You usually won’t need to worry about this though. Different hardware terminals can be emulated for different operating requirements. TERM: This specifies the type of terminal to emulate when running the shell.In most cases, this will be bash by default, but other values can be set if you prefer other options. SHELL: This describes the shell that will be interpreting any commands you type in.Here are some common environmental variables that you will come across: Some environmental and shell variables are very useful and are referenced fairly often. They provide an alternative way of setting persistent values for the session between processes, without writing changes to a file. These variables are used for all sorts of things. This should still give you a good idea of the environmental and shell variables that are set in your session. This will likely still include a few environmental variables, due to the fact that the set command outputs quoted values, while the printenv and env commands do not quote the values of strings. comm -23 We can attempt to compare this output with the output of the env or printenv commands to try to get a list of only shell variables, but this will be imperfect due to the different ways that these commands output information: This will list all of the environmental and shell variables that are defined. We can execute this in a sub-shell so that it does not change our current environment:
We can clean up the output by specifying that set should operate in POSIX mode, which won’t print the shell functions. We probably do not need to know all of the bash functions that are defined, for instance. The amount of additional information that we receive back is a bit overwhelming. You probably want to pipe it into a pager program to more easily deal with the amount of output:
Your shell environment may have more or fewer variables set, with different values than the following output:īASHOPTS=checkwinsize:cmdhist:expand_aliases:extglob:extquote:force_fignore:histappend:interactive_comments:login_shell:progcomp:promptvars:sourcepath In their default state, they should function exactly the same: We can see a list of all of our environmental variables by using the env or printenv commands. We can access these in a few different ways. Printing Shell and Environmental VariablesĮach shell session keeps track of its own shell and environmental variables. This helps users distinguish environmental variables within other contexts. They are often used to keep track of ephemeral data, like the current working directory.īy convention, these types of variables are usually defined using all capital letters. Shell variables are variables that are contained exclusively within the shell in which they were set or defined. Environmental variables are used to pass information into processes that are spawned from the shell. They can be one of two types, environmental variables or shell variables.Įnvironmental variables are variables that are defined for the current shell and are inherited by any child shells or processes. The keys in these scenarios are variables. If the value contains significant white-space, quotations are used: KEY=" value with spaces" Each pair will generally look something like this: KEY= value1: value2. If multiple values are passed, they are typically separated by colon ( :) characters.
The environment is implemented as strings that represent key-value pairs. The environment provides a medium through which the shell process can get or set settings and, in turn, pass these on to its child processes. It obtains the data for these settings from a variety of different files and settings on the system. How the Environment and Environmental Variables WorkĮvery time a shell session spawns, a process takes place to gather and compile information that should be available to the shell process and its child processes. If you’d like to follow along using your local system or a remote server, open a terminal and run the commands from this tutorial there.
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In this guide, we will discuss how to interact with the environment and read or set environmental and shell variables interactively and through configuration files. The environment is an area that the shell builds every time that it starts a session that contains variables that define system properties. One way that the shell keeps track of all of these settings and details is through an area it maintains called the environment. Some of these settings are contained within configuration settings and others are determined by user input. When interacting with your server through a shell session, there are many pieces of information that your shell compiles to determine its behavior and access to resources.
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