Troubleshooting Login Vs Non-Login Shells On Linux

Determining Your Shell Type

Identifying whether you are using a login shell or an interactive non-login shell is key to troubleshooting issues with your shell environment setup and configuration. There are a few methods to definitively determine what type of shell instance you are currently utilizing.

Identifying login vs interactive shells

When a user first logs into a Linux system via the console, SSH, or other terminal, the system initializes a login shell session tied to that user’s credentials and permissions. An interactive non-login shell session is typically started when a user opens a new terminal window after initial login. There are specific variables and behaviors that differ between login and interactive shell sessions.

Set shell variables to test login status

Environment variables can be used to test login status of your current shell. For instance, checking the SHLVL variable will indicate shells nested within other sessions, incrementing for each layer. Typically SHLVL=1 signifies a login shell instance, while higher numbers indicate an interactive shell launched within an existing login session.

$ echo $SHLVL
1

Another variable, the BASH_EXECUTION_STRING contains information about how your current Bash shell was invoked. For login shells this will contain data like “/bin/bash -login” while interactive shells see “/bin/bash”.

Check process tree to validate shell type

Examining the parent process ID (PPID) and process tree can also definitively identify an interactive non-login child process forked from an existing login session. Tracing PPID up the tree until locating PID=1 (systemd init process) will illuminate full origination details.

$ pstree -p $$
bash(12345)---bash(23456)-|-pstree(34567)

Here the 2nd bash process is the interactive instance while the 1st bash is the spawned login shell.

Login Shell Initialization Files

Understanding the order of script execution and environment variable loading during login shell startup is key to proper configuration. Most Linux distributions utilize Bash as the default shell, which loads files from both system directories as well as the user’s home profile upon session initiation.

/etc/profile file execution sequence

When Bash is invoked as a login shell, it preloads system-wide profile scripts before evaluating a user’s personal login files. The /etc/profile script sets default environment paths, variables, locale settings, umask values, and aliases. After running, it sources any scripts in the /etc/profile.d/ folder to apply additional configurations from subsystems and installed applications.

User .profile and .bash_profile usage

Following the system initialization, Bash login shells read the user’s personal .bash_profile file which typically loads custom settings or starts an interactive startup script called .profile rather than directly altering environment values. This allows separation of changes to .profile from the Bash source command in .bash_profile.

# .bash_profile

# Get the aliases and functions
if [ -f ~/.bashrc ]; then
        . ~/.bashrc
fi

# User specific environment and startup programs
PATH=$PATH:$HOME/.local/bin:$HOME/bin

export PATH

. ~/.profile

Sourcing scripts and environment setup

Thanks to the cascading file loading process, Bash login shell environments see all changes made in /etc/profile, /etc/profile.d/*.sh, ~/.bash_profile, ~/.profile, and finally ~/.bashrc in turn. Understanding this sequence enables proper troubleshooting when issues arise during login.

Interactive Non-Login Shells

Non-login shells initialize very differently than those spawned at session startup. These interactive instances generally run default environmental setup via the ~/.bashrc script alone, which can cause unexpected issues.

Default initialization file ~/.bashrc

When launching a new interactive subshell within an existing login session, the child process sources ~/.bashrc to load non-login parameters including user aliases, functions, Bash settings, and application environment variables. There is no cascading system then user file load – just this one script.

Understanding PATH and environment differences

If ~/.bashrc relies on variables, paths,locale definitions or umask values set by system files like /etc/profile, they may not carry over as expected leading to a broken shell experience. Always check that dependency chains are satisfied solely in ~/.bashrc when making changes.

Interactive shell customizations and configs

Limiting new non-login shell tuning, custom prompts, and Bash feature modifications to ~/.bashrc rather than login executed files will ensure consistency across interactive sessions and prevent uninitialized variable issues.

Troubleshooting Issues

When login or non-login shell sessions fail to load your expected configurations there are specific areas to investigate based on understanding their intended boot flow.

Login shell not loading user files

If a login shell fails to source ~/.profile or ~/.bash_profile, check for typos or missing files. Ensure ~/.bashrc is not overriding the intended bifurcation by adding BashDot profile loading. Test Bash environment consistency via a new user account lacking custom scripts.

Interactive shell not applying settings

An interactive non-login shell instance not exhibiting expected configs likely has issues with ~/.bashrc itself or depends too much on login shell environments. Debug ~/.bashrc contents in isolation by commenting out blocks of code until finding problem areas.

Resetting shell startup process for testing

If chasing down shell initialization gremlins, insert set -x debugging or create a new test user to simplify loading flow. When finished remove failed experiments using unset or source commands to avoid side effects. Always comment out code blocks rather than fully deleting during troubleshooting.

Overriding Default Shell Behavior

In certain circumstances like debugging startup flows or reconciling conflicting settings across shell types, overriding aspects of the predetermined initialization sequences may prove useful.

Forcing login vs non-login status

Invoking Bash with the –login option will load login shell files even for non-initial sessions. ThisReplace test is helpful when importing environment from interactive shell into other contexts. –noprofile prevents loading BashDot files, skipping directly to /etc/profile execution.

Skipping or adding initialization scripts

Once bootsrapped, the ENV file can define BASH_ENV to a custom location to override all default script loading. Combining this with a stripped down rcfile containing only known good configs isolates and tests shell initialization.

Interactive shell acting like login shell

Force an interactive shell session to load login scripts instead with BashIncompleteDebug flag. This replaces the default ~/.bashrc load in non-login shells, mimicking full login sequence. Use when moving configs between files.