${memfree#"${memfree%%[! ]*}"}


            Avoid bash though whose pattern matching is very ineficient. With zsh -o extendedglob, you can shorten it to:



            memfree=${${"$(</proc/meminfo)"##*MemFree: #}%%$'n'*}


            Note that ^ is special in many shells (Bourne, fish, rc, es and zsh with the extendedglob option at least), I'd recommend quoting it. Also note that echo can't be used to output arbitrary data (hence my use of printf above).






            share|improve this answer















            grep -oP '^MemFree: *K[0-9]+' /proc/meminfo forks a process that executes grep that opens /proc/meminfo (a virtual file, in memory, no disk I/O involved) reads it and matches the regexp.



            The most expensive part in that is forking the process and loading the grep utility and its library dependencies, doing the dynamic linking, open the locale database, dozens of files that are on disk (but likely cached in memory).



            The part about reading /proc/meminfo is insignificant in comparison, the kernel needs little time to generate the information in there and grep needs little time to read it.



            In:



            a=$(</proc/meminfo)


            In most shells that support that $(<...) ksh operator, the shell just opens the file and read its content (and strips the trailing newline characters). bash is different and much less efficient in that it forks a process to do that reading and passes the data to the parent via a pipe. But here, it's done once so it doesn't matter.



            In:



            printf '%sn' "$a" | grep '^MemFree'


            The shell needs to spawn two processes, which are running concurrently but interact between each other via a pipe. That pipe creation, tearing down, and writing and reading from it has some little cost. The much greater cost is the spawning of an extra process. The scheduling of the processes has some impact as well.



            You may find that using the zsh <<< operator makes it slightly quicker:



            grep '^MemFree' <<< "$a"


            In zsh and bash, that's done by writing the content of $a in a temporary file, that is less expensive than spawning an extra process, but will probably not give you any gain compared to getting the data straight off /proc/meminfo. That's still less efficient than your approach that copies /proc/meminfo on disk, as the writing of the temp file is done at each iteration.



            dash doesn't support here-strings, but its heredocs are implemented with a pipe that doesn't involve spawning an extra process. In:



             grep '^MemFree' << EOF
            $a
            EOF


            The shell creates a pipe, forks a process. The child executes grep with its stdin as the reading end of the pipe, and the parent writes the content at the other end of the pipe.



            But that pipe handling and process synchronisation is still likely to be more expensive than just getting the data straight off /proc/meminfo.



            The content of /proc/meminfo is short and takes not much time to produce. If you want to save some CPU cycles, you want to remove the expensive parts: forking processes and running external commands.



            Like:



            IFS= read -rd '' meminfo < /proc/meminfo
            memfree=${meminfo#*MemFree:}
            memfree=${memfree%%$'n'*}
            memfree=${memfree#"${memfree%%[! ]*}"}


            Avoid bash though whose pattern matching is very ineficient. With zsh -o extendedglob, you can shorten it to:



            memfree=${${"$(</proc/meminfo)"##*MemFree: #}%%$'n'*}


            Note that ^ is special in many shells (Bourne, fish, rc, es and zsh with the extendedglob option at least), I'd recommend quoting it. Also note that echo can't be used to output arbitrary data (hence my use of printf above).







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited 15 hours ago









            terdon

            131k32257436




            131k32257436










            answered 17 hours ago









            Stéphane ChazelasStéphane Chazelas

            306k57579933




            306k57579933








            • 4





              In the case with printf you say the shell needs to spawn two processes, but isn't printf a shell builtin?

              – David Conrad
              11 hours ago






            • 5





              @DavidConrad It is, but most shells don't try to analyze the pipeline for which parts it could run in the current process. It just forks itself and lets the children figure it out. In this case, the parent process forks twice; the child for the left side then sees a built-in and executes it; the child for the right side sees grep and execs.

              – chepner
              10 hours ago














            • 4





              In the case with printf you say the shell needs to spawn two processes, but isn't printf a shell builtin?

              – David Conrad
              11 hours ago






            • 5





              @DavidConrad It is, but most shells don't try to analyze the pipeline for which parts it could run in the current process. It just forks itself and lets the children figure it out. In this case, the parent process forks twice; the child for the left side then sees a built-in and executes it; the child for the right side sees grep and execs.

              – chepner
              10 hours ago








            4




            4





            In the case with printf you say the shell needs to spawn two processes, but isn't printf a shell builtin?

            – David Conrad
            11 hours ago





            In the case with printf you say the shell needs to spawn two processes, but isn't printf a shell builtin?

            – David Conrad
            11 hours ago




            5




            5





            @DavidConrad It is, but most shells don't try to analyze the pipeline for which parts it could run in the current process. It just forks itself and lets the children figure it out. In this case, the parent process forks twice; the child for the left side then sees a built-in and executes it; the child for the right side sees grep and execs.

            – chepner
            10 hours ago





            @DavidConrad It is, but most shells don't try to analyze the pipeline for which parts it could run in the current process. It just forks itself and lets the children figure it out. In this case, the parent process forks twice; the child for the left side then sees a built-in and executes it; the child for the right side sees grep and execs.

            – chepner
            10 hours ago













            6














            In your first case you are just using grep utility and finding something from file /proc/meminfo, /proc is a virtual file system so /proc/meminfo file is in the memory, and it requires very little time to fetch its content.



            But in the second case, you are creating a pipe, then passing the first command's output to the second command using this pipe, which is costly.



            The difference is because of /proc (because it is in memory) and pipe, see the example below:



            time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null

            real 0m0.914s
            user 0m0.032s
            sys 0m0.148s


            cat /proc/meminfo > file
            time for i in {1..1000};do grep ^MemFree file;done >/dev/null

            real 0m0.938s
            user 0m0.032s
            sys 0m0.152s


            time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null

            real 0m1.016s
            user 0m0.040s
            sys 0m0.232s





            share|improve this answer






























              6














              In your first case you are just using grep utility and finding something from file /proc/meminfo, /proc is a virtual file system so /proc/meminfo file is in the memory, and it requires very little time to fetch its content.



              But in the second case, you are creating a pipe, then passing the first command's output to the second command using this pipe, which is costly.



              The difference is because of /proc (because it is in memory) and pipe, see the example below:



              time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null

              real 0m0.914s
              user 0m0.032s
              sys 0m0.148s


              cat /proc/meminfo > file
              time for i in {1..1000};do grep ^MemFree file;done >/dev/null

              real 0m0.938s
              user 0m0.032s
              sys 0m0.152s


              time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null

              real 0m1.016s
              user 0m0.040s
              sys 0m0.232s





              share|improve this answer




























                6












                6








                6







                In your first case you are just using grep utility and finding something from file /proc/meminfo, /proc is a virtual file system so /proc/meminfo file is in the memory, and it requires very little time to fetch its content.



                But in the second case, you are creating a pipe, then passing the first command's output to the second command using this pipe, which is costly.



                The difference is because of /proc (because it is in memory) and pipe, see the example below:



                time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null

                real 0m0.914s
                user 0m0.032s
                sys 0m0.148s


                cat /proc/meminfo > file
                time for i in {1..1000};do grep ^MemFree file;done >/dev/null

                real 0m0.938s
                user 0m0.032s
                sys 0m0.152s


                time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null

                real 0m1.016s
                user 0m0.040s
                sys 0m0.232s





                share|improve this answer















                In your first case you are just using grep utility and finding something from file /proc/meminfo, /proc is a virtual file system so /proc/meminfo file is in the memory, and it requires very little time to fetch its content.



                But in the second case, you are creating a pipe, then passing the first command's output to the second command using this pipe, which is costly.



                The difference is because of /proc (because it is in memory) and pipe, see the example below:



                time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null

                real 0m0.914s
                user 0m0.032s
                sys 0m0.148s


                cat /proc/meminfo > file
                time for i in {1..1000};do grep ^MemFree file;done >/dev/null

                real 0m0.938s
                user 0m0.032s
                sys 0m0.152s


                time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null

                real 0m1.016s
                user 0m0.040s
                sys 0m0.232s






                share|improve this answer














                share|improve this answer



                share|improve this answer








                edited 15 hours ago









                terdon

                131k32257436




                131k32257436










                answered 17 hours ago









                PRYPRY

                2,51031026




                2,51031026






























                    draft saved

                    draft discarded




















































                    Thanks for contributing an answer to Unix & Linux Stack Exchange!


                    • Please be sure to answer the question. Provide details and share your research!

                    But avoid



                    • Asking for help, clarification, or responding to other answers.

                    • Making statements based on opinion; back them up with references or personal experience.


                    To learn more, see our tips on writing great answers.




                    draft saved


                    draft discarded














                    StackExchange.ready(
                    function () {
                    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f501828%2fis-opening-a-file-faster-than-reading-variable-content%23new-answer', 'question_page');
                    }
                    );

                    Post as a guest















                    Required, but never shown





















































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown

































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown







                    Popular posts from this blog

                    Magento 2 controller redirect on button click in phtml file

                    Polycentropodidae