Bash - If condition examples

# If no argument.
    if [ "$#" -eq 0 ]; then
        echo "ERROR: $(basename $0): No argument. Aborted!"
        exit 1;
    fi
 
# If condition with pipes.
    if ! echo 'someString' | grep -qF 'notFound'; then
        echo 'Show negation: string not found.'
    fi
 
# Compare 2 files.
    if cmp -s "$file1" "$file2"; then
        echo "The files are the same."
    else
        echo "The files are differents"
    fi
 
# Elseif
    if [ "${var}" -eq 0 ]; then
        echo "${var}"
    elif [ "${var}" -gt 0 ]; then
        echo "${var}"
    else
        echo "Unknown parameter"
    fi
 
# If test conditions.
    -b filename - Block special file
    -c filename - Special character file
    -d directoryname - Check for directory existence
    -e filename - Check for file existence, regardless of type (node, directory, socket, etc.)
    -f filename - Check for regular file existence not a directory
    -G filename - Check if file exists and is owned by effective group ID.
    -g filename - true if file exists and is set-group-id.
    -k filename - Sticky bit
    -L filename - Symbolic link
    -n string is not null
    -O filename - True if file exists and is owned by the effective user id.
    -r filename - Check if file is a readable
    -S filename - Check if file is socket
    -s filename - Check if file is nonzero size
    -u filename - Check if file set-user-id bit is set
    -w filename - Check if file is writable
    -x filename - Check if file is executable
 
# If string is a number. DON'T double quotes variables in if condition.
    string='bla bla'
    is_number_regex='^[0-9]+$'
    if ! [[ "${string}" =~ ${is_number_regex} ]] ; then
        echo "Error: ${string} is not a number."
    fi
 
# Odd or even?
    if [ $(($i%2)) -eq 0 ]; then
        echo $i is even
    else
        echo $i is odd
    fi
 
# Command not found.
    if ! command -v ffmpeg > /dev/null; then
        printf 'Warning: Skip %s.\n' 'dvdrip: ffmpeg command not found'
        exit 0;
    fi
 
# Interactive mode.   
    echo -n 'Are you sure to delete the above files? [Y/N] '
    read delete_confirm
    if [ "${delete_confirm}" != "Y" ]; then
        echo "OUT! Did nothing."
        exit 0
    fi
 
# If and(-a) / or(-o). Note: Escape parentheses for complex conditions.
    if [ $# -gt 0  -a $# -lt 2 ]; then
        echo "Error: Need 1 or 2 arguments. Aborted!"
        echo "${cmd_eg}"
        exit 1;
    fi  
 
# Modulo operation.
    if ! ((n % 4)); then
        echo "$n divisible by 4."
    fi
 
# String compare
    if [ "${a}" == "${b}" ]; then
        echo "equal"
    else
        echo "not equal"
    fi
 
# If Sat or Sun, then set to Fri.
    date_string=$(date +"%Y-%m-%d")
    day_of_week=$(date +%u)
    if [ "${day_of_week}" -eq 6 ] || ["${day_of_week}" -eq 7 ]; then
        date_string=$(date -d "last Friday" +"%Y-%m-%d")
    fi
 
# Check for empty directory
    if [ -z "$(ls -A /path/to/dir)" ]; then
        echo "Empty"
    else
        echo "Not Empty"
    fi