Enviroment Variables

Enviroment variables are used by the operationing system, programs and possibly your scripts. This variables can be set and access from the command line as well as in scripts/programs.

You can set enviroment variables:

export ENV_NAME=env_value
export ENV_NAME="env_value"

Example:

export MY_DATA=/users/username/datafiles

Viewing the contents of enviroment variables:

echo $ENV_NAME

Example:

> echo $MY_DATA
/users/username/datafiles
>

We cam also append and prepend to an enviromental variable:

export DATA=1,2,3,4
echo $DATA
1,2,3,4
export DATA=$DATA,5,6,7,8
echo $DATA
1,2,3,4,5,6,7,8

Combining environment variables and strings in scripts

By using curl braces, you can concatenate environment variables and strings. For example $MY_DATA becomes ${MY_DATA}:

#!/bin/bash
myprogram ${MY_DATA}/datafile1.dat

This would run the following command:

mprogram /users/username/datafiles/datafile1.dat

Passing variables from the command line to your script

You can pass variables from the command line to your script. Each option you specify after the script is accessible via an environment variables.

They are as follows:

$0 (This is the name of the script)
$1 (1st option)
$2 (2nd option)
and so on i.e. $3,$4,$5....

So the following:

myscript datafile1.dat 20

The variables are populated as:

$0 = myscript
$1 = datafile1.dat
$2 = 20

For example you might write your script so that you can specify which data file to use.:

myscript datafile1.dat

The script looks like this:

#!/bin/bash
myprogram $1

Which would produce:

myprgram datafile1.dat

General

PATH

This is the path the OS will search to find programs

It will search from the first to last entry in order

We can prepend the path to one of our own programs:

export PATH=/dir/to/my/prog:$PATH

JAVA_HOME

The path to java

CFLAGS

C compiler flags

CXXFLAGS

C++ compiler Flags