How to execute command stored in a variable

Question

What is the correct way to call some command stored in variable  Is there any differences between 1 and 2        bin sh cmd  ls -la  APPROOTDIR   grep exception   1  cmd  2 eval   cmd

User · Answer

cmd would just replace the variable with it s value to be executed on command line  eval   cmd  does variable expansion  amp  command substitution before executing the resulting value on command line  The 2nd method is helpful when you wanna run commands that aren t flexible eg   for i in   a   b  format loop won t work because it doesn t allow variables  In this case  a pipe to bash or eval is a workaround   Tested on Mac OSX 10 6 8  Bash 3 2 48

User · Answer

I think you should put         backtick  symbols around your variable

User · Answer

Unix shells operate a series of transformations on each line of input before executing them   For most shells it looks something like this  taken from the bash manpage     initial word splitting brace expansion tilde expansion parameter  variable and arithmetic expansion command substitution secondary word splitting path expansion  aka globbing  quote removal   Using  cmd directly gets it replaced by your command during the parameter expansion phase  and it then undergoes all following transformations   Using eval   cmd  does nothing until the quote removal phase  where  cmd is returned as is  and passed as a parameter to eval  whose function is to run the whole chain again before executing   So basically  they re the same in most cases  and differ when your command makes use of the transformation steps up to parameter expansion   For example  using brace expansion     cmd  echo foo bar baz      cmd foo bar baz    eval   cmd  foobar foobaz

User · Answer

If you just do eval  cmd  when we do cmd  ls -l   interactively and in a script  we get the desired result  In your case  you have a pipe with a grep without a pattern  so the grep part will fail with an error message  Just  cmd will generate a  command not found   or some such  message  So try use eval and use a finished command  not one that generates an error message

[shell] How to execute command stored in a variable?

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