Conditionals and loops

Scripts make decisions with if and repeat work with loops. if [[ -f "$file" ]]; then ... fi tests a condition. Common tests are -f (a file exists), -d (a folder exists), -z (a string is empty), == (two strings match) and -gt (one number is greater than another). case picks between several options neatly.

for f in *.csv; do ... done loops over files. while read -r line; do ... done < file reads a file one line at a time. Keep the logic simple; when a script needs complex data, move to Python.