Quick Guide to UNIX

By default, the CSIF system use a command interpreter (a shell) called tcsh. The shell commands described below work in this shell. They may, or may not, work in others.

File Systems in UNIX

• File hierarchy — directories are arranged in a hierarchy with root (/) at the top. The position of any file within the hierarchy is described by its pathname.
  • Absolute pathname — a pathname which describes the file location starting from the root
  • Relative pathname — a pathname which describes the location of a file or directory relative to the current directory
• File types
  • Files — an ordinary file is a file on the system that contains data, text, or program instructions.
  • Directories — directories store both special and ordinary files and are equivalent to folders.
  • Hard links — directory entry that references the inode of a file. If more than one hard link exists for a file, if the original is deleted, the others can still access the file; it just means that the original reference to the file is deleted.
  • Soft links (also called symbolic links) — a pointer that contains information about the path to another file. If the original file is deleted then the soft link can no longer access the file.
  • Hidden files — files that start with ‘.’ and are usually used to store configuration information
• File permissions — read (‘r’), write (‘w’), and execute (‘r’), for example drwxrwxrwx
  • The first character refers to the file type (‘d’ for directory, ‘-’ for regular file, “l’ for soft link)
  • Characters 2–4 are permissions for the owner of the file
  • Characters 5–7 are permissions for groups that this file belongs
  • Characters 8–10 are permissions for everyone else
• Useful directory related commands
  • ls — provides a list of all the files under the named directory (by default it uses the current directory)
  • mkdir — make a new empty directory mkdir dir_name
  • rmdir — remove a new empty directory rmdir dir_name
  • cd — change directories
    • to go to any directory: cd pathname/directory
    • to go to your home directory: cd ~
    • to move one level up: cd ..
    • to go to the root directory: cd /
  • pwd — present working directory
  • mv — move (or rename) files or directories; mv old_file new_file
  • cp — copy files from one directory to another cp source_file destination_file
  • rm — remove files from directories (be very careful while using variations of this command) rm [option] filename(s)
  • find — to find files and directories based on name, date or other parameters
• Useful file related commands
  • cat — display file contents cat filename
  • chmod — changing file permissions
    • allowing others to read and execute file: chmod o+rx filename
    • allowing user to write into file: chmod u=w filename
    • revoking group execute permission: chmod g-x filename
    • using absolute permissions: 0 = no permission, 1 = –x, 2 = -w-, 3 = -wx, 4 = r–, 5 = r-x, 6 = rw-, 7 = rwx. For example, chmod 755 filename sets the permissions to rwxr-xr-x
  • diff — finds the difference between 2 files diff file1 file2
  • head — display the first few lines of a file head filename
  • tail — displays the last lines of the file
  • touch — update access and modification date of a file.
  • grep — searches for files that have a particular pattern, without a file name it reads the standard input (or from a pipeline) grep [options] pattern filename(s)
  • sort — arranges lines of text in the file alphabetically or numerically sort [options] filename

Processes in UNIX

• Shell — command line interface similar to DOS or the command window in Windows
  • loads when you open a terminal window
  • prompt followed by a cursor ($, %, >)
  • commands are usually words with blank spaces in between; first word is the UNIX command and the others are considered options and arguments.
• Processes — instance of a running program with a 1–5 digit ID called the PID
  • foreground — by default every process that has been started by the user and takes an input from the user and outputs the result.
  • background — a process that isn’t connected to the keyboard and waits when it has to interact with the user.
• Redirection — redirects between standard input and standard output and files
  • uses >, <, >>, and |
  • > — redirects output to a file; creates if not existing; overwrites if exists
  • >> — same as above, but if the file exists, it appends
  • > — redirects standard input, that is, causes the process to take input from a file; you can combine these, so x < y.txt > z.txt tells the command x to read input from the file y.txt and write its output to z.txt
  • | — pipeline, output of command on the left is used as input to the command on the right
• File naming conventions — files can be named anything including special characters, but in order for the shell to be able to distinguish the special characters as being a part of a file name and not as a special character, each special character needs to be escaped by a backslash (‘\’) or the whole file name must be in quotes. For example, ‘?’ is a special character (see the next item). So to give the file name x? y:z (there’s a blank after the ? in the file name) to a command, that file name would be written as either x\?\ y:z or 'x? y:z'
• Wildcards — used for pattern matching
  • * — matches any string of 0 or more characters
  • ? — matches a single character
  • […] — matches any among the list provided between []
    • ls [0-4,a-d,f].doc — lists any of the files 0.doc, 1.doc, 2.doc, 3.doc, 4.doc, a.doc, b.doc, c.doc, d.doc, and f.doc that exist
    • ls G?[^a-c]*x*.doc — lists all files that start with G, has any character in the second position (the ?), has a third character that is not a, b, or c (the [^a-c]; the ^ means “that is not any of”), has an x as the fourth or later character (the *x*), and ends in .doc.
• Variables — can be set to user defined values; called shell or environment variables
  • Local environment variables — these values can only be used in the current shell; to set, say set variable_name = value and to erase, say unset variable_name
  • Global environment variables — these values can be used in child shells too (created by executing shell scripts); to set, say setenv variable_name value and to erase, say unsetenv variable_name
  • To use the value of a variable variable_name in a command, say $variable_name
• File name completion — by pressing TAB, this automatially fills in the rest of the file name if it is unique (and beeps if not)
• History — remembers the last few commands typed (to save time)
  • $history variable that says how many commands are remembered, so set history=50 causes the shell to remember the last 50 commands
  • The command history will produce a list of commands numbered as they were entered; that is, the first command is 1, and so on
  • ! — command completion from history; !r will look for previous commands starting with r and execute the most recent one; !n will execute the command numbered n, if it is in the history
• Aliases — way of creating your own commands that are built up from other commands and options. For example, alias myLS "ls -l" allows you to give the command myLS, which executes ls -l
• Process related commands
  • ps — lists running processes, with options to show processes from all users, extended information, and so forth; see the manual page for details
  • kill — used to stop a background process after getting its PID
  • control-C (hold down the control and C keys simultaneously) — used to stop the foreground process
    • kill PID — terminate the process with process identification number PID
    • kill -9 PID — if a process will not stop after the above, use this; it stops the process immediately (with a few rare exceptions), but the process cannot save any work (so be very careful while using this command!)
  • top — used to list processes sorted by various criteria

Information Gathering Commands

• apropos — searches the manual pages for a keyword or regular expression; call it as<br> apropos [options] keywords and it displays manual pages that match any keyword Some commonly used options are:
  • -a — display items that match all keywords
  • -e — display items that match the keyword(s) exactly
• man — the system’s manual page viewer; gives information on how to use a program or command; call it as man [options] command Some commonly used options are:
  • -k — same as the apropos command
  • -a — show all manual pages that match the command (usually, only one of the pages is displayed)

More Information

See http://www.tutorialspoint.com/unix/ and http://www.computerhope.com/unix.htm

Credit

This was written for ECS 30, Programming and Problem Solving, in Fall 2015 by Pallavi Kudigrama, and modified slightly by Matt Bishop.