Dot operator in Scripting.

-

Using the Unix Dot (.) Operator to Run a Shell Script in the Current Shell

In unix, there is the dot operator. The syntax is a dot (period), followed by a space, and followed by a shell script:

. script

Why would you use the dot operator? 

Normally, when you run a shell script, it executes in a child shell. Once the script completes, the child shell goes away and you are returned to the command prompt on the original shell. This means that the scope of the script doesn't apply to the invoking shell.

In other words, if your script changed the directory or set some variables, those changes won't be reflected after the script ends.

Using a dot space in front of the script means that the scope of the script is the same as the invoking shell.

For example, let's say that you are currently in the /usr directory and you run the script go_tmp, which changes the directory to /tmp.

If you run go_tmp then, after the script ends, you will still be in /usr.

If you run . go_tmp, then you will be in /tmp after it runs.

As another example, suppose you have a script set_java_classpath, which modifies your CLASSPATH environment variable so that you can run java programs. You will need the dot operator (i.e. run . set_java_classpath) so the new path is still there after the script ends.

The dot operator is also used a lot in shell scripts that need to call other scripts (like subroutines).

Comments

Post a Comment

Popular posts from this blog

Standard Error

-
In  addition to standard input and standard output, there is another place known as  standard error. This is where most Unix commands write their error messages. And as with the other two "standard" places, standard error is associated with your terminal by default. In most cases, you never know the difference between standard output and standard error: [root@server1 ~]# ls n* ls: n*: No such file or directory  [root@server1 ~]# ls n* > foo ls: n*: No such file or directory You can also redirect standard error to a file by using the notation command 2> file No space is permitted between the 2 and the > . Any error messages normally intended for standard error will be diverted into the specified file , similar to the way standard output gets redirected. [root@server1 ~]# ls n* 2> errors [root@server1 ~]# cat errors ls: n*: No such file or directory
Read more

Shell Basic Operators.

-
There are various operators supported by each shell. There are following operators which we are going to discuss: Arithmetic Operators. Relational Operators. Boolean Operators. String Operators. File Test Operators. The Bourne shell didn't originally have any mechanism to perform simple arithmetic but it uses external programs, either  awk  or the must simpler program  expr . Here is simple example to add two numbers: #!/bin/sh val=`expr 2 + 2` echo "Total value : $val" This would produce following result: Total value : 4 There are following points to note down: There must be spaces between operators and expressions for example 2+2 is not correct, where as it should be written as 2 + 2. Complete expresssion should be enclosed between  `` , called inverted commas. Arithmetic Operators: There are following arithmatic operators supported by Bourne Shell. Assume variable a holds 10 and variable b holds...
Read more

AWK

-
AWK Structure pattern {action} BEGIN and END patterns in the AWK BEGIN { print “BEGIN” } { print } END   { print “END”  } # cat textfile.txt  Ashish 123 Ashi     456 Ash      789 A          000 # awk '{print $1}' textfile.txt  Ashish Ashi Ash A # awk '{print $2}' textfile.txt  123 456 789 000 # awk '{print $2,$1}' textfile.txt  123 Ashish 456 Ashi 789 Ash 000 A If the file is seperated with some delimeter, then we make use of -F for awk. In the below example, the input file is using the pipe ( | ) as delimiter. # cat textfile.txt  Ashish|123 Ashi|456 Ash|789 A|000 Ashish|007 # awk -F\| '{print $1}' textfile.txt  Ashish Ashi Ash A Ashish # awk -F\| '{print $2}' textfile.txt  123 456 789 000 007 # awk -F\| '{print $2,$1}' textfile.txt  123 Ashish 456 Ashi 789 Ash 000 A 007 Ashish # awk...
Read more