# This shell script demonstrates basic control flow: conditional
# statements, and how to write some basic conditions that you may
# want to test. The basic structure is simple: ‹if condition; then
# commands; else commands; fi›. We can either use semicolons or
# newlines. Different people prefer different styles.

# The ‹condition› part of the ‹if› statement is a command. If the
# command is successful (i.e. if it returns exit code 0), the ‹then›
# branch of the conditional is taken. Otherwise, the ‹else› branch
# is. The ‹else› branch is optional.

echo 'we first try some if statements and the ‹test› program'
echo

# The program that is most often used in ‹if› conditions is called
# ‹test›. You can use it to check basic properties of files
# (existence, whether they are regular files or directories, if one
# file has been modified more recently than another and so on). It
# can also test properties of numbers (basic relational operators)
# and strings (emptiness and equality). A few examples follow.

if test -e file.txt; then
    echo file.txt exists
fi

if test -f file.txt; then
    echo file.txt exists and is a regular file
fi

if test 3 -gt 4; then
    echo "3 is greater than 4"
fi

if test 3 -lt 4; then
    echo "3 is less than 4"
fi

# There is another way to call ‹test›, and that is ‹[›. Yes, there
# is a command with the name ‘left square bracket’. It is usually a
# (sym)link to ‹test›. In some shells, it is also built-in (both
# ‹test› and ‹[›). 

if [ -e file.txt ]; then
    echo file.txt still exists
else
    echo file.txt does not exist
fi

# The closing square bracket is an argument to ‹[› and it is
# required: ‹[› will complain if you don't supply it, like this: 
#
#     [: the last argument must be ']'

echo

# For more details, see ‹man test›. There are two other utilities
# that sometimes come in handy, called ‹true› and ‹false›. Let's try
# running them. I will also mention a new variable (this one is
# quite special) called ‹?›. It holds the exit code of the last
# executed command. It is not possible to assign your own values
# into this variable.

true
echo exit code from true: $?

false
echo exit code from false: $?

if true; then
    echo "if true: then branch";
else
    echo "if true: else branch";
fi

if false; then
    echo "if false: then branch"
else
    echo "if false: else branch"
fi

# I would suggest you run this script now, and also make sure that
# ‹test -f› does what we think it does:
#
#     $ sh test.sh
#     $ touch file.txt
#     $ sh test.sh

# Let's move on:
#
#     $ micro chaining.sh