Shell Variables (Linux Essentials)

A variable is a name that holds a value. Your shell keeps it until you close the terminal.

Set a variable

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learner@tidepool:~$ RELEASE=v2.4.0

RELEASEthe name=no spacesv2.4.0the value

NAME=value, with no spaces

There must be no spaces around =. With spaces, the shell splits the line into words, and the first word becomes a command:

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learner@tidepool:~$ RELEASE = v2.4.0
bash: RELEASE: command not found

A name has letters, numbers and _. It cannot start with a number. Many people use uppercase names for the variables that they set.

Put $ before the name. The shell changes $NAME to the value before the command starts:

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learner@tidepool:~$ echo $RELEASE
v2.4.0

echo $RELEASEyou type

Shellputs in the value

echo v2.4.0echo runs with

echo never sees $RELEASE

This is the same idea as wildcards: the shell changes the line first, then the program starts.

A name that has no value gives an empty result, with no error. A typing error in a name is easy to miss:

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learner@tidepool:~$ echo $RELASE

Set the variable again to change its value. unset removes it:

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learner@tidepool:~$ RELEASE=v2.5.0
learner@tidepool:~$ unset RELEASE

A shell variable exists only in the shell where you set it. A new terminal does not have it.