Bash script - variable content as a command to run

Question

I have a Perl script that gives me a defined list random numbers that correspond to the lines of a file  Next I want to extract those lines from the file using sed      bin bash count   cat last queries txt   wc -l  var   perl test pl test2  count    The variable var returns an output like  cat last queries txt   sed -n  12p 500p 700p   The problem is that I can t run this last command  I tried with  var  but the output is not correct  if I run manually the command it works fine  so no problem there   What is the correct way to do this   P S  Sure I could do all the work in Perl  but I m trying to learn this way  because it could help me in other situations

User · Answer

Your are probably looking for eval  var

User · Answer

You just need to do      bin bash count   cat last queries txt   wc -l    perl test pl test2  count    However  if you want to call your Perl command later  and that s why you want to assign it to a variable  then      bin bash count   cat last queries txt   wc -l  var  perl test pl test2  count    You need double quotes to get your  count value substituted      stuff     eval  var   As per Bash s help      help eval eval  eval  arg          Execute arguments as a shell command       Combine ARGs into a single string  use the result as input to the shell      and execute the resulting commands       Exit Status      Returns exit status of command or success if command is null

User · Answer

line      RANDOM      wc -l  lt   etc passwd    sed -n    line p   etc passwd   just with your file instead   In this example I used the file  etc password  using the special variable   RANDOM   about which I learned here   and the sed expression you had  only difference is that I am using double quotes instead of single to allow the variable expansion

User · Answer

In the case where you have multiple variables containing the arguments for a command you re running  and not just a single string  you should not use eval directly  as it will fail in the following case   function echo arguments       echo  Argument 1   1    echo  Argument 2   2    echo  Argument 3   3    echo  Argument 4   4       Note we are passing 3 arguments to  echo arguments   not 4 eval echo arguments arg1 arg2  Some arg    Result    Argument 1  arg1 Argument 2  arg2 Argument 3  Some Argument 4  arg   Note that even though  Some arg  was passed as a single argument  eval read it as two   Instead  you can just use the string as the command itself     The regular bash eval works by jamming all its arguments into a string then   evaluating the string  This function treats its arguments as individual   arguments to be passed to the command being run  function eval command                 Note the difference between the output of eval and the new eval command function   eval command echo arguments arg1 arg2  Some arg    Result   Argument 1  arg1 Argument 2  arg2 Argument 3  Some arg Argument 4

[bash] Bash script - variable content as a command to run

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