Dynamic variable names in Bash

Question

I am confused about a bash script   I have the following code   function grep search         magic way to define magic variable  1  ls   tail -1      echo  magic variable  1     I want to be able to create a variable name containing the first argument of the command and bearing the value of e g  the last line of ls   So to illustrate what I want     ls   tail -1 stack-overflow txt    grep search   open box stack-overflow txt   So  how should I define declare  magic way to define magic variable  1 and how should I call it within the script   I have tried eval                    but I am still confused

User · Answer

As per BashFAQ 006  you can use read with here string syntax for assigning indirect variables   function grep search       read   1   lt  lt  lt   ls   tail -1       Usage     grep search open box   echo  open box stack-overflow txt

User · Answer

This will work too  my country code  green  x  country   eval z    my   x  code echo  z                    o p  green   In your case  eval final val    magic way to define magic variable   1  echo  final val

User · Answer

Use declare  There is no need on using prefixes like on other answers  neither arrays  Use just declare  double quotes  and parameter expansion   I often use the following trick to parse argument lists contanining one to n arguments formatted as key value otherkey othervalue etc etc  Like     brace expansion just to exemplify for variable in  one foo two bar ninja tip  do   declare    variable       variable      done echo  one  two  ninja    foo bar tip   But expanding the argv list like  for v in       do declare    v       v       done     Extra tips    parse argv s leading key value parameters for v in       do   case   v  in        declare    v       v           break   esac done   consume argv s leading key value parameters while           do   case   v  in        declare    v       v           break   esac   shift done

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Even though it s an old question  I still had some hard time with fetching dynamic variables names  while avoiding the eval  evil  command  Solved it with declare -n which creates a reference to a dynamic value  this is especially useful in CI CD processes  where the required secret names of the CI CD service are not known until runtime  Here s how    Bash v4 3    -----------------------------------------------------------   Secerts in CI CD service  injected as environment variables   AWS ACCESS KEY ID DEV  AWS SECRET ACCESS KEY DEV   AWS ACCESS KEY ID STG  AWS SECRET ACCESS KEY STG   -----------------------------------------------------------   Environment variables injected by CI CD service   BRANCH NAME  quot DEV quot    ----------------------------------------------------------- declare -n  AWS ACCESS KEY ID REF AWS ACCESS KEY ID   BRANCH NAME  declare -n  AWS SECRET ACCESS KEY REF AWS SECRET ACCESS KEY   BRANCH NAME   export AWS ACCESS KEY ID    AWS ACCESS KEY ID REF  export AWS SECRET ACCESS KEY    AWS SECRET ACCESS KEY REF   echo  AWS ACCESS KEY ID  AWS SECRET ACCESS KEY aws s3 ls

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I ve been looking for better way of doing it recently  Associative array sounded like overkill for me  Look what I found   suffix bzz declare prefix  suffix mystr      and then     varname prefix  suffix echo    varname

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For indexed arrays  you can reference them like so   foo  a b c  bar  d e f   for arr var in  foo   bar   do     declare -a  arr        arr var                do something with  arr     echo     arr var contains       for char in    arr       do         echo   char      done done   Associative arrays can be referenced similarly but need the -A switch on declare instead of -a

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This should work   function grep search         declare magic variable  1    ls   tail -1       echo    tmpvar magic variable  1  amp  amp  echo    tmpvar      grep search var    calling grep search with argument  var

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Combining two highly rated answers here into a complete example that is hopefully useful and self-explanatory     bin bash  intro  quot You know what  quot  pet1  quot cat quot  pet2  quot chicken quot  pet3  quot cow quot  pet4  quot dog quot  pet5  quot pig quot     Setting and reading dynamic variables for i in  1  5   do         pet  quot pet i quot          declare  quot sentence i  intro I have a pet    pet  at home quot  done    Just reading dynamic variables for i in  1  5   do         sentence  quot sentence i quot          echo  quot    sentence  quot  done  echo echo  quot Again  but reading regular variables  quot  echo  sentence1 echo  sentence2 echo  sentence3 echo  sentence4 echo  sentence5  Output  You know what  I have a pet cat at home You know what  I have a pet chicken at home You know what  I have a pet cow at home You know what  I have a pet dog at home You know what  I have a pet pig at home Again  but reading regular variables  You know what  I have a pet cat at home You know what  I have a pet chicken at home You know what  I have a pet cow at home You know what  I have a pet dog at home You know what  I have a pet pig at home

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An extra method that doesn t rely on which shell bash version you have is by using envsubst  For example   newvar   echo   magic variable     dynamic part     envsubst

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Example below returns value of  name of var  var name of var echo   eval echo     var

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Wow  most of the syntax is horrible  Here is one solution with some simpler syntax if you need to indirectly reference arrays     bin bash  foo 1  fff ddd    foo 2  ggg ccc     for i in 1 2   do     eval mine      foo  i             echo   mine     quot   quot    done    For simpler use cases I recommend the syntax described in the Advanced Bash-Scripting Guide

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for varname  prefix suffix format  just use   varname   prefix  suffix

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Beyond associative arrays  there are several ways of achieving dynamic variables in Bash  Note that all these techniques present risks  which are discussed at the end of this answer  In the following examples I will assume that i 37 and that you want to alias the variable named var 37 whose initial value is lolilol  Method 1  Using a    pointer    variable You can simply store the name of the variable in an indirection variable  not unlike a C pointer  Bash then has a syntax for reading the aliased variable     name  expands to the value of the variable whose name is the value of the variable name  You can think of it as a two-stage expansion     name  expands to  var 37  which expands to lolilol  name  quot var  i quot  echo  quot  name quot            outputs    var 37    echo  quot    name  quot         outputs    lolilol    echo  quot    name lol  quot     outputs    loli      etc   Unfortunately  there is no counterpart syntax for modifying the aliased variable  Instead  you can achieve assignment with one of the following tricks  1a  Assigning with eval eval is evil  but is also the simplest and most portable way of achieving our goal  You have to carefully escape the right-hand side of the assignment  as it will be evaluated twice  An easy and systematic way of doing this is to evaluate the right-hand side beforehand  or to use printf  q   And you should check manually that the left-hand side is a valid variable name  or a name with index  what if it was evil code       By contrast  all other methods below enforce it automatically    check that name is a valid variable name    note  this code does not support variable name index  shopt -s globasciiranges     quot  name quot      a-zA-Z     a-zA-Z 0-9         exit  value  babibab  eval  quot  name quot    value     carefully escape the right-hand side  echo  quot  var 37 quot     outputs    babibab     Downsides   does not check the validity of the variable name  eval is evil  eval is evil  eval is evil   1b  Assigning with read The read builtin lets you assign values to a variable of which you give the name  a fact which can be exploited in conjunction with here-strings  IFS  read -r -d     quot  name quot   lt  lt  lt   babibab  echo  quot  var 37 quot     outputs    babibab n     The IFS part and the option -r make sure that the value is assigned as-is  while the option -d    allows to assign multi-line values  Because of this last option  the command returns with an non-zero exit code  Note that  since we are using a here-string  a newline character is appended to the value  Downsides   somewhat obscure  returns with a non-zero exit code  appends a newline to the value   1c  Assigning with printf Since Bash 3 1  released 2005   the printf builtin can also assign its result to a variable whose name is given  By contrast with the previous solutions  it just works  no extra effort is needed to escape things  to prevent splitting and so on  printf -v  quot  name quot    s   babibab  echo  quot  var 37 quot     outputs    babibab     Downsides   Less portable  but  well    Method 2  Using a    reference    variable Since Bash 4 3  released 2014   the declare builtin has an option -n for creating a variable which is a    name reference    to another variable  much like C   references  Just as in Method 1  the reference stores the name of the aliased variable  but each time the reference is accessed  either for reading or assigning   Bash automatically resolves the indirection  In addition  Bash has a special and very confusing syntax for getting the value of the reference itself  judge by yourself     ref   declare -n ref  quot var  i quot  echo  quot    ref  quot     outputs    var 37    echo  quot  ref quot        outputs    lolilol    ref  babibab  echo  quot  var 37 quot     outputs    babibab     This does not avoid the pitfalls explained below  but at least it makes the syntax straightforward  Downsides   Not portable   Risks All these aliasing techniques present several risks  The first one is executing arbitrary code each time you resolve the indirection  either for reading or for assigning   Indeed  instead of a scalar variable name  like var 37  you may as well alias an array subscript  like arr 42   But Bash evaluates the contents of the square brackets each time it is needed  so aliasing arr   do evil   will have unexpected effects    As a consequence  only use these techniques when you control the provenance of the alias  function guillemots       declare -n var  quot  1 quot    var  quot     var    quot     arr   aaa bbb ccc   guillemots  arr 1      modifies the second cell of the array  as expected guillemots  arr   date gt  gt date out 1      writes twice into date out                once when expanding var  once when assigning to it   The second risk is creating a cyclic alias  As Bash variables are identified by their name and not by their scope  you may inadvertently create an alias to itself  while thinking it would alias a variable from an enclosing scope   This may happen in particular when using common variable names  like var   As a consequence  only use these techniques when you control the name of the aliased variable  function guillemots         var is intended to be local to the function      aliasing a variable which comes from outside   declare -n var  quot  1 quot    var  quot     var    quot     var  lolilol  guillemots var    Bash warnings     var  circular name reference    echo  quot  var quot        outputs anything   Source   BashFaq 006  How can I use variable variables  indirect variables  pointers  references  or associative arrays  BashFAQ 048  eval command and security issues

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For zsh  newers mac os versions   you should use real var  quot holaaaa quot  aux var  quot real var quot  echo    P aux var  holaaaa  Instead of  quot   quot

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Use an associative array  with command names as keys     Requires bash 4  though declare -A magic variable     function grep search         magic variable  1     ls   tail -1       echo   magic variable  1       If you can t use associative arrays  e g   you must support bash 3   you can use declare to create dynamic variable names   declare  magic variable  1   ls   tail -1     and use indirect parameter expansion to access the value   var  magic variable  1  echo     var     See BashFAQ  Indirection - Evaluating indirect reference variables

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I want to be able to create a variable name containing the first argument of the command  script sh file     usr bin env bash function grep search       eval  1   ls   tail -1     Test    source script sh   grep search open box   echo  open box script sh   As per help eval   Execute arguments as a shell command    You may also use Bash    var  indirect expansion  as already mentioned  however it doesn t support retrieving of array indices   For further read or examples  check BashFAQ 006 about Indirection   We are not aware of any trick that can duplicate that functionality in POSIX or Bourne shells without eval  which can be difficult to do securely  So  consider this a use at your own risk hack   However  you should re-consider using indirection as per the following notes   Normally  in bash scripting  you won t need indirect references at all  Generally  people look at this for a solution when they don t understand or know about Bash Arrays or haven t fully considered other Bash features such as functions  Putting variable names or any other bash syntax inside parameters is frequently done incorrectly and in inappropriate situations to solve problems that have better solutions  It violates the separation between code and data  and as such puts you on a slippery slope toward bugs and security issues  Indirection can make your code less transparent and harder to follow

[bash] Dynamic variable names in Bash

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