How to run Unix shell commands and programs.pptx

How to run Unix shell commands and programs.pptx

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  Unix
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  Vi editor  viis generally considered the de facto standard in Unix editors because −  It's usually available on all the flavours of Unix system.  Its implementations are very similar across the board.  It requires very few resources.  It is more user-friendly than other editors such as the ed or the ex.
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  Vi editor  Tocreate a new file testfile if it already does not exist in the current working directory − $vi testfile  The above command will generate the following output − | ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ "testfile" [New File] A tilde represents an unused line. If a line does not begin with a tilde and appears to be blank, there is a space, tab, newline, or some other non-viewable character present.
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  The Unix Architecture  Kerneland shell  The kernel interacts with the machine’s hardware and the shell with the user.  The kernel is the core of the operating system - a collection of routines mostly written in C.  The kernel is loaded into memory when the system is booted and communicates directly with the hardware.
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  What exactly is a“shell”?  The shell acts as an interface between the user and the kernel.  The shell is a command line interpreter(CLI).  User commands are often entered on a command line interface that is provided by a “shell”.  The shell is a program that reads the user commands, evaluates it, and then prints the result.  For evaluating the command, the shell may execute other commands, or pass them to the ‘kernel’.  Bourne shell If you are using a Bourne-type shell, the $ − character is the default prompt.  C shell If you are using a C-type shell, the % character is the − default prompt.  The most popular shells are: bash, tcsh,csh, and korn.  Shell commands are case sensitive.
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  Operating System Organization
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  Shell variables  Theshell supports variables that are useful in both the command line and shell script. Eg. $ x=10 No $ required for assignment $ echo $x but needed for evaluation  A variable can also be assigned the value of another variable Eg. $ y=$x $ echo $y  All shell variables are initialized to null string by default. x = A null string  A variable can be removed with unset and protected from reassignment by readonly. unset x x is now undefined readonly x x can’t be reassigned
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  Effect of quoting and escaping message=Youdidn ’t enter the filename Message= “You didn’t enter the filename” $ echo The average pay is $1000 The average pay is $1000 $ echo ‘The average pay is $1000’ The average pay is $1000 $ echo “ The PATH is $PATH and the current directory is pwd” The path is /bin:usr/bin: and the current directory is /home/a/b $ echo The average pay is $1000 The average pay is 000 $1 belongs to a set of parameter “positional parameters”
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  Where to use shell variables Setting Pathnames – if a pathname is used several times in a script, we should assign it to a variable $ progs = ‘/home/a/c_progs’ $ cd $progs ; pwd pwd – print working directory /home/a/c_progs  Using Command Substitution $ mydir = “pwd” ; echo $mydir /home/a/c_progs  Concatenating Variables and Strings base=foo ; ext=.c Two assignments in a line file=$base$ext or file=$(base)$ext cc -o $base $file. Creates executable foo from foo.c
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  shell variables file=$base’.c’ This isfoo.c; quotes not required file=${base}.c Same but more readable file=$base’01’ This is foo01; quotes required