Running Unix Shell Commands

Foreground & Background Processes

Normally, commands are run in the foreground

• Unix assigns the running process a process ID (PID)
• Takes control of your terminal until it is done

Running a process in the background

• put '&' at end of the command
• Unix returns process ID and job number
• shell prompt returns immediately

Job Control Viewing 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)
• Suspend key (usually ^Z) stops the foreground job and puts it in the background to be resumed later

Killing a job

• terminate a specified process id or job number

• job number prefixed with '%'
• process number has no prefix
  

• -TERM is default
• -QUIT
• -KILL

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Command Line Editing

Editing the Current Line

Your account is setup to use vi editing mode.

At prompt, start out in insert mode.

• Some editing is available; check out special characters with stty

• <DEL> deletes the previous character
• ^W erases the previous word
• ^V quotes the next character
• ^U erases the entire command line
• ^R reprints the current command line

Press <ESC> key to enter control mode

• Moving around the current line
• h/l moves left/right by one character
• b/w moves left/right by one word
• 0/$ moves to beginning/end of line
• Deleting
• X/dh deletes one character back
• x/dl deletes one character forward
• db/df deletes one word back/forward
• D/d$ deletes to end of line
• d0 deletes to beginning of line
• dd deletes entire line
• C deletes to end of line, then enters input mode
• cc deletes entire line, then enters input mode
• Returning to insert mode
• i/a/I/A returns to insert mode
• R overwrites existing text

Editing Previous Lines

You may also edit commands stored in your $HISTORY file

• Moving around the history file
• k/- move up one line
• j/+ move down one line
• Gn move to line n
• Searching the history file
• ?string search backward for string
• /string search foreward for string
• n repeat search in same direction as previous
• N repeat search in opposite direction as previous

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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, tabs, and spaces are ignored
• # precedes comments
• #! /bin/ksh indicates which shell to use

You need to make the file executable with chmod

.profile

Executed automatically when you login, this customizes your shell environment with

• aliases define a shorthand for shell commands
• options change the shell's behavior
• variables are names with associated values
• user-created
• keyword
• environment

Aliases

Aliases let you define a shorthand for shell commands that you use frequently

• alias [-x] new=original
• new is a short mnemonic starting with a letter (usually lower-case)
• original is a string that gets substituted when new is typed
• -x option exports alias to child processes
• use single quotes around original when it contains >1 word
• alias rm='rm -I'
• cannot contain arguments

Options

Options are set either on or off

• set -o optionname # turns option on
• set +o optionname # turns option off
• set -o # shows options that are set

User-Created Variables

You can make up variables to represent useful string values

• varname=value
• varname uses letters (typically lower-case), digits, and '_'
• value is some string
• $varname or ${varname} represent the value of the variable

Quoting

How a string is represented determines how the shell interprets it

• No quotes: all special characters are evaluated
• Double quotes: some special characters are evaluated
• variables
• command substitution
• tilde
• Single quotes: no special characters are evaluated

Keyword Shell Variables

Built-in variables have specific meaning to the Unix shell. They generally use all caps.

• Editing mode variables
  COLUMNS, LINES, HISTFILE, EDITOR, VISUAL
• Mail variables
  MAIL, MAILCHECK
• Prompting variables
  PS1, PS2
• Terminal type
  TERM
• Command search path
  PATH
• Others
  HOME, SHELL, PWD

Environment Shell Variables

Carry over to all sub-processes

• Many built-in variables are also environment variables:
  HISTFILE, HOME, LOGNAME, MAIL, MAILPATH, PATH, PWD, SHELL, TERM
• Any variable can be made an environment variable with the export command
• May define variables to be in environment of specific subprocesses only
• ENV file defines aliases, options, and variables for all subprocesses

Read-Only Shell Variables

Some variables may only be read by shell scripts

• Command line arguments
• $1, ..., $9
• $* and $@
• $#
• PID numbers
• $$
• $!
• Exit status
• $?