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| There are two halves to this: evaluating variables, and assigning values. We'll take each half separately: === Evaluating indirect/reference variables === |
This is a complex page, because it's a complex topic. It's been divided into roughly three parts: associative arrays, evaluating indirect variables, and assigning indirect variables. There are discussions of programming issues and concepts scattered throughout. <<TableOfContents>> === Associative Arrays === We introduce associative arrays first, because in the majority of cases where people are trying to use indirect variable assignments/evaluations, they ought to be using associative arrays instead. For instance, we frequently see people asking how they can have a bunch of related variables like `IPaddr_hostname1`, `IPaddr_hostname2` and so on. A more appropriate way to store this data would be in an associative array named `IPaddr` which is indexed by the hostname. To map from one string to another, you need arrays indexed by a string instead of a number. These exists in AWK as "associative arrays", in Perl as "hashes", and in Tcl simply as "arrays". They also exist in [[KornShell|ksh93]], where you'd use them like this: {{{ # 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 supports them from version 4 and up: {{{ # Bash 4 and up declare -A homedir homedir[jim]=/home/jim # or homedir=( [jim]=/home/jim [silvia]=/home/silvia [alex]=/home/alex ) ... }}} 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 subservient 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: {{{ source "$conf" for host in "${!commands[@]}"; do ssh "$host" "${commands[$host]}" done # Where "$conf" is a file like this: declare -A commands commands=( [host1]="mvn clean install && cd webapp && mvn jetty:run" [host2]="..." ) }}} This is the kind of approach we'd expect in a high-level language, where we can store hierarchical information in advanced data structures. The difficulty here is that we really want each element of the associative array to be a ''list'' or ''another array'' of command strings. But the shell simply doesn't permit 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: {{{ # A series of conf files named for the hosts we need to run our commands on: for conf in /etc/myapp/*; do host=${conf##*/} ssh "$host" bash < "$conf" done # /etc/myapp/hostname is just a script: mvn clean install && cd webapp && mvn jetty:run }}} 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: {{{ 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: 1. It's really hard to read, to keep track of, 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 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.) 4. If the program handles unsanitized user input, it can be [[BashFAQ/048|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. === Indirection === ==== Think before using indirection ==== Putting variable names or any other [[BashFAQ/050|bash syntax inside parameters]] is generally a bad idea. It violates the separation between code and data, and as such puts you on a slippery slope toward bugs, security issues, etc. ''Even'' when you know you "got it right", because you "know and ''understand'' exactly what you're doing", bugs happen to all of us and it pays to respect separation practices to minimize the extent of damage they can cause. Aside from that, it also makes your code non-obvious and non-transparent. 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 [[BashGuide/Arrays|Bash Arrays]] or haven't fully considered other Bash features such as functions. ==== Evaluating indirect/reference variables ==== |
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Unfortunately, for shells other than Bash and ksh93, there is no syntax for ''evaluating'' a referenced variable. You would have to use [[BashFAQ/048|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). |
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| We are not aware of any trick that can duplicate that functionality in Bash, POSIX or Bourne shells (short of using [[BashFAQ/048|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 [[BashFAQ/048|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 |
We are not aware of any trick that can duplicate that functionality in POSIX or Bourne shells (short of using [[BashFAQ/048|eval]], which is extremely difficult to do securely). 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. {{{ # Bash -- trick #1. Seems to work in bash 2 and up. realarray=(...) ref=realarray; index=2 tmp="$ref[$index]" echo "${!tmp}" # gives array element [2] # Bash -- trick #2. Seems to work in bash 3 and up. # Does NOT work in bash 2.05b. tmp="$ref[@]" printf "<%s> " "${!tmp}"; echo # Iterate whole array. }}} It is not possible to retrieve array indices directly using the Bash ''${!var}'' indirect expansion. ==== Assigning indirect/reference variables ==== Sometimes you'd like to "point" from one variable to another, for purposes of writing information to a dynamically configurable place. Typically this happens when you're trying to write a "reusable" function or library, and you want it to [[BashFAQ/084|put its output]] in a variable of the caller's choice instead of the function's choice. Various traits of Bash make safe ([[BashWeaknesses|reusability of Bash functions]] difficult at best, so this is something that should not happen ''often''.) Assigning a value "through" a reference (I'm going to use "ref" from now on) is more widely possible, but the means of doing so are usually extremely shell-specific. All shells with the sole exception of AT&T ksh93 lack real reference variables or pointers. Indirection can '''only''' be achieved by indirectly evaluating variable names. IOW, you can never have a real unambiguous reference to an object in memory, the best you can do is use the name of a variable to try simulating the effect. Therefore, '''you must control the value of the ref''' and ensure side-effects such as globbing, user-input, and conflicting local parameters can't affect parameter names. Names must either be deterministic or validated in a way that makes certain guarantees. If an end user can populate the ref variable with arbitrary strings, the result can be unexpected code injection. We'll show an example of this at the end. In ksh93, we can use `nameref` again: {{{ # ksh93/mksh/Bash 4.3 |
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| ref="contents" | ref=contents |
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| In Bash, we can use {{{read}}} and Bash's ''here string'' syntax: {{{ |
In Bash, we can use {{{read}}} and Bash's [[HereDocument|here string]] syntax: {{{ # Bash/ksh93/mksh/zsh ref=realvariable IFS= read -rd '' "$ref" <<<contents # realvariable now contains the string "contents" }}} If you need to assign multiline values, keep reading. A similar trick works for Bash array variables too: {{{ # Bash/ksh93/mksh/zsh typeset readFix=${BASH_VERSION+a} aref=realarray IFS=' ' read -d '' -r${readFix:-A} "$aref" <<<'words go into array elements' echo "${realarray[1]}" # prints "go" }}} [[IFS]] is used to delimit words, so you may or may not need to set that. Another trick is to use Bash's {{{printf -v}}} (only available in [[BashFAQ/061|recent versions]]): {{{ # Bash 3.1 or higher ONLY. Array assignments require 4.2 or higher. ref=realvariable printf -v "$ref" %s "contents" }}} You can use all of {{{printf}}}'s formatting capabilities. This trick also permits any string content, including embedded and trailing newlines. Yet another trick is Korn shell's {{{typeset}}} or Bash's {{{declare}}}. The details of `typeset` vary greatly between shells, but can be used in compatible ways in limited situations. Both of them cause a variable to become ''locally scoped'' to a function, if used inside a function; but if used outside all functions, they can operate on global variables. {{{ # Bash/ksh (any)/zsh typeset "${ref}=contents" |
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| 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 [[BashFAQ/061|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 $ref="contents" |
declare "${ref}=contents" }}} Bash 4.2 adds `declare -g` which assigns variables to the global scope from any context. There is very little advantage to `typeset` or `declare` over `eval` for scalar assignments, but many drawbacks. `typeset` cannot be made to not affect the scope of the assigned variable. This trick does preserve the exact contents, like `eval`, if correctly escaped. You must still be careful about what is on the ''left''-hand side of the assignment. Inside square brackets, expansions are still performed; thus, with a tainted ref, `declare` can be just as dangerous as `eval`: {{{ |
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| declare $ref="contents" }}} The advantage of using `typeset` or `declare` over `eval` is that the right hand side of the assignment is ''not'' parsed by the shell. If you used `eval` here, you would have to sanitize/escape the entire right hand side first. If you aren't using Bash or Korn shell, you can still do assignments to referenced variables using ''here document'' syntax: |
ref='x[$(touch evilfile; echo 0)]' ls -l evilfile # No such file or directory declare "${ref}=value" ls -l evilfile # It exists now! }}} 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. If you aren't using Bash or Korn shell, you can do assignments to referenced variables using HereDocument syntax: |
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| read $ref <<EOF | read -r "$ref" <<EOF |
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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". [[KornShell|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. 1. 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. 1. 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. 1. If the program handles unsanitized user input, it can be [[BashFAQ/048|VERY dangerous]]! There is, howver, no danger if the right-hand variable expansion is done on the second pass: |
(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. ===== eval ===== {{{ # Bourne ref=myVar eval "${ref}=\$value" }}} This expands to the statement that is executed: {{{ myVar=$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 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: |
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| var=myVar eval "$var=\$value" |
eval ${ref}=\"$value\" # WRONG! eval "$ref='$value'" # WRONG! eval "${ref}=\$value" # Correct (curly braced PE used for clarity) eval "$ref"'=$value' # Correct (equivalent) |
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| This expands to the statment that is executed: | The good news is that if you can sanitize the right hand side correctly, this trick is fully portable, has no variable scope issues, and allows all content including newlines. The bad news is that if you fail to sanitize the right hand side correctly, you have a massive security hole. Use `eval` if you know what you're doing and are very careful. The following is inefficient, ugly, easy to do incorrectly, and usually unnecessary. This simply documents the single-quote escaping method. It may be useful in cases where there are large numbers of metacharacters that must be put into a string literal and eval'd. You're almost always better off constructing the string in advance using e.g. a heredoc with an escaped sentinel and then just expanding a single variable on the RHS of the assignment as demonstrated in the above example. |
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| myVar=$value | # POSIX evil="';" value='echo fail' ref=x { eval "${ref}='$(sed -e "s/'/'\\\''/g")'"; } <<EOF ${evil}${value} EOF [ "${evil}${value}" = "$x" ] && echo success |
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| The right-hand side is not parsed by the shell, and there is no danger of unwanted side effects. | {{{ # Bash/ksh93/zsh eval "$(printf %s=%q "$ref" "$value")" }}} === See Also === * [[http://wiki.bash-hackers.org/syntax/arrays#indirection | More advanced indirection on arrays]] * [[BashFAQ/005]] * [[BashGuide/Arrays]] * [[BashSheet#Arrays|BashSheet Array reference]] ---- CategoryShell |
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 three parts: associative arrays, 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 in the majority of cases where people are trying to use indirect variable assignments/evaluations, they ought to be using associative arrays instead. For instance, we frequently see people asking how they can have a bunch of related variables like IPaddr_hostname1, IPaddr_hostname2 and so on. A more appropriate way to store this data would be in an associative array named IPaddr which is indexed by the hostname.
To map from one string to another, you need arrays indexed by a string instead of a number. These exists in AWK as "associative arrays", in Perl as "hashes", and in Tcl simply as "arrays". They also exist in ksh93, where you'd use them like this:
# 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 supports them from version 4 and up:
# Bash 4 and up declare -A homedir homedir[jim]=/home/jim # or homedir=( [jim]=/home/jim [silvia]=/home/silvia [alex]=/home/alex ) ...
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 subservient 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:
source "$conf" for host in "${!commands[@]}"; do ssh "$host" "${commands[$host]}" done # Where "$conf" is a file like this: declare -A commands commands=( [host1]="mvn clean install && cd webapp && mvn jetty:run" [host2]="..." )
This is the kind of approach we'd expect in a high-level language, where we can store hierarchical information in advanced data structures. The difficulty here is that we really want each element of the associative array to be a list or another array of command strings. But the shell simply doesn't permit 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:
# A series of conf files named for the hosts we need to run our commands on: for conf in /etc/myapp/*; do host=${conf##*/} ssh "$host" bash < "$conf" done # /etc/myapp/hostname is just a script: mvn clean install && cd webapp && mvn jetty:run
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:
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 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.
Indirection
Think before using indirection
Putting variable names or any other bash syntax inside parameters is generally a bad idea. It violates the separation between code and data, and as such puts you on a slippery slope toward bugs, security issues, etc. Even when you know you "got it right", because you "know and understand exactly what you're doing", bugs happen to all of us and it pays to respect separation practices to minimize the extent of damage they can cause.
Aside from that, it also makes your code non-obvious and non-transparent.
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.
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
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.
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).
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 ${#ref[*]} elements" } realarray=(...) myfunc realarray
We are not aware of any trick that can duplicate that functionality in POSIX or Bourne shells (short of using eval, which is extremely difficult to do securely). Bash can almost do it -- some indirect array tricks work, and others do not, and we do not know whether the syntax involved will remain stable in future releases. So, consider this a use at your own risk hack.
# Bash -- trick #1. Seems to work in bash 2 and up. realarray=(...) ref=realarray; index=2 tmp="$ref[$index]" echo "${!tmp}" # gives array element [2] # Bash -- trick #2. Seems to work in bash 3 and up. # Does NOT work in bash 2.05b. tmp="$ref[@]" printf "<%s> " "${!tmp}"; echo # Iterate whole array.
It is not possible to retrieve array indices directly using the Bash ${!var} indirect expansion.
Assigning indirect/reference variables
Sometimes you'd like to "point" from one variable to another, for purposes of writing information to a dynamically configurable place. Typically this happens when you're trying to write a "reusable" function or library, and you want it to put its output in a variable of the caller's choice instead of the function's choice. Various traits of Bash make safe (reusability of Bash functions difficult at best, so this is something that should not happen often.)
Assigning a value "through" a reference (I'm going to use "ref" from now on) is more widely possible, but the means of doing so are usually extremely shell-specific. All shells with the sole exception of AT&T ksh93 lack real reference variables or pointers. Indirection can only be achieved by indirectly evaluating variable names. IOW, you can never have a real unambiguous reference to an object in memory, the best you can do is use the name of a variable to try simulating the effect. Therefore, you must control the value of the ref and ensure side-effects such as globbing, user-input, and conflicting local parameters can't affect parameter names. Names must either be deterministic or validated in a way that makes certain guarantees. If an end user can populate the ref variable with arbitrary strings, the result can be unexpected code injection. We'll show an example of this at the end.
In ksh93, we can use nameref again:
# ksh93/mksh/Bash 4.3 nameref ref=realvariable ref=contents # realvariable now contains the string "contents"
In Bash, we can use read and Bash's here string syntax:
# Bash/ksh93/mksh/zsh ref=realvariable IFS= read -rd '' "$ref" <<<contents # realvariable now contains the string "contents"
If you need to assign multiline values, keep reading.
A similar trick works for Bash array variables too:
# Bash/ksh93/mksh/zsh typeset readFix=${BASH_VERSION+a} aref=realarray IFS=' ' read -d '' -r${readFix:-A} "$aref" <<<'words go into array elements' echo "${realarray[1]}" # prints "go"
IFS is used to delimit words, so you may or may not need to set that.
Another trick is to use Bash's printf -v (only available in recent versions):
# Bash 3.1 or higher ONLY. Array assignments require 4.2 or higher. ref=realvariable printf -v "$ref" %s "contents"
You can use all of printf's formatting capabilities. This trick also permits any string content, including embedded and trailing newlines.
Yet another trick is Korn shell's typeset or Bash's declare. The details of typeset vary greatly between shells, but can be used in compatible ways in limited situations. Both of them cause a variable to become locally scoped to a function, if used inside a function; but if used outside all functions, they can operate on global variables.
# Bash/ksh (any)/zsh typeset "${ref}=contents" # Bash declare "${ref}=contents"
Bash 4.2 adds declare -g which assigns variables to the global scope from any context.
There is very little advantage to typeset or declare over eval for scalar assignments, but many drawbacks. typeset cannot be made to not affect the scope of the assigned variable. This trick does preserve the exact contents, like eval, if correctly escaped.
You must still be careful about what is on the left-hand side of the assignment. Inside square brackets, expansions are still performed; thus, with a tainted ref, declare can be just as dangerous as eval:
# Bash: ref='x[$(touch evilfile; echo 0)]' ls -l evilfile # No such file or directory declare "${ref}=value" ls -l evilfile # It exists now!
This problem also exists with typeset in mksh and pdksh, but apparently not ksh93. This is why the value of ref must be under your control at all times.
If you aren't using Bash or Korn shell, you can do assignments to referenced variables using HereDocument syntax:
# Bourne ref=realvariable read -r "$ref" <<EOF contents EOF
(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.
eval
# Bourne ref=myVar eval "${ref}=\$value"
This expands to the statement that is executed:
myVar=$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 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:
eval ${ref}=\"$value\" # WRONG!
eval "$ref='$value'" # WRONG!
eval "${ref}=\$value" # Correct (curly braced PE used for clarity)
eval "$ref"'=$value' # Correct (equivalent)The good news is that if you can sanitize the right hand side correctly, this trick is fully portable, has no variable scope issues, and allows all content including newlines. The bad news is that if you fail to sanitize the right hand side correctly, you have a massive security hole. Use eval if you know what you're doing and are very careful.
The following is inefficient, ugly, easy to do incorrectly, and usually unnecessary. This simply documents the single-quote escaping method. It may be useful in cases where there are large numbers of metacharacters that must be put into a string literal and eval'd. You're almost always better off constructing the string in advance using e.g. a heredoc with an escaped sentinel and then just expanding a single variable on the RHS of the assignment as demonstrated in the above example.
# POSIX
evil="';"
value='echo fail'
ref=x
{ eval "${ref}='$(sed -e "s/'/'\\\''/g")'"; } <<EOF
${evil}${value}
EOF
[ "${evil}${value}" = "$x" ] && echo success# Bash/ksh93/zsh eval "$(printf %s=%q "$ref" "$value")"