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Shells

Shell interpretive cycle:

  • shell issue prompt and wait for you to enter command
  • after a command is entered, the shell scans for the metacharacters and expands abbreviations
  • the shell waits for the command to execute

Types

  • The Bourne family comprising the Bourne shell (/bin/sh) and its derivatives
    • the Korn shell (/bin/ksh) and
    • Bash (/bin/bash)
  • the C shell (/bin/csh) and its derivative,
    • Tcsh (/bin/tcsh)

Wildcard Meaning

  • * Any Number of characters including none
  • ? A single character
  • [ijk] A single character- either i, j, or k
  • [x-z] A single character within the ASCII range of character x and z
  • [!ijk] A single character that is not i, j, k
  • [!x-z] A single character not in the ASCII range of character x and z
  • {pat1, pat2, ...} pat1, pat2, ...

Redirection

  • 0-Standard Input - The file (or stream) representing input, connected to the keyboard.
  • 1-Standard Output - The file (or stream) representing output, connected to the display.
  • 2-Standard Error - The file(or stream) representing error, connected to display.

Three standard file are represented by a number called file descriptor. 1,2,3.

  • < - file redirection i.e. taking contents of one file as input
  • | - pipe i.e. giving output of one command to another

Taking input from both file and standard input::

  • cat ~ foo first from standard input and then from foo

  • cat foo ~ bar first from foo, the stdin then bar

  • > - redirecting output to a file

  • >> - appending output to a file

File descriptors are implicitly prefixed to the redirection symbols. e.g. > is same as 1>, < is same as <0.

  • 2> - redirecting error stream
  • 2>> - appending error stream

(tyy - stands for teletype)

  • tty - tells the filename of terminal you are using

(stty - settings of tty)

  • stty - displays basic settings
  • stty -a - displays all tty settings
    • its first line contains speed, rows, columns, lines of the terminal
    • the it shows the settings of the control characters, in form keyword = value
    • rest line contains the keyword or -keywords :: the prefix - implies that the option is turned off

Output of stty -a:

speed 38400 baud; rows 41; columns 159; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; 
werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
-parenb -parodd -cmspar cs8 -hupcl -cstopb cread -clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl -ixon -ixoff -iuclc -ixany -imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke -flusho -extproc
  • stty sane - restores default settings
  • stty -echoe -set the echoe off i.e. now backspace won't erase the characters
  • stty -echo -sets the keyboard input display off used for passwords
  • stty intr \^c - sets the interrrupt control character

Control characters

  • ^s - stop scrolling the output and lock keyboard
  • ^q - start scrolling and unlock keyboard
  • ^u - kills the line without executing the command
  • ^c - send interrrupt signal
  • ^\ - end the sigkill signal - but creates a core file containing the memory dump of the program
  • ^z - suspends the program and return to the shell
  • ^d - eof cahracter (terminates the login session)
  • ^h - erases text (backspace)
  • ^j - alternative to the enter (carriage return)
  • ^m - alternative to the enter
  • sitty sane - restores the terminal