UNIX BASIC CONCEPTS

See also Unix File Management
In Unix every process has at least 3 communication Channel: Standard Input (STDIN)-0, Standard Output(STDOUT)-1 and Standard Error (STDERR)-2
SHELL BASICS: If you like vi, put your shell's command line editing into vi mode like
$set -o vi

Command line arguments and functions: command line arguments to a script become variable whose names are number.
$1 -> First command line argument
$2 -> Second command line argument and so on
$#  ->  variable contains the number of command line arguments that were supplied.
$*  ->  variable contains all the argument at once
Neither of these variables counts $0.

Regular Expressions:
^ --> Matches Beginning of the line
$ --> End of the line
* --> One or more matches of the proceeding element
. --> Matches any character
? --> zero or one match of the proceeding element
[char] --> Matches any character from a given set
[^char] --> Matches any character not in a given set
[n] --> Matches exactly n instance of the proceeding element
Click here to learn more about regex

File Evaluation Operators
-d file ---> File exists and is a directory
-e file ---> File exists
-f file ---> File exists and is a regular file
-r file ---> You have read permission on file
-s file ---> File exists and is not empty
-w file ---> You have write permission on file
file1 -nt file2---> File1 is newer than File2
file1 -ot file2 ---> File1 is older than File2


Understanding each column of ps -ef command

[oracle@usha ~]$ ps -ef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 Jan23 ?        00:00:14 /sbin/init
root         2     0  0 Jan23 ?        00:00:00 [kthreadd]
root         3     2  0 Jan23 ?        00:00:32 [ksoftirqd/0]
root         6     2  0 Jan23 ?        00:00:00 [migration/0]
root         7     2  0 Jan23 ?        00:00:03 [watchdog/0]
root        13     2  0 Jan23 ?        00:00:00 [cpuset]
root        14     2  0 Jan23 ?        00:00:00 [khelper]
root     31487     2  0 11:04 ?        00:00:00 [kworker/0:1]
oracle   31488  2207  0 11:05 pts/1    00:00:00 ps -ef

...................................
......................
UID => User ID that owns the process
PID => The Process ID for the tasks
PPID => The Parent process. if the parent is one the process is created by init process.
STIME =>The Start time of the process
TTY =>Terminal associated with the process
TIME =>The amount of CPU time used by the process so for. This value will increase until the process is complete
CMD => The Unix command that is being executed


Unix - Special Variables

posted Apr 22, 2013, 6:36 PM by Sachchida Ojha

$ character represents the process ID number, or PID, of the current shell:
$echo $$

Above command would write PID of the current shell:

29949

The following table shows a number of special variables that you can use in your shell scripts:

VariableDescription
$0The filename of the current script.
$nThese variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1, the second argument is $2, and so on).
$#The number of arguments supplied to a script.
$*All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2.
$@All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2.
$?The exit status of the last command executed.
$$The process number of the current shell. For shell scripts, this is the process ID under which they are executing.
$!The process number of the last background command.

Command-Line Arguments:

The command-line arguments $1, $2, $3,...$9 are positional parameters, with $0 pointing to the actual command, program, shell script, or function and $1, $2, $3, ...$9 as the arguments to the command.

Following script uses various special variables related to command line:

#!/bin/sh

echo "File Name: $0"
echo "First Parameter : $1"
echo "First Parameter : $2"
echo "Quoted Values: $@"
echo "Quoted Values: $*"
echo "Total Number of Parameters : $#"

Here is a sample run for the above script:

$./test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
First Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2

Special Parameters $* and $@:

There are special parameters that allow accessing all of the command-line arguments at once. $* and $@ both will act the same unless they are enclosed in double quotes, "".

Both the parameter specifies all command-line arguments but the "$*" special parameter takes the entire list as one argument with spaces between and the "$@" special parameter takes the entire list and separates it into separate arguments.

We can write the shell script shown below to process an unknown number of command-line arguments with either the $* or $@ special parameters:

#!/bin/sh

for TOKEN in $*
do
   echo $TOKEN
done

There is one sample run for the above script:

$./test.sh Zara Ali 10 Years Old
Zara
Ali
10
Years
Old

Note: Here do...done is a kind of loop which we would cover in subsequent tutorial.

Exit Status:

The $? variable represents the exit status of the previous command.

Exit status is a numerical value returned by every command upon its completion. As a rule, most commands return an exit status of 0 if they were successful, and 1 if they were unsuccessful.

Some commands return additional exit statuses for particular reasons. For example, some commands differentiate between kinds of errors and will return various exit values depending on the specific type of failure.

Following is the example of successful command:

$./test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
First Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2
$echo $?
0
$

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