Quick Guide to Linux
By default, the CSIF system use a command interpreter (called a shell) called bash (it’s a version of sh). 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. When 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 (‘x’), 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 ~ or 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)
Normally, you should give the option -i, which will ask you to confirm each file’s deletion: rm -i 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 pipe): 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 (\textbackslash) 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 character among the list provided between [] unless the first character is ^, in which case it matches any character not between the ^ and the ]
- 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, have any character in the second position (the ?), have a third character that is not a, b, or c (the [^a-c]; the ^ means “that is not any of”), have an x as the fourth or later character (the *x*), and end 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 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 variable_name=value; export variable_name and to erase, say unset variable_name. Doing so does not affect the variable in any subprocesses.
- 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
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
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. Thanks to Hongyi Chai for spotting an error (fixed).
