The UNIX Shells

1
The UNIX Shells

• 1. What is a Unix shell?
• 2. A few common shells in the Unix Linux.
• A. Bourne shell
• B. Korn shell
• C. C shell
• D. Bash-the default shell on most Linux

2
Korn Shell
C Shell
Common Core
Bourne Shell
3
Shell
User
Resident modules
Kernel
Hardware
4
Shell Functions (Core)

• Built-in commands, Scripts, Redirection
• Wildcards, Pipes, Subshells
• Background processing
• Command substitution
• Variables (Local Environment)
• Sequences (Conditional Unconditional)

5
Selecting A Shell

• 1. Password is required to change shell
• 2. Utility use chsh input password
• Bunrne --- /bin/sh
• C --- /bin/csh
• Korn --- /bin/ksh (not avl. Here)
• Bash --- /bin/bash (Default for most
  
• of the
  Linux)

6
Utilities Shell Commands

• In general, we dont differentiate utilities
  commands. In most of the books, both mean
• the same thing.
• They are utility programs to fulfill certain
  functions such as
• Text editing, File manipulation, email

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Metacharacters

• gt --- Output redirection
• gtgt --- Output redirection-append to a file
• lt --- Input redirection (Read standard
• input from a
  file)
• --- File-substitution (similar to DOS)
• ? --- File-substitution (single character)
• --- Pipe symbol (one out to another in)

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More Metacharacters

• --- Conditional execution (executes a
• command if the previous one fails)
• --- Opposite to
• ---Runs a command in the background
• --- Comment
• --- Expends the value of a variable
• \ --- Disables the special meaning of next Char.

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More Metacharacters

• --- To sequence commands
• () --- Groups commands
• --- File substitution wildcard
• command --- Command substitution replaced
• by the output from
  command
• ltlttok --- Input redirection reads standard input
  
• from script up to tok

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Examples

• 1. cat gt file_1 You already familiar with
• 2. cat gtgt file_1 appends new typed
• contents to the existing file_1
• 3. ls file list all files with file the first
  part
• of the name
• 4. ls ?ile2
• 5. ls file3 wc
• 6. sleep 30 sleep 30 second in the background

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More examples of using metacharacters

• date pwd ls cal--execute them in sequence
• ls ac -- list any string beginning with a or
  c
• echo the date today is date
• ls /.c --- list all files ending in .c in any
  
• subdirectory
• ls A-Za-z --- list any string begin with a
  letter