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CS CS-- 344344 - - Unix OperatingUnix Operating
System Fundamentals System Fundamentals
Lecture 7
Specifying Instructions to the Shell
Based on slides created by
Dr. Bangalore for the
Spring 2005 offering of
the course
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Declaring and Using Variables in theDeclaring and Using Variables in the
ShellShell
Declaring Variables: variable_name =variable_name =
valuevalue
Display Variables: echo $variable_nameecho $variable_name
Examples:
a=Hello
b=date
$b
c=‘date’
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Declaring and Using Variables in the Declaring and Using Variables in the
Shell Shell
Difference between single and double quotes
Double quotes ignore most special characters, but command and variable substitution is performed Single quotes, no substitution performed: Examples: d=“You are currently logged on to hostname” You are currently logged on to vulcan5.cis.uab.edu
e=“Today’s date is: $b” (b=date) f=“Today’s date is: $c” (c=‘date’) echo $e $f Today’s date is: date Today's date is: Fri Oct 13 10:03:37 CDT 2006
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Overwriting Existing FilesOverwriting Existing Files
When output is redirected to a file, if the file
already exists it’s overwritten.
To avoid overwriting existing files set the
noclobber variable: setset – –o noclobbero noclobber
When noclobber variable is set, the shell
complains that the file exists
To overwrite a file when noclobber variable is set,
use >!>! instead of >> for output redirection
Example: date >! filenamedate >! filename
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Avoiding Accidental Removal of Files Avoiding Accidental Removal of Files
Unlike Windows, in UNIX when files are deleted
they cannot be undeleted
noclobbernoclobber feature is an instruction to the shell
not to the UNIX commands (e.g., cp, mv, rm)
To avoid accidental removal of files use – –ii option
with cp, mv, rm
set alias for cp, mv, rm, etc. to avoid using – –ii
every time that you use those commands
alias rm=’rm –i’ alias cp=’cp –i’ alias mv=’mv –i’
To override an alias use “\commandcommand” instead of
command (e.g., \rm filenamerm filename)
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Redirecting Error MessagesRedirecting Error Messages
Utilities write to the error stream when an error occurs during their execution (^) lsls – –ll filename (if the filename does not exist or the file permissions are not sufficient)
We have seen output redirection using “>>” and “>>>>”. To redirect error messages use “2>2>” or “2>>2>>” (in bash) The shell assigns a 1 for the standard output stream and 2 for the standard error stream (“>>” is same as “1>1>”)
To redirect both error and output together use “2>&12>&1” after the filename PrintPrint stdoutstdout to screen,to screen, stderrstderr toto outerrouterr lsls^ – –l myfile xyz 2>l myfile xyz 2>^ outerrouterr
PrintPrint stdoutstdout ANDAND stderrstderr toto outerrouterr lsls – –ll myfilemyfile xyz >xyz > outerrouterr 2>&12>& 10/13/2006 8 Department of Com
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Communicative Shell Communicative Shell
The expansion and modification made by the
shell when multiple commands or special
characters are used can be viewed by starting the
shell with – –xx option
We can start a new shell by typing “bashbash – –xx” or
execute a script “bashbash – –x myscriptx myscript” from the
current shell
$ bash -x $ tr '\t' ':' < quizscores | sort
$ bash -x $ c="You are currently logged on to hostname" ++ hostname
- c=You are currently logged on to blazer $ echo $c
- echo You are currently logged on to blazer You are currently logged on to blazer
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Shell Command Line Expansion (I)Shell Command Line Expansion (I)
Wildcard character – **
Matches zero or more characters List all files that end with .java – ls .javals .java List all files that start with cs – (^) ls csls cs
Matching character – ??
Matches one character List all files that start with chapter followed by any character – (^) ls chapter?ls chapter?
** and ?? are special characters called meta-
characters. These characters are interpreted by
the shell and should not be used in filenames
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Shell Command Line Expansion (II) Shell Command Line Expansion (II)
To specify filename within a specified range use [][]
Only one character is matched List all files that start with chapter and has a number following it within the range 1-5 – ls chapter[1ls chapter[1--5]5]
To match and create multiple filenames from one
pattern use {}{}
Curly braces match files specified in the braces, but will NOT expand ranges List all files that start with chapter and has specific numbers following it – ls chapter{2,3,4}ls chapter{2,3,4}
All meta-characters can be combined together
ls *[0ls *[0--9]?{java,c,cpp}9]?{java,c,cpp}
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Local & Environment VariablesLocal & Environment Variables
Use “export variable_nameexport variable_name”
a=Helloa=Hello
export aexport a
export b=Worldexport b=World
export PATH=${PATH}:~/binexport PATH=${PATH}:~/bin
Remove variable – unsetunset
unset aunset a
List all variables – setset
List environment variables – envenv
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Miscellaneous Miscellaneous
viewview: similar to vi with the read-only flag set
splitsplit: split long files into smaller files
splitsplit – –l linecount filename suffixl linecount filename suffix splitsplit – –l 20 longfile file+l 20 longfile file+
