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eval

shell builtinLinux/Unix
The eval command is one of the most frequently used commands in Linux/Unix-like operating systems. eval Execute arguments as a shell command
See ExamplesView Syntax

Quick Reference

Command Name:

eval

Category:

shell builtin

Platform:

Linux/Unix

Basic Usage:

eval [options] [arguments]

Common Use Cases

  • 1

    Dynamic command execution

    Execute commands constructed at runtime

  • 2

    Variable indirection

    Access variables whose names are stored in other variables

  • 3

    Command output processing

    Execute the output of a command as shell code

  • 4

    Complex command construction

    Build and execute complex commands programmatically

Syntax

eval [arguments]

Options

Option Description
eval doesn't accept options like standard commands. It simply evaluates its arguments as a command.

Examples

How to Use These Examples

The examples below show common ways to use the eval command. Try them in your terminal to see the results. You can copy any example by clicking on the code block.

# Basic Examples Basic
eval "echo Hello, World"
Execute the command "echo Hello, World".
eval "ls -la"
Execute the command "ls -la".
# Advanced Examples Advanced
cmd="ls -la" eval $cmd Store a command in a variable and execute it with eval. eval "for i in {1..3}; do echo $i; done" Execute a complex command, in this case a for loop. eval $(ssh-agent) Execute the output of ssh-agent as commands. var="my_value" eval "echo \$$var" Use eval for variable indirection (evaluating the contents of a variable named in another variable).

Try It Yourself

Practice makes perfect! The best way to learn is by trying these examples on your own system with real files.

Understanding Syntax

Pay attention to the syntax coloring: commands, options, and file paths are highlighted differently.

Notes

The eval command is a shell built-in that concatenates its arguments into a single string, and then executes that string as a command in the current shell environment. This makes it a powerful tool for dynamic command execution. Eval is particularly useful in several scenarios: 1. Variable indirection: Accessing the value of a variable whose name is stored in another variable 2. Dynamic command construction: Building and executing commands on the fly 3. Processing command output as shell commands (like with ssh-agent) 4. Executing complex commands stored in variables However, eval should be used with caution, as it can introduce security vulnerabilities if used with untrusted input. Malicious code within the evaluated string will be executed with the permissions of the current user. Principles for safe eval usage: - Never use eval on untrusted input - Carefully quote variables used within eval - Consider alternatives like arrays or functions when possible Eval is a powerful tool, but should be used sparingly and carefully in shell scripts.

Tips & Tricks

1

Always quote variables used within eval to prevent unexpected word splitting

2

Avoid using eval with untrusted input as it can lead to code injection

3

Use eval with ssh-agent to set up the SSH agent environment

4

For variable indirection, consider using ${!varname} syntax in bash when possible

5

Use eval when processing complex command strings with nested variables

Common Use Cases

Dynamic command execution

Execute commands constructed at runtime

Variable indirection

Access variables whose names are stored in other variables

Command output processing

Execute the output of a command as shell code

Complex command construction

Build and execute complex commands programmatically

Shell initialization

Process configuration commands from initialization scripts

Related Commands

These commands are frequently used alongside eval or serve similar purposes:

source
exec
bash
sh
command
type
alias
set
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Use Cases

1

Dynamic command execution

Execute commands constructed at runtime

2

Variable indirection

Access variables whose names are stored in other variables

3

Command output processing

Execute the output of a command as shell code

4

Complex command construction

Build and execute complex commands programmatically

5

Shell initialization

Process configuration commands from initialization scripts

Learn By Doing

The best way to learn Linux commands is by practicing. Try out these examples in your terminal to build muscle memory and understand how the eval command works in different scenarios.

See ExamplesTry Lab Exercises
$ eval
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