Help for Commands

There are several ways to find out more about commands.

Man-Pages

If you already know which command you want to use, but are not sure how exactly to use it, you can read its man-page (man stands for manual). For instance, to learn more about the passwd command, type

man passwd

man passwd

The command reveals this:

PASSWD(1)                 BSD General Commands Manual                PASSWD(1)

NAME
     passwd -- modify a user's password

SYNOPSIS
     passwd [-i infosystem [-l location]] [-u authname] [user]

DESCRIPTION
     The passwd utility changes the user's password.  If the user is not
     the super-user, passwd first prompts for the current password and will
     not continue unless the correct password is entered.
...

PASSWD(1)                 BSD General Commands Manual                PASSWD(1)

NAME
     passwd -- modify a user's password

SYNOPSIS
     passwd [-i infosystem [-l location]] [-u authname] [user]

DESCRIPTION
     The passwd utility changes the user's password.  If the user is not
     the super-user, passwd first prompts for the current password and will
     not continue unless the correct password is entered.
...

You can return to the command line by pressing q (for quit). You can also browse man-pages online.

apropos Command

Let's say you want to copy a file but are unsure what the proper command is called. You can search in your local man-pages using the apropos command, for instance:

apropos copy

apropos copy

reveals

FcCacheCopySet(3)        - Returns a copy of the fontset from cache
FcCharSetCopy(3)         - Copy a charset
FcLangSetCopy(3)         - copy a langset object
FcMatrixCopy(3)          - Copy a matrix
FcPatternDuplicate(3)    - Copy a pattern
FcRangeCopy(3)           - Copy a range object
FcStrCopy(3)             - duplicate a string
FcStrCopyFilename(3)     - create a complete path from a filename
FcValueSave(3)           - Copy a value
tiffcp(1)                - copy (and possibly convert) a TIFF file
tiffcrop(1)              - select, copy, crop, convert, extract, and/or process one or more TIFF files
asr(8)                   - Apple Software Restore; copy volumes (e.g. from disk images)
copy(9), copyin(9), copyinstr(9), copyout(9), copystr(9) - kernel copy functions
copyops(n), transfer::copy(n) - Data transfer foundation
cp(1)                    - copy files
cpio(1)                  - copy files to and from archives
...

FcCacheCopySet(3)        - Returns a copy of the fontset from cache
FcCharSetCopy(3)         - Copy a charset
FcLangSetCopy(3)         - copy a langset object
FcMatrixCopy(3)          - Copy a matrix
FcPatternDuplicate(3)    - Copy a pattern
FcRangeCopy(3)           - Copy a range object
FcStrCopy(3)             - duplicate a string
FcStrCopyFilename(3)     - create a complete path from a filename
FcValueSave(3)           - Copy a value
tiffcp(1)                - copy (and possibly convert) a TIFF file
tiffcrop(1)              - select, copy, crop, convert, extract, and/or process one or more TIFF files
asr(8)                   - Apple Software Restore; copy volumes (e.g. from disk images)
copy(9), copyin(9), copyinstr(9), copyout(9), copystr(9) - kernel copy functions
copyops(n), transfer::copy(n) - Data transfer foundation
cp(1)                    - copy files
cpio(1)                  - copy files to and from archives
...

From there you can read that cp is probably the right command you were looking for.

Command Help

The man pages are quite extensive, and sometimes you want to have a shorter description. Then you can use the help text for the command, which is often available when you use the --help flag for the command:

cd --help

cd --help

This reveals the following:

cd: cd [-L|[-P [-e]] [-@]] [dir]
    Change the shell working directory.
   
    Change the current directory to DIR.  The default DIR is the value of the
    HOME shell variable.
   
    The variable CDPATH defines the search path for the directory containing
    DIR.  Alternative directory names in CDPATH are separated by a colon (:).
    A null directory name is the same as the current directory.  If DIR begins
    with a slash (/), then CDPATH is not used.
   
    If the directory is not found, and the shell option `cdable_vars' is set,
    the word is assumed to be  a variable name.  If that variable has a value,
    its value is used for DIR.
...

cd: cd [-L|[-P [-e]] [-@]] [dir]
    Change the shell working directory.
   
    Change the current directory to DIR.  The default DIR is the value of the
    HOME shell variable.
   
    The variable CDPATH defines the search path for the directory containing
    DIR.  Alternative directory names in CDPATH are separated by a colon (:).
    A null directory name is the same as the current directory.  If DIR begins
    with a slash (/), then CDPATH is not used.
   
    If the directory is not found, and the shell option `cdable_vars' is set,
    the word is assumed to be  a variable name.  If that variable has a value,
    its value is used for DIR.
...
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