Termux is not FHS compliant
Termux does things a bit differently from regular Linux systems, which can
affect how you use it. Unlike most Linux distributions, Termux doesn't stick
to the Filesystem Hierarchy Standard (FHS). This means you won't find
familiar directories like /bin, /etc, or /usr in their usual places. Because
of this, programs need to be adjusted and recompiled to work correctly in
the Termux environment, or they won't be able to find their configuration
files or other necessary data.
If you try to run scripts with standard shebangs (like #!/bin/sh), you might
encounter issues. To fix this, you can use the
termux-fix-shebang script to modify these files before
running them. Alternatively, recent versions of Termux include a special
package (termux-exec) that allows the use of standard shebangs.
Many packages also have dependencies on shared libraries, which Termux
installs in $PREFIX/lib. On devices running Android versions before 7,
Termux uses the $LD_LIBRARY_PATH variable to tell the linker where to find
these shared library files. For Android 7 and higher, the DT_RUNPATH ELF
header attribute is used instead of LD_LIBRARY_PATH.
If you really need a more traditional Linux file system layout for some
reason, you can use the termux-chroot utility from the 'proot' package. This
utility sets up a chroot environment that provides standard paths like /tmp,
/etc, and /usr:
$ pkg install proot
$ termux-chroot
$ ls /usr
bin doc etc include lib libexec share tmp var
Using termux-chroot can be helpful if you're using custom software that
requires these standard paths to be available.
Termux uses Bionic libc
|
| Shell could seem to be stupid saying that file does not exist but you know it exists. However in this case shell is actually right. The dynamic linker is not available at standard path. Use strace to see what's going on. |
To ensure our packages work well with Android and to avoid the need for
custom toolchains, we compile everything using the Android NDK. The
resulting binaries are linked against the Bionic libc (files like libc.so,
libm.so, libdl.so from /system/lib or /system/lib64).
Due to the differences between Android's libc and the standard Linux libc,
it's not possible to directly run native packages copied from Linux
distributions:
1. Dynamically linked programs won't run because the linker is expected in a
location that doesn't exist (/lib), and the libc Application Binary
Interface (ABI) doesn't match.
2. Statically linked programs (only networking ones) won't be able to
resolve DNS names because GNU libc doesn't normally allow static linking
with the resolver. Additionally, the file /etc/resolv.conf, which is used
for DNS resolution, doesn't exist on Android.
3. On non-rooted Android devices running version 8 or newer, statically
linked programs won't run due to issues with the seccomp filter, a security
feature.
To work around these restrictions, you can set up a Linux distribution root
filesystem using PRoot, which allows you to run Linux binaries on Android.
Root file system is stored as ordinary application data
The root file system and user home directory in Termux are stored in a
private application data directory on the /data partition. This directory is
accessible through the $PREFIX and $HOME variables. It's crucial to note
that you cannot move $PREFIX to another location because all programs expect
it to remain unchanged. Additionally, you cannot store binaries, symlinks,
or other files from $PREFIX on an sdcard due to the filesystem not
supporting Unix permissions, symlinks, sockets, etc.
It's important to back up all important data before uninstalling the Termux
application or wiping its data, as this will also remove the directories
$PREFIX and $HOME, along with their contents.
Termux is single-user
Android applications, including Termux, are sandboxed and run with their own
Linux user ID and SELinux label. In Termux, everything is executed with the
same user ID as the Termux application itself, which typically looks like
u0_a231 and cannot be changed.
To enhance security, all Termux packages (except root-only ones) are patched
to drop any multiuser, setuid/setgid, and similar functionality.
Additionally, default ports for server packages like ftpd, httpd, and sshd
are changed to 8021, 8080, and 8022, respectively.
You have free read-write access to all application components, including
$PREFIX. However, be cautious, as it's easy to accidentally delete or
overwrite files in $PREFIX, which can lead to issues.
Conclusion
Termux provides a powerful environment on Android for running Linux packages and commands. While it mimics a Linux distribution in many ways, there are important differences to be aware of, such as its non-compliance with the Filesystem Hierarchy Standard. Understanding these distinctions and the limitations of Termux, such as its sandboxed nature and restrictions on file system access, is crucial for effective and safe use of the platform.