ldd Command Linux: Display Shared Library Dependencies

The ldd command is an essential Linux utility that displays the shared library dependencies of executable files and shared libraries. Understanding how to use ldd is crucial for system administrators, developers, and anyone working with Linux systems to troubleshoot library issues, analyze dependencies, and ensure proper application deployment.

What is the ldd Command?

The ldd (List Dynamic Dependencies) command is a shell script that prints the shared libraries required by each program or shared library specified on the command line. It works by invoking the dynamic linker to resolve dependencies and display the mapping between library names and their actual file locations.

Syntax and Basic Usage

The basic syntax of the ldd command is:

ldd [options] file...

Let’s start with a simple example:

$ ldd /bin/ls
	linux-vdso.so.1 (0x00007fff8b7fe000)
	libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f8b2c8e0000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f8b2c6f0000)
	libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007f8b2c650000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f8b2c640000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f8b2c920000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f8b2c620000)

This output shows that the ls command depends on several shared libraries, including the C library (libc.so.6), SELinux library (libselinux.so.1), and others.

Command Options

The ldd command supports several useful options:

-v (Verbose Mode)

The -v option provides verbose output, showing version information for libraries:

$ ldd -v /bin/ls
	linux-vdso.so.1 (0x00007fff8b7fe000)
	libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f8b2c8e0000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f8b2c6f0000)
	libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007f8b2c650000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f8b2c640000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f8b2c920000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f8b2c620000)

	Version information:
	/bin/ls:
		libc.so.6 (GLIBC_2.3.4) => /lib/x86_64-linux-gnu/libc.so.6
		libc.so.6 (GLIBC_2.2.5) => /lib/x86_64-linux-gnu/libc.so.6

-d (Data Relocations)

The -d option performs relocations and reports missing objects:

$ ldd -d /usr/bin/gcc

-r (Function Relocations)

The -r option performs relocations for both data objects and functions:

$ ldd -r /usr/bin/python3

-u (Unused Dependencies)

The -u option shows unused direct dependencies:

$ ldd -u /bin/cat

Understanding the Output

The ldd output format consists of several components:

• Library name: The name of the shared library
• Arrow (=>): Indicates the mapping to the actual file
• Full path: The complete path to the library file
• Memory address: The virtual memory address where the library is loaded

Special cases in the output include:

• linux-vdso.so.1: Virtual Dynamic Shared Object, a kernel mechanism
• ld-linux-x86-64.so.2: The dynamic linker/loader
• statically linked: Indicates a statically linked executable

Practical Examples

Checking System Binaries

Let’s examine the dependencies of common system binaries:

$ ldd /bin/bash
	linux-vdso.so.1 (0x00007ffd123df000)
	libtinfo.so.6 => /lib/x86_64-linux-gnu/libtinfo.so.6 (0x00007f8a1c8b0000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f8a1c8a0000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f8a1c6b0000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f8a1c8f0000)

Analyzing Custom Applications

For a custom compiled program:

$ gcc -o hello hello.c
$ ldd ./hello
	linux-vdso.so.1 (0x00007ffd5b3fe000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9a2c8b0000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f9a2c8f0000)

Checking Library Dependencies

You can also use ldd on shared libraries themselves:

$ ldd /usr/lib/x86_64-linux-gnu/libssl.so.1.1
	linux-vdso.so.1 (0x00007ffd123df000)
	libcrypto.so.1.1 => /usr/lib/x86_64-linux-gnu/libcrypto.so.1.1 (0x00007f8a1c6b0000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f8a1c4c0000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f8a1c4b0000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f8a1c490000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f8a1c8f0000)

Troubleshooting with ldd

Missing Libraries

When a library is missing, ldd will show it clearly:

$ ldd ./broken_app
	linux-vdso.so.1 (0x00007ffd123df000)
	libmissing.so.1 => not found
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f8a1c6b0000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f8a1c8f0000)

This indicates that libmissing.so.1 cannot be found in the library search path.

Version Conflicts

Use the verbose option to identify version conflicts:

$ ldd -v /usr/bin/application
	Version information:
	/usr/bin/application:
		libssl.so.1.1 (OPENSSL_1_1_0) => not found

Security Considerations

Important security considerations when using ldd:

• Never run ldd on untrusted executables: ldd may execute the target program to determine dependencies
• Use alternative tools for untrusted files: Consider using objdump or readelf instead
• Check for suspicious library paths: Libraries loaded from unusual locations may indicate security issues

For safer analysis of untrusted binaries, use:

$ objdump -p /path/to/binary | grep NEEDED
$ readelf -d /path/to/binary | grep NEEDED

Alternative Tools and Methods

Using objdump

The objdump command provides similar information without executing the binary:

$ objdump -p /bin/ls | grep NEEDED
  NEEDED               libselinux.so.1
  NEEDED               libc.so.6

Using readelf

The readelf command is another safe alternative:

$ readelf -d /bin/ls | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libselinux.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]

Advanced Usage Scenarios

Dependency Analysis Scripts

You can create scripts to analyze dependencies across multiple files:

#!/bin/bash
for file in /usr/bin/*; do
    if [ -x "$file" ] && [ -f "$file" ]; then
        echo "Dependencies for $file:"
        ldd "$file" 2>/dev/null | grep -E "=> /"
        echo "---"
    fi
done

Finding All Executables Using a Specific Library

$ find /usr/bin -executable -type f -exec sh -c 'ldd {} 2>/dev/null | grep -q "libssl"' \; -print

Performance Considerations

When working with ldd, keep in mind:

• Large binaries: Complex applications may have many dependencies, making output lengthy
• Network filesystems: Libraries on network mounts may slow down ldd execution
• Recursive dependencies: Use scripts carefully to avoid infinite loops when analyzing interdependent libraries

Common Use Cases

Software Deployment

Before deploying applications to new systems, use ldd to ensure all dependencies are available:

$ ldd /opt/myapp/bin/application
$ # Check if all libraries are found and paths are correct

System Maintenance

Identify which applications depend on libraries you plan to update:

$ find /usr/bin -type f -executable -exec sh -c 'ldd {} 2>/dev/null | grep -q "libcurl"' \; -print

Development and Debugging

During development, use ldd to verify your application links correctly:

$ gcc -o myapp myapp.c -lcurl
$ ldd ./myapp | grep curl

Best Practices

• Always verify library paths before deploying applications
• Use version-specific checks with the -v option for critical applications
• Document dependencies for your applications to aid in deployment
• Regular dependency audits help maintain system security and stability
• Combine with package management tools for comprehensive dependency management

Conclusion

The ldd command is an indispensable tool for Linux system administration and development. It provides crucial insights into shared library dependencies, helping with troubleshooting, deployment planning, and system maintenance. By understanding how to use ldd effectively, you can better manage your Linux systems, resolve dependency issues, and ensure smooth application deployment.

Remember to always exercise caution when using ldd on untrusted binaries, and consider using alternative tools like objdump and readelf for security-sensitive scenarios. With practice, ldd becomes an essential part of your Linux toolkit for understanding and managing shared library dependencies.