linux crash course chapter 09 2

Chapter 9:The TC Shell Everything you thought you knew is now wrong... • TC Shell - an expansion of the C Shell from BSD • The ‘T’ comes from TENEX and TOPS-20 OSes • Meant to add featu

Chapter 9:

The TC Shell

Everything you thought you

knew is now wrong

• TC Shell - an expansion of the C Shell from BSD

• The ‘T’ comes from TENEX and TOPS-20 OSes

• Meant to add features such as command

Shell Scripting Caveat

• Recall that we can specify which shell to use for a script by starting the first line with

#!/path_to_shell

• Without this line, tcsh will use sh to execute the script unless you run the script with tcsh explicitly

• Different than bash and other shells

• Even tcsh admits it’s not great

Accessing tcsh

• Easiest way is to just issue tcsh

• Want to change your login shell to tcsh?

– chsh

• Exiting tcsh

– exit

– logout (only if login shell)

– CTRL-D (if ignoreeof not set)

Things in common with bash

• Command line expansion

– Called substitution in tcsh docs

• History

– history builtin works the same

– ! history references work the same

– Instead of HISTSIZE and HISTFILESIZE, tcsh uses history and savehist

– If variable histlit is set, history shows literal commands before command line substitution

tcsh vs bash con’t

• Aliases

– Syntax: alias name “value”

– Allows you to reference command line

arguments using \!* or \!:n

– Special aliases

• beepcmd – instead of bell

• cwdcmd – whenever you change directories

• periodic – a periodic command to run (tperiod)

• precmd – runs just before shell prompt

• shell – absolute path to use for scripts w/o #!

tcsh vs bash con’t

• Job control

– Almost identical, slightly different for multiple

processes spawned at once

• Filename substitution

– *, ?, [], {}, ~ all the same

– ~+,~- not available

• Directory stack – same

• Command Substitution $() – NOT available

– Use `command` instead

Standard Error

• tcsh doesn’t have an easy way to capture

standard error like bash (i.e 2> )

• Instead we have to use >& to combine

standard error and standard out

• So grab standard out first, then combine out and error to capture error

• Ex:

(cat x y > results) >& errors

Word Completion

• Tab completion is similar in tcsh

• Start with an ambiguous reference then hit tab

• If there are multiple matches, it will maximize the length of the prefix

• Will *not* show a list of possible matches

unless you press CTRL-D

• In tcsh, there are two scopes, local and global

• To declare a local variable, use:

set variable = value

– Note the spaces – different than bash

• To declare a local integer variable:

@ variable = value

• To declare a global (avail to child procs):

setenv variable value

Variables con’t

• Just like bash, use a $ to reference a

variable’s contents (also ${ } )

• unset, unsetenv removes variables

• To declare an array of strings:

set variable = ( values … )

• To reference single entries use [] operator (base 1)

echo variable[2]

set variable[4] = “Value”

Numeric Variables con’t

• To make an array of numeric variables, you actually have to use set

set variable = (1 2 3 4 5)

• Then use @ to access the array

@ variable[2] = 2

More variable goodness

• $#variable – displays the number of

elements in a variable array

• $?variable – displays whether the variable

is set (1 for set, 0 for not set)

• To read user input into a variable, set the

variable to “$<“

– E.g in a script

• echo –n “Enter a value:”

set usr_input = “$<“

Important Shell Variables

• autologout – set timeout period

• cwd – contains current working directory

• histfile – contains location of history

• history – how many history items to keep

• home – your home directory

• mail – where your mail is stored

• owd – previous (old) working directory

Shell Variables, con’t

• histlit – show literal commands in history

• ignoreeof – ignore CTRL-D

• nobeep – disables shell beeps

• noglob – turns of file globbing

• rmstar – prompt for rm * commands

• visiblebell – causes screen to flash for bells

Shell Variables, con’t

• argv – array of command line args

– $n , $*

– Use $#argv to get number of args

• bindkey

• Control structures