1. Shell Features

2. Section Overview
I/O Substitution
Pathname Substitution
Parameter/Variable Substitution
Command Substitution

3. UNIX Shell
Enables users to enter commands
Text based
Similar to MSDOS Command
Prompt (but are much more
powerful)
Common Shells
 Bourne: sh, ksh, bash
 C: csh, tcsh

4. Shell Features
Environment variables and aliases
History of commands used
Programming constructs
Added features in newer shells
 Command line editing
 Command line completion
Shell configuration files

5. Command History/Editing
Most shells maintain a history of
previously entered commands
history: View list of commands
!#: Rerun command number #
Command line editing
 Up/down arrow keys to cycle through list
 Left/right arrow keys to move within listed
command
 <ctrl><a>: move to start of command line
 <ctrl><e>: move to end of command line

6. Command Aliases
Shortcut for command line
Set/view using the alias
command
Examples:
 alias ls='ls --color=tty'
 alias rm='rm –i'

7. Shell Variables
Customize environment
Shared settings for programs
Assigning Variables: var=“value”
Referencing Variables: $var
Viewing variables: set, printenv
Variables are case sensitive

8. Special Shell Variables
HOME – User’s home directory path
SHELL – Current shell
UID – User’s UID number
PATH – Program search path
PS1/PS2 – Format for command
prompt (prompt for csh/tcsh)
Must use “export” to set variables in shell

9. Parsing the Command Line
Breaks the command line into
components
Special Characters modify
operation
 How command runs
 Filename Expansion
 I/O redirection
 Quotes

10. Command Run Modifiers
Write Space – Separate arguments
Newline (<cr>) – End of line
Special Characters
Character Meaning
; Separate multiple command on same line
& Run command in the background
At end of line, continue on next line
|| && Logical operators based on exit status

11. Filename Expansion
Special characters representing
multiple filenames
Also referred to as globbing
Character Matches
* 0 or more characters
? 1 character
[ ] Matches any 1 character in [ ]
(including ranges)
[^ ] Matches any 1 character not in [ ]
(including ranges)

12. I/O Redirection
Modifies how input and output is
used
Chain command to solve bigger
Characters Result
tasks < file Use file as input for command
command
command > file Store command output in file
command >> file Append command output to file
command << label Input from standard input until label
`command` Execute command then replace command
name with its output
cmd1 | cmd2 Use output from cmd1 as input to cmd2

13. Standard File Descriptors
Special file pointers for input and
output
Extension from C language
Name Descriptor Default Device
Standard input (stdin) 0 Keyboard
Standard output (stdout) 1 Screen
Standard error (stderr) 2 Screen

14. Using File Descriptors
Finer control over input/output
redirection
stdout and stderr can be
redirected to same file
Characters Result
command 2> file stderr output stored in file
cmd >& <file-descriptor> Output redirected to <file-descriptor>
Cmd > file 2>&1 Stderr & stdout redirected to file

15. Quote Characters
Prevent special characters from
being interpreted by the shell
Character Name Action
‘ Single Quote Shell ignores all special
characters enclosed ‘ ’
“ Double Quote Shell ignores all special
characters enclosed “ ”
except for $ ‘
Backslash Shell ignores character
immediately following