# This is a shell script that will demonstrate basic variable
# substitution. We will be ahead of the lecture a fair amount, but
# the things we will do are not hard. I hope you remember pipes from
# the first week (we also used those in the second week, they are
# written as ‹program1 | program2›).

# Let's also ask the shell to tell us what is going on. We saw this
# last week:

set -x

# There are two types of variables that we are interested in. First
# are «shell» variables, the second are «environment» variables.
# There is a fair amount of overlap, which can be confusing. We need
# to be careful about the distinction.

variable=value

# The above syntax sets the value of VARIABLE to be value. In a
# traditional shell, there are no types: everything is a string.
# There must be no spaces around the ‹=› ‘assignment operator’.

# To use a variable, we need to use special syntax. Compare

echo variable
echo $variable

# You can also set a variable to the output of a program.
# Incidentally, the ‹$(command)› construct can be used in other
# places too, we may encounter that later.

variable=$(echo hello from echo)
: $variable

# The command ‹:› does nothing. It is useful when we are using ‹set
# -x› because it shows up in the trace, along with all the arguments
# (which are ‹$›-expanded).

# You should run this script now. The invocation is:
#
#     $ sh variables.sh

: "That's it for now. Continue to: env.sh"