How can I use variable variables (indirect variables, pointers, references) or associative arrays?
This is a complex page, because it's a complex topic. It's been divided into roughly four parts: associative arrays, name references, evaluating indirect variables, and assigning indirect variables. There are discussions of programming issues and concepts scattered throughout.
Contents
Associative Arrays
We introduce associative arrays first, because we observe that inexperienced programmers often conjure arcane solutions to problems that would be solved more cleanly with associative arrays.
An associative array is an unordered collection of key-value pairs. A value may be retrieved by supplying its corresponding key. Since strings are the only datatype most shells understand, associative arrays map strings to strings, unlike indexed arrays, which map integers to strings. Associative arrays exist in AWK as "associative arrays", in Perl as "hashes", in Tcl as "arrays", in Python and C# as "dictionaries", in Java as a "Map", and in C++11 STL as std::unordered_map.
1 # Bash 4 / ksh93
2
3 typeset -A homedir # Declare associative array
4 homedir=( # Compound assignment
5 [jim]=/home/jim
6 [silvia]=/u/silvia
7 [alex]=/net/home/alex
8 )
9
10 homedir[ormaaj]=/home/ormaaj # Ordinary assignment adds another single element
11
12 for user in "${!homedir[@]}"; do # Enumerate all indices (user names)
13 printf 'Home directory of user %q is: %q\n' "$user" "${homedir[$user]}"
14 done
Prior to Bash 4 or if you can't use ksh93, your options are limited. Either move to another interpreter (awk, perl, python, ruby, tcl, ...) or re-evaluate your problem to simplify it. There are certain tasks for which associative arrays are a powerful and completely appropriate tool. There are others for which they are overkill, or simply unsuitable.
Suppose we have several remote hosts with slightly different configuration, and that we want to ssh to each one and run slightly different commands. One way we could set it up would be to hard-code a bunch of ssh commands in per-hostname functions in a single script and just run them in series or in parallel. (Don't reject this out of hand! Simple is good.) Another way would be to store each group of commands as an element of an associative array keyed by the hostname:
This solution works, because we're encoding a very short shell script in a string, and storing it as an array element. When we call ssh, it passes the string directly to the remote host, where a shell evaluates it and executes it. But what if the scripts were much longer, or more complicated?
If we want to get fancy and store each sub-command (cd webapp for example) as an element of a list, and then have each hostname map to a list of sub-commands, we'd quickly find that we can't do that in a shell. That's the kind of approach we'd expect in a high-level language, where we can store hierarchical information in advanced data structures. We want each element of the associative array to be a list or another array of command strings. But the shell simply doesn't offer that kind of data structure.
So, often it pays to step back and think in terms of shells rather than other programming languages. Aren't we just running a script on a remote host? Then why don't we just store the configuration sets as scripts? Then it's simple:
Now we've removed the need for associative arrays, and also the need to maintain a bunch of extremely horrible quoting issues. It is also easy to parallelize using GNU Parallel:
1 parallel ssh -- {/} bash "<" {} ::: /etc/myapp/*
Associative array hacks in older shells
Before you think of using eval to mimic associative arrays in an older shell (probably by creating a set of variable names like homedir_alex), try to think of a simpler or completely different approach that you could use instead. If this hack still seems to be the best thing to do, consider the following disadvantages:
- It's really hard to read, to keep track of, and to maintain.
The variable names must be a single line and 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.
Quoting is hard to get right. If a content string (not a 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. (Bash's printf %q helps, but nothing analogous is available in POSIX shells.)
If the program handles unsanitized user input, it can be VERY dangerous!
Read BashGuide/Arrays or BashFAQ/005 for a more in-depth description and examples of how to use arrays in Bash.
If you need an associative array but your shell doesn't support them, please consider using AWK instead.
Name References
Bash 4.3 introduced name references, which are variables that work like symbolic links. That is, the content of a nameref variable is the name of a second variable. Any assignments to the nameref variable, or any expansions of it, are performed on the variable whose name it "points to".
This is fine in concept, but it has a few pitfalls that you must be aware of:
The name that's pointed to is resolved using the same dynamic scoping rules as any other variable name. For example, if you have declare -n ref=foo and you try to use $ref, the shell will look for a variable named foo in the current function, then in the calling function, and so on up to the global scope if necessary.
There is no way to override this. You cannot say "this nameref points to the variable foo in the caller's scope". If there's a local variable foo, it will be used instead, whether you like it or not.
Therefore, any function which uses a nameref (especially if it's meant to contain the name of an output variable in the caller's scope) must actively defend against namespace collisions. If the caller passes a variable name like n for you to store information into, then your function must not use its own local variable n. A good policy is to prefix all of your local variables (not just the nameref!) with some unique string, such as your function's name.
KornShell (ksh93) has them as well, using typeset instead of declare:
As long as you avoid namespace collisions, namerefs can be extremely useful. They give the "indirection" that many people are looking for:
Indirection
In this section, we discuss various tricks available in shells older than bash 4.3, where namerefs aren't available.
Think before using indirection
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.
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 (indexed or associative) or haven't fully considered other Bash features such as functions.
Evaluating indirect/reference variables
BASH allows for expanding parameters indirectly -- that is, one variable may contain the name of another variable. Name reference variables are the preferred method for performing variable indirection. Older versions of Bash could also use a ! prefix operator in parameter expansions for variable indirection. Namerefs should be used unless portability to older bash versions is required. No other shell uses ${!variable} for indirection and there are problems relating to use of that syntax for this purpose. It is also less flexible.
Zsh allows you to access a parameter indirectly with the parameter expansion flag P:
zsh's ability to nest parameter expansions allow for referencing arrays too:
Unfortunately, for shells other than Bash, ksh93, and zsh there is no syntax for evaluating a referenced variable. You would have to use eval, which means you would have to undergo extreme measures to sanitize your data to avoid catastrophe.
It's difficult to imagine a practical use for this that wouldn't be just as easily performed by using an associative array. But people ask it all the time (it is genuinely a frequently asked question).
We are not aware of any trick that can duplicate that functionality in POSIX or Bourne shells without eval, which can be difficult to do securely. Older versions of Bash can almost do it -- some indirect array tricks work, and others do not, and we do not know whether the syntax involved will remain stable in future releases. So, consider this a use at your own risk hack.
1 # Bash -- trick #2. Seems to work in bash 3 and up.
2 # Can't be combined with special expansions until 4.3. e.g. "${!tmp##*/}"
3 # Does NOT work in bash 2.05b -- Expands to one word instead of three in bash 2.
4 tmp=${ref}[@]
5 printf '<%s> ' "${!tmp}"; echo # Iterate whole array as one word per element.
It is not possible to retrieve array indices directly using the Bash ${!var} indirect expansion.
Assigning indirect/reference variables
Sometimes you'd like to "point" from one variable to another, for purposes of writing information to a dynamically configurable place. Typically this happens when you're trying to write a "reusable" function or library, and you want it to put its output in a variable of the caller's choice instead of the function's choice. (Various traits of Bash make safe reusability of Bash functions difficult at best, so this is something that should not happen often.)
Assigning a value "through" a reference (I'm going to use "ref" from now on) is more widely possible, but the means of doing so are usually extremely shell-specific. All shells with the sole exception of AT&T ksh93 lack real reference variables or pointers. Indirection can only be achieved by indirectly evaluating variable names. IOW, you can never have a real unambiguous reference to an object in memory; the best you can do is use the name of a variable to try simulating the effect. Therefore, you must control the value of the ref and ensure side-effects such as globbing, user-input, and conflicting local parameters can't affect parameter names. Names must either be deterministic or validated in a way that makes certain guarantees. If an end user can populate the ref variable with arbitrary strings, the result can be unexpected code injection. We'll show an example of this at the end.
In ksh93, we can use nameref again:
In zsh, using parameter expansions ::= and expansion flags P:
In Bash, if you only want to assign a single line to the variable, you can use read and Bash's here string syntax:
If you need to assign multiline values, you can use a HereDocument:
A similar trick works for Bash array variables too:
IFS is used to delimit words, so you may or may not need to set that. Also note that the read command will return failure because there is no terminating NUL byte for the -d '' to catch. Be prepared to ignore that failure.
Another trick is to use Bash's printf -v, available in bash 3.1 and newer:
You can use all of printf's formatting capabilities. This trick also permits any string content, including embedded and trailing newlines.
Yet another trick is Korn shell's typeset or Bash's declare. The details of typeset vary greatly between shells, but can be used in compatible ways in limited situations. Both of them cause a variable to become locally scoped to a function, if used inside a function; but if used outside all functions, they can operate on global variables.
Bash 4.2 adds declare -g which assigns variables to the global scope from any context.
If you aren't using Bash or Korn shell, you can do assignments to referenced variables using HereDocument syntax:
Alas, read without -d means we're back to only getting at most one line of content. This is the most portable trick, but it's limited to single-line content.
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.
Security Concerns
Some people mistakenly believe that typeset or declare is "safer" than eval. It turns out, they're just as dangerous. Possibly even more so, because people think they're safe. An eval merits an immediate scrutiny, but declare is often overlooked.
Another drawback of typeset or declare is that they always affect the scope of the assigned variable. eval leaves the scope untouched.
With any indirect assignment, you must be careful about what you're assigning to. Inside square brackets, expansions are still performed; thus, with a tainted ref, declare or printf -v can be just as dangerous as eval:
This problem also exists with typeset in mksh and pdksh, but apparently not ksh93. This is why the value of ref must be under your control at all times.
eval
This expands to the statement that is executed:
1 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 quoted/escaped carefully. In the example shown here, there was only one. In a more complex situation, there could be dozens.
This is very often done incorrectly. Permutations like these are seen frequently all over the web even from experienced users that ought to know better:
The good news is that if you can sanitize the right hand side correctly, this trick is fully portable, has no variable scope issues, and allows all content including newlines. The bad news is that if you fail to sanitize the right hand side correctly, you have a massive security hole. Use eval if you know what you're doing and are very careful.
The following code demonstrates how to correctly pass a scalar variable name into a function by reference for the purpose of "returning" a value:
1 # POSIX
2
3 f() {
4 # Check that the referenced variable name is not empty, and
5 # is a valid variable name.
6 if [ "$#" != 1] || [ -z "$1" ]; then
7 echo >&2 "usage: f varname"
8 return 1
9 fi
10
11 if printf '%s\n' "$1" | LC_ALL=C grep -v -q '^[A-Za-z_][A-Za-z0-9_]*$' ||
12 [ "$(printf '%s\n' "$1" | wc -l)" != 1 ]; then
13 echo >&2 "f: invalid varname argument"
14 return 2
15 fi
16
17 # Code goes here that eventually sets the variable "x".
18 # In shells with local variables, x should be local.
19 # x contains the value we'd like to return to the caller.
20 x=foo
21
22 # Return the value into the caller's variable.
23 eval "${1}=\$x"
24 }