Environment

Environment variables

Environment variables are a set of variables which can be used internaly in some programs, or just as shortcuts in the terminal.

Listing environment variables

To get the list of all environment variables with their values, type env.

Question: what’s the value of the variable HOME

Solution:

$ env
LC_ALL=en_US.UTF-8
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=00:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.m4a=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.oga=00;36:*.opus=00;36:*.spx=00;36:*.xspf=00;36:
SSH_CONNECTION=10.0.2.2 51025 10.0.2.15 22
LESSCLOSE=/usr/bin/lesspipe %s %s
LANG=en_US.UTF-8
XDG_SESSION_ID=11
USER=fish
PWD=/data
HOME=/home/fish
LC_CTYPE=UTF-8
SSH_CLIENT=10.0.2.2 51025 22
SSH_TTY=/dev/pts/0
MAIL=/var/mail/fish
TERM=xterm-256color
SHELL=/bin/bash
SHLVL=1
LOGNAME=fish
XDG_RUNTIME_DIR=/run/user/1000
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
LESSOPEN=| /usr/bin/lesspipe %s
OLDPWD=/
_=/usr/bin/env

From this list, we can read that HOME has the value /home/fish.

Displaying a variable’s value

To display a single variable’s value, use echo, which basically prints a message. So echo coucou will simply print “coucou”, while echo $HOME prints the value of the HOME variable ($HOME refers to the value for the HOME variable):

$ echo $HOME
/home/fish

$ echo "This is the value of HOME: $HOME"
This is the value of HOME: /home/fish

Importantly, trying to use a environment variable that hasn’t been defined is equivalent to using a variable set to empty string:

$ echo "hello: '$SOME_UNDEFINED_ENVIRONMENT_VARIABLE'"
hello: ''

Setting/unsetting a variable value

To set an envionement varible:

$ export MY_VARIABLE=12
$ # Check the result
$ echo $MY_VARIABLE
12

To unset this environment variable, use the command unset:

$ export FOO="I am Groot"
$ echo "FOO is: '$FOO'"
FOO is: 'I am Groot'
$ unset FOO
$ echo "FOO is: '$FOO'"
FOO is: ''

Important environment variables

The most important environment variable would be

HOME, path to your home directory which is the place were numerous hidden files are stored
PS1, defines the look of your command-line prompt
PATH, stores the list of directories where executables are to be found.

Configuring the environment

The `$HOME/.bashrc` file

A file is often used to configure the environment. This file, $HOME/.bashrc, is read by bash, the terminal interpreter upon start.

Aliases

Aliases are pseudo-commands you defined as shortcuts for other commands. As an example, you’ll probably want to list a directory’s content and getting file sizes quite often. We saw in chapter Manipulating files and directories that the command for that is ls -lh. A way to get faster at typing this command is by creating an alias, i.e. a shorter pseudo-command that will do the same thing.

The command to create an alias is…alias:

$ # Create an alias for ls -lh
$ alias ll='ls -lh'
$ # Now, typing 'll' as the same effect as 'ls -lh'

Setting global environment varialbles

To set an environment variable value on a global level, the command to use is export.

For example, a current practice is to store some executables in your home directory, more precisely in $HOME/.local/bin. To append this directory to the list of directories where executable should be looked for, type: export PATH=$HOME/.local/bin:$PATH

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