Environment variables, PATH, and shell initialisation

Every running process carries a set of environment variables — key/value strings inherited from its parent. Misconfigured environment variables cause an outsized share of production issues: missing PATH, wrong LANG, forgotten LD_LIBRARY_PATH, and the infamous EDITOR=vi left over from a personal shell.

Shell variables vs environment variables

Bash has two related concepts that are easy to confuse:

• Shell variable: a name that lives only in the current shell. Set with name=value (no export). Not inherited by child processes.
• Environment variable: a shell variable that has been exported with export name. Inherited by every child process the shell starts.

$ MY_LOCAL=hello; export MY_EXPORTED=world; bash -c 'echo MY_LOCAL=$MY_LOCAL; echo MY_EXPORTED=$MY_EXPORTED'

MY_LOCAL=
MY_EXPORTED=world

PATH — the most important environment variable

PATH is a colon-separated list of directories. When you type a command name (not a path), the shell searches each directory in order and runs the first executable match.

$ echo "$PATH" | tr ':' '
'

/usr/local/sbin
/usr/local/bin
/usr/sbin
/usr/bin
/sbin
/bin

The order is significant. A directory listed earlier in PATH is preferred. Common layout on Debian-family systems:

• /usr/local/sbin, /usr/local/bin — locally-installed software, by convention.
• /usr/sbin, /usr/bin — vendor packages.
• /sbin, /bin — legacy directories, often symlinks to /usr/sbin and /usr/bin on modern systems.

Shell initialisation files

When a bash shell starts, it reads a sequence of files depending on how it was invoked:

The most common confusion: ~/.bashrc vs ~/.bash_profile. The rule of thumb is to put interactive-only configuration (aliases, prompts, shell functions) in ~/.bashrc, and environment setup (PATH, EDITOR, LANG) in ~/.profile or ~/.bash_profile. Then have ~/.bash_profile source ~/.bashrc so interactive login shells also get the rc configuration.

$ ls -la /etc/profile /etc/bash.bashrc ~/.bashrc ~/.profile 2>/dev/null

-rw-r--r-- 1 root root  745 Apr  2  2024 /etc/profile
-rw-r--r-- 1 root root 2265 Apr  2  2024 /etc/bash.bashrc
-rw-r--r-- 1 root root  352 Apr  9 11:11 ~/.bashrc
-rw-r--r-- 1 root root  807 Apr  9 11:11 ~/.profile

Systemd unit environment

Services started by systemd do not read ~/.bashrc. They get their environment from:

1. The unit file’s Environment= and EnvironmentFile= directives.
2. The system manager’s default environment (/etc/default/ on Debian-family, /etc/sysconfig/ on RHEL-family).
3. The manager-environment generator, which copies a curated subset of /etc/environment into the system manager.

The canonical place for service environment is the unit file’s EnvironmentFile=, pointing at a file in /etc/default/<service> or /etc/sysconfig/<service>:

[Service]
EnvironmentFile=/etc/default/myapp
ExecStart=/usr/bin/myapp

The environment file is a simple key=value list, one per line:

MYAPP_PORT=8080
MYAPP_LISTEN=0.0.0.0

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