Honestly google is a better tool unless you already kind of know what you're looking for.

Google "linux how much space on drive" and the first hit tells you exactly what you need.

Another question someone might ask is "how do I change my password". Google for "linux change password" and there is a special help box right at the top of the search results.

apropos password and you get this enormous list:

apg (1) - generates several random passwords
chage (1) - change user password expiry information
chgpasswd (8) - update group passwords in batch mode
chpasswd (8) - update passwords in batch mode
cpgr (8) - copy with locking the given file to the password or gr...
cppw (8) - copy with locking the given file to the password or gr...
cracklib-check (8) - Check passwords using libcrack2
create-cracklib-dict (8) - Check passwords using libcrack2
crypt (3) - password and data encryption
crypt_r (3) - password and data encryption
des_read_2passwords (3ssl) - Compatibility user interface functions
des_read_password (3ssl) - Compatibility user interface functions
doveadm-pw (1) - Dovecot's password hash generator
endpwent (3) - get password file entry
endspent (3) - get shadow password file entry
EVP_BytesToKey (3ssl) - password based encryption routine
expiry (1) - check and enforce password expiration policy
fgetpwent (3) - get password file entry
fgetspent (3) - get shadow password file entry
fgetspent_r (3) - get shadow password file entry
getpass (3) - get a password
getpw (3) - reconstruct password line entry
getpwent (3) - get password file entry
getpwnam (3) - get password file entry
getpwnam_r (3) - get password file entry
getpwuid (3) - get password file entry
getpwuid_r (3) - get password file entry
getspent (3) - get shadow password file entry
getspent_r (3) - get shadow password file entry
getspnam (3) - get shadow password file entry
getspnam_r (3) - get shadow password file entry
git-credential-cache (1) - Helper to temporarily store passwords in memory
gitcredentials (7) - providing usernames and passwords to Git
grpconv (8) - convert to and from shadow passwords and groups
grpunconv (8) - convert to and from shadow passwords and groups
grub-mkpasswd-pbkdf2 (1) - generate hashed password for GRUB
lckpwdf (3) - get shadow password file entry
login.defs (5) - shadow password suite configuration
lppasswd (1) - add, change, or delete digest passwords.
Net::LDAP::Control::PasswordPolicy (3pm) - LDAPv3 Password Policy control object
Net::LDAP::Extension::SetPassword (3pm) - LDAPv3 Modify Password extension ob...
pam_pwhistory (8) - PAM module to remember last passwords
pam_unix (8) - Module for traditional password authentication
passwd (1) - change user password
passwd (1ssl) - compute password hashes
passwd (5) - the password file
passwd2des (3) - RFS password encryption
putpwent (3) - write a password file entry
putspent (3) - get shadow password file entry
pwck (8) - verify integrity of password files
pwconv (8) - convert to and from shadow passwords and groups
pwunconv (8) - convert to and from shadow passwords and groups
seahorse (1) - Passwords and Keys
setpwent (3) - get password file entry
setspent (3) - get shadow password file entry
sgetspent (3) - get shadow password file entry
sgetspent_r (3) - get shadow password file entry
shadow (5) - shadowed password file
shadowconfig (8) - toggle shadow passwords on and off
smbpasswd (5) - The Samba encrypted password file
smbpasswd (8) - change a user's SMB password
ulckpwdf (3) - get shadow password file entry
unix_chkpwd (8) - Helper binary that verifies the password of the curren...
unix_update (8) - Helper binary that updates the password of a given user
vigr (8) - edit the password, group, shadow-password or shadow-gr...
vipw (8) - edit the password, group, shadow-password or shadow-gr...
xcrypt (3) - RFS password encryption
xdecrypt (3) - RFS password encryption
xencrypt (3) - RFS password encryption

If you didn't already know you were looking for the "passwd" command it might be hard to pick it out of the list.