eval

Execute arguments as a shell command.

Source: src/execution/eval_builtin.f90:10-38

Synopsis

eval [string ...]

Description

The eval builtin concatenates its arguments, separated by spaces, and executes the result as a shell command. This allows for dynamic command construction and indirect variable access.

Usage

Basic Evaluation

cmd="echo hello"
eval $cmd
# hello

Variable Construction

name="PATH"
eval echo \$$name
# /usr/bin:/bin:...

Dynamic Variable Names

prefix="user"
eval "${prefix}_name='John'"
echo $user_name
# John

Examples

Indirect Variable Access

get_value() {
    local varname="$1"
    eval echo "\$$varname"
}

myvar="hello"
get_value myvar
# hello

Command Building

# Build command with conditionals
cmd="ls"
[[ $verbose ]] && cmd+=" -l"
[[ $all ]]     && cmd+=" -a"
eval $cmd

Array From String

items="one two three"
eval "arr=($items)"
echo "${arr[1]}"
# two

Dynamic Assignment

set_var() {
    local name="$1"
    local value="$2"
    eval "$name='$value'"
}

set_var greeting "Hello, World"
echo $greeting
# Hello, World

Pipeline Construction

build_pipeline() {
    local cmd="cat file.txt"
    [[ $filter ]] && cmd+=" | grep '$filter'"
    [[ $sort ]]   && cmd+=" | sort"
    eval $cmd
}

Security Considerations

eval is dangerous with untrusted input!

# DANGEROUS - command injection
user_input="; rm -rf /"
eval "echo $user_input"   # Executes rm!

# SAFER - validate input first
if [[ $user_input =~ ^[a-zA-Z0-9_]+$ ]]; then
    eval "echo $user_input"
fi

Safe Alternatives

# Instead of eval for indirect access:
declare -n ref=varname    # Nameref (if supported)
echo "$ref"

# Instead of eval for arrays:
read -ra arr <<< "$string"

When to Use eval

Appropriate uses:

Indirect variable expansion
Dynamic command generation from trusted sources
Complex quoting scenarios
Shell-generated code execution

Avoid when:

User input is involved
Simpler alternatives exist
Security is a concern

Implementation

Source: eval_builtin.f90:10-38

subroutine execute_eval(cmd, shell)
  ! Concatenate all arguments
  ! Parse as shell command
  ! Execute in current context
end subroutine

Exit Status

Status	Condition
0	Success (or empty string)
Other	Exit status of evaluated command

Common Patterns

Variable Indirection

# Get value
indirect() {
    eval echo "\"\$$1\""
}

# Set value
set_indirect() {
    eval "$1=\"\$2\""
}

Export Dynamic Variable

export_var() {
    local name="$1"
    local value="$2"
    eval "export $name='$value'"
}

Parse Key=Value Pairs

while IFS='=' read -r key value; do
    eval "$key='$value'"
done < config.txt

Quoting with eval

# Single expansion
eval echo '$HOME'        # Literal $HOME
eval echo "\$HOME"       # Expands $HOME

# Double expansion
var='$HOME'
eval echo $var           # Expands to home directory

# Preserve spaces
text="hello   world"
eval "echo \"$text\""    # Preserves spacing