You can reasonably view a Unix file as a stream of bytes; as Kernighan and Pike put it in the The Unix Programming Environment, "A file is a sequence of bytes. ... No structure is imposed on a file by the system, and no meaning is attached to its contents — the meaning of the bytes depends solely on the programs that interpret the file. ... Magnetic tapes, mail messages, characters typed on the keyboard, line printer output, data flowing in pipes — each of these files is just a sequence of bytes as far as the system and the programs in it are concerned." (page 41)
Although this seems quite normal to us these days, back in the early days of Unix, such a simple view of files was not the norm; instead, many files were structured in some manner, generally by some sort of "record" scheme, though there were many other ideas floating around.
So what are some of the common ways that we interpret the contents of a file? The simplest of course for human being is simply as a text file. The simplest for a computer might be as a binary program, where its contents are loaded into memory and directly executed by a processor. Other types include PDF files, spreadsheets, SVG files, database files, and a host of other types.
We aggregate these files into what we call "directories". Directories are recursive structures, since they can also contain other directories.
Windows has generally called the concept "Folders" rather than "directories".
We need to be able to name the locations of our files.
The "root" is the starting point, referred to with "/"
Sub-directories identified by name, separated by "/"
Absolute pathnames always start with the "root" (i.e., the slash character)
Relative pathnames are from current directory
".." refers to directory above
"." refers to the current working directory
Referencing user directories:
~USER represents the absolute path to USER's home directory
"~/" represents the absolute path to your own home directory
Absolute pathnames simply give directions on how to get to a node
Examples of absolute pathnames:
Relative pathnames are relative to the current working directory
Examples of relative pathnames
COMMANDNAME [FLAGS] [PARAMETERS] FLAGS: Commands often accept one or more flags after command name. Each flag starts with "-" or with "--", and is separated from other flags by spaces. Individual flags often may be combined with a single "-". ls -l -a ls -la PARAMETERS: Commands often accept one or more parameters. Parameters are often pathnames representing files or directories. Parameters are separated by spaces. cp -a /home/carol/Mail /home/carol/Mail-Backup
Generally, you don't want to type the entire pathname for a program like Perl: /usr/bin/perl
Instead, it would be nice to use the "relative" syntax of simply perl — but you don't want to have to also first cd to /usr/bin either.
How do we solve this dilemma? A fundamental construct of most shells is the idea of a PATH variable. The PATH variable simply lists a number of directories that we will search for commands that have no explicit path information.
$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin
This PATH means that each of the paths /usr/kerberos/bin, /usr/local/bin, /bin, and /usr/bin will be searched when an unadorned command name is invoked.
|ls||List files and directories|
|cat||Output a bytestream|
|less||Paginate a bytestream|
|pr||Paginate a bytestream|
|touch||Update the timestamp on a file; if the file does not exist, it is created|
|cd||Change the shell's current working directory (built-in)|
|cp||Copy files and directories|
|mv||Move files and directories|
|rm||Remove files and directories|
|rmdir||Remove empty directories|
|wc||Count lines, words, and characters in a file|
|grep||Search a file|
|sort||Sort a file|
|head||Show the initial lines of a file|
|tail||Show the terminal lines of a file|
|cmp||Find the first difference in two files|
|diff||Find the differences in two files|
|man||Read the fine manual|
|date||Find date and time|
|who||List the people logged in|
Every file and directory has a set of permissions associated with it.
$ ls -l /etc/hosts -rw-r--r-- 1 root root 905 Jun 16 2010 /etc/hosts $ ls -dl /etc drwxr-xr-x 105 root root 12288 Jan 15 14:24 /etc $ ls -l /usr/bin/emacs-21.4-x -rwxr-xr-x 2 root root 6649776 Apr 28 2011 /usr/bin/emacs-21.4-x $ ls -l /usr/bin/emacs lrwxrwxrwx 1 root root 19 Feb 8 2012 /usr/bin/emacs -> /usr/bin/emacs-21.4
The first component tells us about the item in the filesystem; an unadorned "-" indicates a regular file; "d" indicates a directory; "l" indicates a soft link; "s" indicates a socket; "c" is a character device file; "b" is a block device file; "p" is a named pipe.
$ ls -l /etc/hosts -rw-r--r-- 1 root root 905 Jun 16 2010 /etc/hosts $ ls -dl /etc drwxr-xr-x 105 root root 12288 Jan 15 14:24 /etc $ ls -l /usr/bin/emacs-21.4-x -rwxr-xr-x 2 root root 6649776 Apr 28 2011 /usr/bin/emacs-21.4-x $ ls -l /usr/bin/emacs lrwxrwxrwx 1 root root 19 Feb 8 2012 /usr/bin/emacs -> /usr/bin/emacs-21.4The next 3 characters indicate whether the owner of a file has (1) read (2) write or (3) execute permission (execute in the case of a directory means being able to cd into the directory). The second set of three characters indicate whether users in the same group as this item have read/write/execute permission; the last group of three indicates if all users (the "world") have read/write/execute permission.
ls [-a][-l][-p][-r][-R][-x] [pathname] Description: Lists the files in a directory. Options: [-a] Display all files [-l] Displays all information [-r] Reverses order [-R] Includes sub-directories Examples: ls ls -al ls -al *.exe
touch filename Description: Immediately creates an empty file, or updates the time stamp on an existing file.
cp [-i][-R][-r] source destination Description: Copies the contents of a file or directory to another file or directory. Options: [-a] Archive mode; preserve ownership, permissions, and any special attributes [-i] Ask before you replace [-r] Recursive copy Parameters: source - What you want to copy target - New location Example: cp .plan .plan.backup cp -r ~/public_html/* /tmp cp -a /home/carol /home/carol-backup
mv [-i] source target Description: Renames or moves a file from one directory to another, either with the same name or a different one. Note the original file (and name) will no longer exist. This is not a copy. Options: [-i] Prompt you before replacing a file Parameters: source - what you want to copy target - where to put it Examples: mv x y mv x dir/ mv -i /etc/passwd-old /etc/passwd
rm [-i][-r][-f] NAMES Description: Delete files. Options: [-i] Prompt you before replacing a file [-r] Recursive, deletes an entire directory and all contents and subdirectories. [-f] Forcible removal. Don't give any error messages or ask questions even if files don't exist or permissions are awkward. Parameters: NAMES - the names of what to remove Example: rm -rf / rm -i this.* rm -r /home/carol
less [filename] Description: paginates a file or stdin.
wc [-c][-l][-w] [NAMES] Description: Counts characters, lines, and/or words in files or stdin. Options: [-c] Number of characters [-l] Number of lines [-w] Number of words
pwd Description: Displays the current working directory.
cd [directory] Description: Changes current working directory. Examples: cd / cd cd /etc cd ../public_html/classes
mkdir DIRECTORIES Description: Creates new directories. Examples: mkdir classes mkdir ../classes/new
rmdir DIRECTORIES Description: Removes the specified empty directories. Will not remove a directory that has any contents. Examples: rmdir classes/ rmdir /home/carol/
man command Description: Displays the fine manual page for a command.