eval

eval [argument...]

eval is a POSIX special shell builtin that concatenates its arguments separated by spaces, then reads and executes the resulting string as a shell command. This enables dynamic command construction and double expansion of variables.The command is useful when command strings are built programmatically or stored in variables. It allows variable indirection (accessing a variable whose name is in another variable). A common real-world use is initializing ssh-agent with `eval "$(ssh-agent -s)"`.If there are no arguments or only null arguments, eval returns exit status 0. Otherwise it returns the exit status of the executed command.

ARGUMENT

Arguments to concatenate and execute.

Security risk with untrusted input. Because arguments are re-parsed by the shell, eval with user-supplied strings is functionally equivalent to system(unsanitized) in C - it can run arbitrary commands, including via metacharacters and command substitution. Quoting must be planned in two layers (the layer eval sees and the layer the resulting command sees), which makes debugging difficult. Prefer arrays ("$cmd[@]" in bash/zsh) or printf -v when only variable indirection is needed, and reserve eval for cases like eval "$(ssh-agent -s)" where the input is trusted.

eval is a standard POSIX special shell builtin defined in IEEE Std 1003.1 (POSIX.1), present in the original Bourne shell and all its derivatives including bash, zsh, ksh, and dash. It provides essential metaprogramming capabilities for shell scripts.

bash(1), sh(1), exec(1), set(1)