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Loops

Loop over a (small) range of numbers

Alternative to seq and only suitable for small and static number ranges. The number list can also be replaced with a list of words, variables etc.

# Loop from 0-10.
for i in 0 1 2 3 4 5 6 7 8 9 10; do
    printf '%s\n' "$i"
done

Loop over a variable range of numbers

Alternative to seq.

# Loop from var-var.
start=0
end=50

while [ "$start" -le "$end" ]; do
    printf '%s\n' "$start"
    start=$((start+1))
done

Loop over the contents of a file

while IFS= read -r line || [ -n "$line" ]; do
    printf '%s\n' "$line"
done < "file"

Loop over files and directories

Don’t use ls.

CAVEAT: When the glob does not match anything (empty directory or no matching files) the variable will contain the unexpanded glob. To avoid working on unexpanded globs check the existence of the file contained in the variable using the appropriate file conditional. Be aware that symbolic links are resolved.

# Greedy example.
for file in *; do
    [ -e "$file" ] || [ -L "$file" ] || continue
    printf '%s\n' "$file"
done

# PNG files in dir.
for file in ~/Pictures/*.png; do
    [ -f "$file" ] || continue
    printf '%s\n' "$file"
done

# Iterate over directories.
for dir in ~/Downloads/*/; do
    [ -d "$dir" ] || continue
    printf '%s\n' "$dir"
done