Raspberry pi Part 4

Raspberry pi Part 4

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  www.techvilla.org.in TECHVILLA www.techvilla.org.in
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  www.techvilla.org.in Working with theShell  What is shell?  Different shells in linux.  Programming in shell.  Shell vs windows cmd
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  www.techvilla.org.in What’s Shell?  It’sacts an interface between the user and OS (kernel).It’s known as “ command interpreter”.  For Linux, the Bash is the default  Why Shell?  For routing jobs, such as system administration, without writing programs  However, the shell script is not efficient, therefore, can be used for prototyping the ideas  For example, % ls –al | more (better format of listing directory) % man bash | col –b | lpr (print man page of man)
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  www.techvilla.org.in Shell basics  Shellis the interface between end user and the Linux system, similar to the commands in Windows  Bash is installed as in /bin/sh  Check the version  % /bin/sh --version Kernel Other programs X window bash csh
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  www.techvilla.org.in Pipe and Redirection Redirection (< or >) % ls –l > lsoutput.txt (save output to lsoutput.txt) % ps >> lsoutput.txt (append to lsoutput.txt) % more < killout.txt (use killout.txt as parameter to more) % kill -l 1234 > killouterr.txt 2 >&1 (redirect to the same file) % kill -l 1234 >/dev/null 2 >&1 (ignore std output)  Pipe (|)  Process are executed concurrently % ps | sort | more % ps –xo comm | sort | uniq | grep –v sh | more % cat mydata.txt | sort | uniq | > mydata.txt (generates an empty file !)
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  www.techvilla.org.in Shell as aLanguage  We can write a script containing many shell commands  Interactive Program:  grep files with POSIX string and print it % for file in * > do > if grep –l POSIX $file > then > more $file  fi  done Posix There is a file with POSIX in it  ‘*’ is wildcard % more `grep –l POSIX *` % more $(grep –l POSIX *) % more –l POSIX * | more
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  www.techvilla.org.in Writing a Script Use text editor to generate the “first” file #!/bin/sh # first # this file looks for the files containing POSIX # and print it for file in * do if grep –q POSIX $file then echo $file fi done exit 0 % /bin/sh first % chmod +x first %./first (make sure . is include in PATH parameter)
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  www.techvilla.org.in Syntax  Variables  Conditions Control  Lists  Functions  Shell Commands  Result  Document
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  www.techvilla.org.in Variables  Variables neededto be declared, note it is case-sensitive (e.g. foo, FOO, Foo)  Add ‘$’ for storing values % salutation=Hello % echo $salutation Hello % salutation=7+5 % echo $salutation 7+5 % salutation=“yes dear” % echo $salutation yes dear % read salutation Hola! % echo $salutation Hola!
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  www.techvilla.org.in Quoting  Edit a“vartest.sh” file #!/bin/sh myvar=“Hi there” echo $myvar echo “$myvar” echo `$myvar` echo $myvar echo Enter some text read myvar echo ‘$myvar’ now equals $myvar exit 0 Output Hi there Hi there $myvar $myvar Enter some text Hello world $myvar now equals Hello world
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  www.techvilla.org.in Environment Variables  $HOMEhome directory  $PATH path  $PS1 第一層提示符號 (normally %)  $PS2 第二層提示符號 (normally >)  $$ process id of the script  $# number of input parameters  $0 name of the script file  $IFS separation character (white space)  Use ‘env’ to check the value
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  www.techvilla.org.in Shell vs windowscmd  The difference of Windows and Linux  Case sensitive in Linux  / in Linux  No relationship between executable file and extension  don't search current path  Security policy is more strict in Linux  link file VS shortcut