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Finally, some people just ''cannot'' resist throwing `eval` into the picture:

 {{{
 # Bourne
 var=myVar
 eval "$var=\$value"
 }}}

This expands to the statement that is executed:

 {{{
 myVar=$value
 }}}

The right-hand side is not parsed by the shell, so there is no danger of unwanted side effects. The drawback, here, is that every single shell metacharacter on the right hand side of the `=` must be escaped carefully. In the example shown here, there was only one. In a more complex situation, there could be dozens.
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There is, however, no danger if the right-hand variable expansion is done on the second pass:

{{{
 var=myVar
 eval "$var=\$value"
}}}

This expands to the statment that is executed:

{{{
 myVar=$value
}}}

The contents of {{{$value}}} is not parsed by the shell, and there is no danger of unwanted side effects.

How can I use variable variables (indirect variables, pointers, references) or associative arrays?

There are two halves to this: evaluating variables, and assigning values. We'll take each half separately:

Evaluating indirect/reference variables

BASH allows you to expand a parameter indirectly -- that is, one variable may contain the name of another variable:

  •  # Bash
     realvariable=contents
     ref=realvariable
     echo "${!ref}"   # prints the contents of the real variable

KornShell (ksh93) has a completely different, more powerful syntax -- the nameref command (also known as typeset -n):

  •  # ksh93
     realvariable=contents
     nameref ref=realvariable
     echo "$ref"      # prints the contents of the real variable

ksh93's nameref allows us to work with references to arrays, as well as regular scalar variables. For example,

  •  # ksh93
     myfunc() {
       nameref ref=$1
       echo "array $1 has ${#ref[*]} elements"
     }
     realarray=(...)
     myfunc realarray

We are not aware of any trick that can duplicate that functionality in Bash, POSIX or Bourne shells (short of using eval, which is extremely difficult to do securely).

Unfortunately, for shells other than Bash and ksh93, there is no syntax for evaluating a referenced variable. You would have to use eval, which means you would have to undergo extreme measures to sanitize your data to avoid catastrophe.

Assigning indirect/reference variables

Assigning a value "through" a reference (or pointer, or indirect variable, or whatever you want to call it -- I'm going to use "ref" from now on) is more widely possible, but the means of doing so are extremely shell-specific.

In ksh93, we can just use nameref again:

  •  # ksh93
     nameref ref=realvariable
     ref="contents"
     # realvariable now contains the string "contents"

In Bash, we can use read and Bash's here string syntax:

  •  # Bash
     ref=realvariable
     read $ref <<< "contents"
     # realvariable now contains the string "contents"

This works equally well with Bash array variables too:

  •  # Bash
     aref=realarray
     read -a $aref <<< "words go into array elements"
     echo "${realarray[1]}"   # prints "go"

Another trick is to use Bash's printf -v (only available in recent versions):

  •  # Bash 3.1 or higher
     ref=realvariable
     printf -v $ref "contents"

The printf -v trick is handy if your contents aren't a constant string, but rather, something dynamically generated. You can use all of printf's formatting capabilities.

Yet another trick is Korn shell's typeset or Bash's declare. These are roughly equivalent to each other. Both of them cause a variable to become locally scoped to a function, if used inside a function; but if used outside a function, they can operate on global variables.

  •  # Korn shell (all versions):
     typeset $ref="contents"
    
     # Bash:
     declare $ref="contents"

The advantage of using typeset or declare over eval is that the right hand side of the assignment is not parsed by the shell. If you used eval here, you would have to sanitize/escape the entire right hand side first.

If you aren't using Bash or Korn shell, you can still do assignments to referenced variables using here document syntax:

  •  # Bourne
     ref=realvariable
     read $ref <<EOF
     contents
     EOF

Remember that, when using a here document, if the sentinel word (EOF in our example) is unquoted, then parameter expansions will be performed inside the body. If the sentinel is quoted, then parameter expansions are not performed. Use whichever is more convenient for your task.

Associative Arrays

Sometimes it's convenient to have associative arrays, arrays indexed by a string. Awk has associative arrays. Perl calls them "hashes", while Tcl simply calls them "arrays". ksh93 supports this kind of array:

  •  # ksh93
     typeset -A homedir             # Declare ksh93 associative array
     homedir[jim]=/home/jim
     homedir[silvia]=/home/silvia
     homedir[alex]=/home/alex
     
     for user in ${!homedir[@]}     # Enumerate all indices (user names)
     do
         echo "Home directory of user $user is ${homedir[$user]}"
     done

BASH version 4.0 finally supports them, though older versions do not.

  •  # bash 4.0
     declare -A homedir
     homedir[jim]=/home/jim
     ... (same as the ksh93 example, other than declare vs. typeset)

If you can't use ksh93 or bash 4.0, consider switching to awk, perl, ksh93, tcl, etc. if you need this type of data structure to solve your problem.

Before you think of using eval to mimic this behavior in a shell (probably by creating a set of variable names like homedir_alex), try to think of a simpler approach that you could use instead. If this hack still seems to be the best thing to do, have a look at the following disadvantages:

  1. It's hard to read and to maintain.
  2. The variable names must match the RegularExpression ^[a-zA-Z_][a-zA-Z_0-9]* -- i.e., a variable name cannot contain arbitrary characters but only letters, digits, and underscores. We cannot have a variable's name contain Unix usernames, for instance -- consider a user named hong-hu. A dash '-' cannot be part of a variable name, so the entire attempt to make a variable named homedir_hong-hu is doomed from the start.

  3. Quoting is hard to get right. If content strings (not variable name) can contain whitespace characters and quotes, it's hard to quote it right to preserve it through both shell parsings. And that's just for constants, known at the time you write the program.

  4. If the program handles unsanitized user input, it can be VERY dangerous!

There is, however, no danger if the right-hand variable expansion is done on the second pass:

 var=myVar
 eval "$var=\$value"

This expands to the statment that is executed:

 myVar=$value

The contents of $value is not parsed by the shell, and there is no danger of unwanted side effects.

BashFAQ/006 (last edited 2023-04-14 06:52:11 by ormaaj)