1. Shell Features CSCI N321 – System and Network Administration
Copyright © 2000, 2005 by Scott Orr and the Trustees of Indiana University

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

3. References CQU 85321 System Administration Course Textbook Lectures
Chapter 7
Lectures
2002 #6

4. 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

5. 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

6. 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

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

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

9. 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

10. bash Shell Logins /bin/bash /etc/profile.d/ /etc/bashrc /etc/profile
~/.bash_profile
~/.bashrc or
~/.bash_login
~/.profile or

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

12. 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

13. 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 [ ]

14. command << label
I/O Redirection
Modifies how input and output is used
Chain command to solve bigger tasks
Characters
Result
command < file
Use file as input for 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

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

16. 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

17. 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 \