Unix for Artists: Fun with FFMPEG is a workshop hosted by the Echo Park Film Center.
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unix4artists/bash/ifelse.sh

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#!/bin/bash -e
#####################################################
#
# If/else statements allow you to run select commands
# if conditions are met or if they are not.
#
# Usage: bash bash/ifelse.sh
#
#####################################################
NUMBER=3
COMPARE="Compare this string"
SUBSTRING="not"
SUBSTRING2="this"
# Evaluate whether two numbers are equal with the "-eq" operator
if [ ${NUMBER} -eq 3 ]; then
echo "Variable \$NUMBER is equal to 3"
else
echo "Variable \$NUMBER is NOT equal to 3"
fi
sleep 2
# Evaluate whether a string contains substrings. Uses the
# "elif" operator to check if a second condition is met if
# the first one fails.
if [[ "${COMPARE}" == *"${SUBSTRING}"* ]]; then
echo "String \$COMPARE contains substring \"${SUBSTRING}\""
elif [[ "${COMPARE}" == *"${SUBSTRING2}"* ]]; then
echo "String \$COMPARE contains substring \"${SUBSTRING2}\""
else
echo "String \$COMPARE contains none of the substrings"
fi
sleep 2
# Evaluate whether a file exists with the "-f" operator
if [ -f "text/cat1.txt" ]; then
echo "File text/cat1.txt exists. Here are the contents:"
cat text/cat1.txt
else
echo "File text/cat1.txt does NOT exist"
fi
sleep 2
# Evaluate whether a directory exists with the "-d" operator
if [ -d "not_here" ]; then
echo "Directory not_here exists. Here are the contents:"
ls not_here
else
echo "Directory not_here does NOT exist"
fi
# Evaluate whether or not one condition OR another
# is met using the "||" operators.
# The "-gt" operator determines if a number is
# greater than a compared number and "-lt" for
# less than.
if [ $NUMBER -gt 1 ] || [ $NUMBER -lt 5]; then
echo "Variable \$NUMBER is greater than 1 or less than 5"
fi
# Evaluate whether both conditions are met using
# the AND operator "&&"
if [[ "${COMPARE}" == *"${SUBSTRING}"* ]] && [[ "${COMPARE}" == *"${SUBSTRING2}"* ]]; then
echo "String \$COMPARE contains both substrings"
else
echo "String \$COMPARE does not contain both substrings"
fi