CS155: Shells Lecture 10

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CS155 ShellsLecture 10
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Review

• Use tar and zip to group a set of files into one
  archive file.
• Use gzip and bzip2 to compress files and
  archives.
• Use sed to do simple editing on streams of text.

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Introduction to Shells

• The shell manages communication between the user
  and UNIX.
• Shells also provide facilities for completing
  tasks more efficiently
• Environment Variables
• Shell Variables
• History
• Scripting Language

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Environment Variables

• A variable is a symbol with an associated value.
• Environment variables are special variables that
  are accessible globally (i.e. by all programs).
• Environment variables are used to customize the
  behavior of the OS and many programs.
• They are also accessible to all shells attached
  to the current login session.

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Environment Variables

• List current environment variables
• printenv
• env

gt printenv USERcs155 LOGNAMEcs155 HOME/s/bach/a
/class/cs155 PATH/s/bach/a/class/cs155/bin/usr/u
cb/bin/usr/bin/usr/local/bin/usr/local/gcc/bin
/usr/bin/X11/usr/X/bin/usr/local/java/bin/s/ch
opin/f/proj/eclipse. PWD/s/bach/a/class/cs155 GR
OUPclass HOSTpreisner LANGen_US.UTF-8 SUPPORTED
en_US.UTF-8en_USen HOSTNAMEpreisner MANPATH/u
sr/share/man/usr/man/usr/local/man/usr/local/gc
c/man PAGERless
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Environment Variables

• List current environment variables
• Use to reference one of the variables
• Use double-quotes or no quotes

gt echo I am user and my group is group I am
user and my group is group gt echo I am user
and my group is group I am cs155 and my group
is class gt echo I am user and my group is
group I am cs155 and my group is class gt echo
path /s/bach/a/class/cs155/bin /usr/ucb /bin
/usr/bin /usr/local/bin /usr/local/gcc/bin
/usr/bin/X11 /usr/X/bin /usr/local/java/bin .
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Environment Variables

• setenv set an environment variable
• Note, changing PWD does not change the current
  directory.

gt printenv USERcs155 LOGNAMEcs155 PWD/s/bach/a/
class/cs155 ... gt setenv PWD /tmp gt
setenv USERcs155 LOGNAMEcs155 PWD/tmp ... gt
pwd /s/bach/a/class/cs155
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Shell Variables

• Shell variables are specific to a current shell
  session.
• A shell is opened when you login, but also at
  other times such as when running typescript.
• Some shell variables overlap with environment
  variables. Their values are generally inherited
  from the environment.

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Setting Shell Variables

• set can be used to list and change shell
  variables
• Changing prompt immediately changes the shell
  prompt.

gt set path (/s/bach/a/class/cs155/bin /usr/ucb
/bin /usr/bin /usr/local/bin /usr/local/gcc/bin
/usr/bin/X11 /usr/X/bin /usr/local/java/bin
/s/chopin/f/proj/eclipse .) prompt gt shell
/usr/local/bin/tcsh term xterm tty
pts/2 uid 2543 user cs155 ... gt set prompt
hello, beautifulgt hello, beautifulgt
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Common Variables

• history number of lines to keep in history
• home current working directory on login
• path list of directories in which to search for
  executables
• shell path of the shell executable
• prompt the text displayed on prompt
• user the current username

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On/Off Variables

• Some shell variables do not have values but
  affect programs by the existence or lack of
  existence.
• These variables can be turned on by
• set var
• and turned off by
• unset var

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Quoting and Shell Variables

• The symbols n and are special symbols
  interpreted by the shell
• n references your login name
• references your current pwd

gt set promptuser PWDgt user PWDgt set
promptuser PWDgt cs155 /s/bach/a/class/cs155gtc
d .. cs155 /s/bach/a/class/cs155gt cs155
/s/bach/a/class/cs155gtset promptn gt cs155
/s/bach/a/classgtcd cs155 gt
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Escape Sequences

• Escape sequences are special characters that
  stand for difficult to type character codes.
• \n new line
• \t tab
• Escape sequences are often used with echo or
  other output functions to format the output
  string.

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.login and .cshrc files

• Two files exist to set shell variables upon
  login .login .cshrc
• .login gets execute only when a new terminal
  session is started
• .cshrc is executed every time a new shell is
  opened

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.forward

• If you dont check your mail on your CS account,
  forward your mail!
• .forward file
• One line in the file email address

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alias

• alias sets a string to be equivalent to a command
  (usually with options)
• unalias removes the alias from a string

gt alias rmj rm . dir (ls -Alg) help
man l. ls -d . --colortty la ls
-AlgF ll ls -l --colortty mlprint mp -o -l
! lpr -h mprint mp -o ! lpr -h pprint
(enscript -Gh -fNewCenturySchlbk-Roman8
-FTimes-Bold9) print lpr -h vi vim rmj
rm . gt unalias rmj
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umask

• umask sets the default permissions for new files.
• The default permissions are
• specified by a 3 digit number.
• Example umask 017
• 017 ? 000 001 111
• ? 111 110 000 (complement the digits)
• ? rwx rw- ---
• So, the default permission is urwx, grw

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Your .cshrc file
unalias ls set path ( HOME/bin /usr/ucb /bin
/usr/bin /usr/local/bin /usr/local/gcc/bin
/usr/bin/X11 /usr/X/bin /usr/local/java/bin
/s/chopin/f/proj/eclipse .) set the default
permissions umask 077 alias dir ls
-Alg alias la 'ls -AlgF' alias help
man setenv PRINTER guitar

• A .cshrc file is a shell script.
• at the beginning of a line signifies that the
  line is a comment and should be ignored by the
  shell.

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Summary

• Shells can be customized using Environment and
  Shell variables.
• To use the value of a particular variable in a
  command put a at the beginning of the name.
• Shell variables are treated differently inside
  and .
• .login and .cshrc are shell scripts executed on
  logging in or starting a new shell.