wiki.bash-hackers.org/commands/builtin/eval.md

# The eval builtin command

## Synopsis

    eval: eval [arg ...]

## Description

`eval` takes its arguments, concatenates them separated by spaces, and
executes the resulting string as Bash code in the current execution
environment. `eval` in Bash works in essentially the same way as most
other languages that have an `eval` function. Perhaps the easiest way to
think about `eval` is that it works in the same way as running ''bash -c
"bash code..." ''from a script, except in the case of `eval`, the given
code is executed in the current shell environment rather than a child
process.

## Examples

In this example, the literal text within the
[here-document](/syntax/redirection.md#here_documents) is executed as Bash
code exactly as though it were to appear within the script in place of
the `eval` command below it.

    #!/usr/bin/env bash
    { myCode=$(</dev/stdin); } <<\EOF
    ... arbitrary bash code here ...
    EOF

    eval "$myCode"

### Expansion side-effects

Frequently, `eval` is used to cause side-effects by performing a pass of
expansion on the code before executing the resulting string. This allows
for things that otherwise wouldn't be possible with ordinary Bash
syntax. This also, of course, makes `eval` the most powerful command in
all of shell programming (and in most other languages for that matter).

This code defines a set of identical functions using the supplied names.
`eval` is the only way to achieve this effect.

    main() {
        local fun='() { echo "$FUNCNAME"; }' x

        for x in {f..n}; do
            eval "${x}${fun}"
        done

        "$@"
    }

    main "$@"

### Using printf %q

The `printf %q` format string performs shell escaping on its arguments.
This makes `printf %q` the "anti-eval" - with each pass of a string
through printf requiring another `eval` to peel off the escaping again.

    while (( ++n <= 5 )) || ! evalBall="eval $evalBall"; do
        printf -v evalBall 'eval %q' "printf $n;${evalBall-printf '0\n'}"
    done
    $evalBall

The above example is mostly fun and games but illustrates the
`printf %q` property.

### Higher-order functions

Since all current POSIX-compatible shells lack support for [first-class
functions](http://en.wikipedia.org/wiki/First-class_function), it can be
tempting and sometimes useful to simulate some of their effect using
`eval` to evaluate a string containing code.

This example shows [partial
application](http://en.wikipedia.org/wiki/Partial_application) using
`eval`.

    function partial {
        eval shift 2 \; function "$1" \{ "$2" "$(printf '%q ' "${@:3}")" '"$@"; }'
    }

    function repeat {
        [[ $1 == +([0-9]) ]] || return
        typeset n
        while ((n++ < $1)); do
            "${@:2}"
        done
    }

    partial print3 repeat 3 printf '%s ' # Create a new function named print3
    print3 hi                            # Print "hi" 3 times
    echo

This is very easy to do incorrectly and not usually considered idiomatic
of Bash if used extensively. However abstracting eval behind functions
that validate their input and/or make clear which input must be
controlled carefully by the caller is a good way to use it.

## Portability considerations

- Unfortunately, because eval is a **special builtin**, it only gets its
  own environment in Bash, and only when Bash is not in POSIX mode. In
  all other shells plus Bash in POSIX mode, the environment of eval will
  leak out into the surrounding environment. It is possible to work
  around this limitation by prefixing special builtins with the
  `command` regular builtin, but current versions of ~~ksh93~~ and zsh
  don't do this properly
  ([fixed](http://article.gmane.org/gmane.comp.programming.tools.ast.devel/686)
  in ksh 93v- 2012-10-24 alpha). Earlier versions of zsh work (with
  `setopt POSIX_BUILTINS` -- looks like a regression). This works
  correctly in Bash POSIX mode, Dash, and mksh.

<!-- -->

- `eval` is another one of the few Bash builtins with keyword-like
  conditional parsing of arguments that are in the form of compound
  assignments.

<!-- -->

     $ ( eval a=( a b\\ c d ); printf '<%s> ' "${a[@]}"; echo ) # Only works in Bash.
    <a> <b c> <d>
     $ ( x=a; eval "$x"=( a b\\ c d ); printf '<%s> ' "${a[@]}"; echo ) # Argument is no longer in the form of a valid assignment, therefore ordinary parsing rules apply.
    -bash: syntax error near unexpected token `('
     $ ( x=a; eval "$x"'=( a b\ c d )'; printf '<%s> ' "${a[@]}"; echo ) # Proper quoting then gives us the expected results.
    <a> <b c> <d>

We don't know why Bash does this. Since parentheses are metacharacters,
they must ordinary be quoted or escaped when used as arguments. The
first example above is the same error as the second in all non-Bash
shells, even those with compound assignment.

In the case of `eval` it isn't recommended to use this behavior, because
unlike e.g. [declare](commands/builtin/declare), the initial expansion
is still subject to all expansions including
[word-splitting](syntax/expansion/wordsplit) and [pathname
expansion](syntax/expansion/glob).

     $ ( set -x; touch 'x+=(\[[123]\]=*)' 'x+=([3]=yo)'; eval x+=(*); echo "${x[@]}" )
    + touch 'x+=(\[[123]\]=*)' 'x+=([3]=yo)'
    + eval 'x+=(\[[123]\]=*)' 'x+=([3]=yo)'
    ++ x+=(\[[123]\]=*)
    ++ x+=([3]=yo)
    + echo '[[123]]=*' yo
    [[123]]=* yo

Other commands known to be affected by compound assignment arguments
include: [let](commands/builtin/let),
[declare](commands/builtin/declare),
[typeset](commands/builtin/typeset), [local](commands/builtin/local),
[export](commands/builtin/export), and
[readonly](commands/builtin/readonly). More oddities below show both
similarities and differences to commands like
[declare](commands/builtin/declare). The rules for `eval` appear
identical to those of [let](commands/builtin/let).

## See also

- [BashFAQ 48 - eval and security
  issues](http://mywiki.wooledge.org/BashFAQ/048) -- **IMPORTANT**
- [Another eval
  article](http://fvue.nl/wiki/Bash:_Why_use_eval_with_variable_expansion%3F)
- [Indirection via
  eval](http://mywiki.wooledge.org/BashFAQ/006#Assigning_indirect.2BAC8-reference_variables)
- [More indirection via
  eval](http://fvue.nl/wiki/Bash:_Passing_variables_by_reference)
- [Martin Väth's "push"](https://github.com/vaeth/push) -- `printf %q`
  work-alike for POSIX.