Unix File Structure

File Hierarchy

Unix file system is a hierarchy of files

• Tree structure
• / is the root

Absolute pathname

• Files represented by their position in the hierarchy
• File names separated by '/'

Directory files vs. ordinary files

Directories, data, executable code and I/O devices are all files

File names

• typically contain letters, numbers, underscore, period, comma
• names are case sensitive
• extensions indicate file contents, format, or purpose

Hidden files have names that start with '.'

• .profile
• .sh_history
• .mailrc

Special characters have special meanings to the shell

• & ; | * ? ' " ` [ ] ( ) $ < > { } ^ # / \
• <return> <space> <tab>
• control characters (view with stty)

Special characters may be quoted with '\'

Directories

Working directory is the directory you are RinS

• check the current working directory with pwd
• change the current working directory with cd

Home directory

• set as value of HOME
• generally referred to as ~username
• return to your home directory with cd

Relative path names

• file references that don't start with '/' are relative to the working directory
• . refers to the current working directory
• .. refers to the parent directory

File Operations

Creating and removing directories

• mkdir creates a new empty directory (relative or absolute)
• rmdir removes a directory

File manipulation

• cp copies files
• rm deletes files
• mv renames files and/or moves them to different directories

Listing Files Listing files in a directory with ls

• ls -a lists all files (including hidden ones)
• ls -l provides detailed information about files
  -rwxr--r-- 1 john staff 335 Jan 22 13:23 myfile
  • access permissions
  • number of links (for ordinary files) or number of subdirectories (for directories)
  • owner of the file
  • group to which the file belongs
  • size of the file in bytes
  • date and time file was last modified
  • filename
• ls -t sorts files by time last modified
• ls -F uses symbols to indicate file types

Access Permissions

Checking access permissions (ls -l)

• 1 = type of file:ordinary (-), symbolic link (l), directory (d), or device (b,c)
• 2-4 = read, write, execute permissions of user (owner)
• 5-7 = read, write, execute permissions group
• 8-10 = read, write, execute permissions of others (everyone else)

Changing access permissions

• chmod permissions filename(s)
• absolute permissions represented by a 3-digit octal number
• add permissions by specifying who+permission
  • who is one or more of user (u), group (g), other (o), or all (a)
  • permission is one or more of read (r), write (w), or execute (x)
• remove permissions by specifying who-permission

Links

Hard links

• represented by absolute file pointers
• files must be local
• only root can set hard links to directories

Soft links

• represented by full pathname
• may refer to files on other mounted systems
• anyone can point to directories

Creating/removing links

• ln [-s] pathname linkname

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Using the Shell

Command line basics

Syntax: command [arg1] [arg2] ... [argn]

Arguments

• Input to the program
• Options

Options

• modify the original meaning of the command
• usually preceded by '-'
• collections of options generally only need one '-'

Executing the command

Operating system searches for command

• if relative path is given, searches directories listed in PATH
• if absolute path is given, goes directly to that location
• files prefixed with './' are treated as absolute pathnames

Commands generally open 3 files represented by their file descriptors

• 0: standard input
• 1: standard output
• 2: standard error output

The terminal is a file

• represented as a file in /dev/
• by default, the terminal is standard input, output and error output

Redirection

Indicates other file(s) are to be used as standard input/output/error

Standard input

• < filename reads all standard input from the file

Standard output

• > filename writes all standard output to a new file (creates or overwrites it)
• >> filename appends all standard output to the end of the file

Standard error

• 2> filename writes all standard error output to the file
• 2>&1 writes all standard error output to the same file used for standard output

Pipes

Pipes pass standard output of one process to the standard input of another

• pipes represented by '|'
• no limits on number of pipes

Filters

• modify the standard output of one command before passing it to the next
• interactive programs cannot be used as filters

Sending output to terminal AND a file with tee

Background processes

Running a process in the background

• put '&' at end of the command
• operating system returns process ID (PID) and job number
• shell prompt returns immediately

Process status

• ps lists process id, terminal, time running, and command name
• jobs lists job number, status, and command name, and indicates most recently (and next-most recently) jobs put in the background
• fg puts most recent job in the foreground (unless job number is specified)

Killing a job

• terminate a specified process id or job number
• can specify signal
  • -QUIT
  • -KILL

Filename generation

Metacharacters or wild cards represent variations on file names

• ? represents a single character that could be anything
• * represents 0 or more characters that could be anything
• [] surrounds a list of possible single characters that may be substituted

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Shell Scripts

Simple Shell Scripts

A shell script is a file that contains commands to be executed by the shell.

• Commands are executed sequentially
• Blank lines are ignored
• # precedes comments
• #! /bin/ksh indicates which shell to use

You need to make the file executable with chmod