Viewing Directories

ls

ls [-aAdglorRSt1] [-I <pattern>] <file-name>

Options:
    a : List all entries, including hidden files (those that start with a ".")
    A : Like "a", but don't display the "." and ".." directories
    d : Display the directory but not its contents (doesn't work recursively) 
    g : Display the group ownership
    l : Display a long listing (including permission mode, number of links, owner,
        size in bytes, time last modified)
    o : Display without colors
    r : Reverse the sort order
    R : Recursive
    S : Sort by size
    t : Sort by time

    1 : (the number one) List one file per line

    I <pattern> : Ignore files matching the shell pattern (unless they're listed on the command line)

ls       # Short listing
ls -al   # Long listing
ls -alt  # Sort by time (long listing)

ls ??*         # List files with names that are two or more characters long
ls .[^.]*      # List hidden files (skipping the "." and ".." directories)

ls [a-zA-Z]*   # List files that begin with a letter
ls [^0-9]*     # List files that don't begin with a number (same as: ls [!0-9]*)

Misc

# Count the number of files in the current directory
find -maxdepth 1 -type f -name "*" | wc -l

# If there are jpg files in the current directory
if ls *.jpg 2> /dev/null | grep -q jpg; then ... fi
if [ "`find -maxdepth 1 -type f -name "*.jpg" | wc -l`" -gt "0" ]; then ... fi