You will probably have seen references to The Environment in various contexts relating to shells, shell scripts, scripts in other languages and compiled programs.
In Unix and Unix-like operating systems an environment is maintained by the shell, and we will be looking at how Bash deals with this in this episode. When a script, program or subprocess is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of the form name=value
.
The environment is used to convey various pieces of information to the executing script or program. For example, two standard variables provided by the shell are 'HOME'
, which is set to the current user’s home directory and 'PWD
, set to the current working directory. The shell user can set, change, remove and view environment variables for their own purposes as we will see in this episode. The Bash shell itself creates and in some cases manages environment variables.
The environment contains global data which is passed down to subprocesses (child processes) by copying. However, it is not possible for a subprocess to pass information back to the superior (parent) process.
You can view the environment in a number of ways.
From the command line the command printenv
can do this (this is usually but not always a stand-alone command: it’s /usr/bin/printenv
on my Debian system). We will look at this command later.
The command env
without any arguments does the same thing as printenv
without arguments. This is actually a tool to run a program in a modified environment which we will look at later. The environment printing capability can be regarded as more of a bonus feature.
Scripting languages like awk
(as well as Python and Perl, to name just a few) can view and manipulate the environment.
Compiled languages such as C
can do this too of course.
There are other commands that will show the environment, and we will look at some of these briefly.
The variables in the environment are not significantly different from the shell parameters we have seen throughout this Bash Tips series. The only difference is that they are marked for export to commands and sub-shells. You will often see variables (or parameters) in the environment referred to as environment variables. The Bash manual makes a distinction between ordinary parameters (variables) and environment variables, but many other sources are less precise about this in my experience.
The standard variables in the environment have upper-case names (HOME
, SHELL
, PWD
, etc), but there is no reason why a variable you create should not be in lower or mixed case. In fact, the Bash manual suggests that you should avoid using all upper-case names so as not to clash with Bash’s variables.
Variables can be created and changed a number of ways.
export
commanddeclare
command with the -x
optionmyvar=42
, just as for a normal variableexport
command can also be used to turn off the export marker on a variableunset
command (as seen earlier in the series)We will look at all of these features in more detail later in the episode.
I have provided detailed notes as usual for this episode, and these can be viewed here.
"
Bash parameter manipulation"
"
Some Bash tips"
"
Some more Bash tips"
"
Some further Bash tips"
"
Some additional Bash tips"
"
Some other Bash tips"
"
Some supplementary Bash tips"
"
More supplementary Bash tips"
"
Some ancillary Bash tips - 9"
"
More ancillary Bash tips - 10"
"
Further ancillary Bash tips - 11"
"
Additional ancillary Bash tips - 12"
"
Extra ancillary Bash tips - 13"
"
Bash Tips - 14 (Some auxiliary Bash tips)"
"
Bash Tips - 15 (More auxiliary Bash tips)"
"
Bash Tips - 16 (Further auxiliary Bash tips)"
"
Bash Tips - 17 (Additional auxiliary Bash tips)"
"
Bash Tips - 18 (Extra auxiliary Bash tips)"
"
Bash Tips - 19 (Supplemental auxiliary Bash tips)"
"
Bash Tips - 20 (Some collateral Bash tips)"
Unless otherwise stated, our shows are released under a Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.
The HPR Website Design is released to the Public Domain.