Scripting

grep
sed
awk
perl

Here I will discuss some scripting languages which I am familiar with. I generally do stuff with sed and awk when it is simple and use perl when I find that the task is complex. I start with grep which is a quite powerful tool as far as searching is concerned. Note that at the beginning of code > is not the part of code.

grep

grep is a very powerful tool for searching patterns. If we want to print all the lines from a file named " file.txt " which have the word " foo " we use the following:

> grep "foo" file.txt

sed

sed is an acronym for stream-editor. It can be used to do cool things with a string of characters, which may come from a file or from the terminal output of a linux command or an executable. The main things for which people use sed are as follows:

Replaces an old pattern of characters by a new one.
```
> echo "sed is hot " | sed s/hot/cool/ 
```
replaces " hot " by " cool " . In the above " s " stands for substitution. If we want to replace " hot " by " cool " in a file named " file.txt " we use the following:
```
> cat file.txt | sed s/hot/cool/
```
Note that for global replacement (replacing everywhere) we use the option " g "
```
> echo "sed is hot " | sed s/hot/cool/g
```
Note that the above commands will write the output on the terminal without changing the input file. However, we can direct the output in a new file:
```
> cat file.txt | sed s/hot/cool/g > file1.txt 
```
will replace everywhere " hot " by " cool " in the file " file.txt " and will write output in a new file " file1.txt " .

There is a particular way for writing patterns and is universally followed by almost all scripting languages. The patterns written in this way are called " regular expression " or regexp . A regexp starts and end with a delimiter. In this tutorial we will be using " / " delimiters. I have a file called " mb.js " which I can see using Linux command " ls " . I want to see " mc " in place of " mb " for which I do the following:
```
>  ls -l *.* | sed s/mb/mc/
```
If we use sed with option " -n " it does not print anything
```
> cat file.txt | sed -n s/hot/cool/
```
however, if we use the option " p " it prints only the lines in which the pattern was found
```
> cat file.txt | sed -n s/hot/cool/p
```
Delete a pattern
In order to delete the word " foo " from a file " file.txt " we use the following:
```
> cat file.txt | sed /foo/d 
```
please note that " cat " is not a part of sed , it is a linux command to see the content of a file without opening it in an editor like " vi " and " | " is called " pipe " which is used to combine more than one linux commands.

For more detail visit here or here .

awk

awk is an amazing tool which can be used to manipulate string of characters which may be in the form of content of a file or terminal output. Some of the most interesting things about awk are that it recognizes rows and columns in a file and is compatible with the syntax of the c programming. In order to print the third column of a multicolum file named " file.txt " we use the following:

> cat file.txt | awk '{print $3}'

> cat file.txt | awk '{printf(" %s\n",$3)}'

note that here we have used the syntax of c which makes awk very powerful. For example, we can print the average of third and four columns using the following:

> cat file.txt | awk '{printf(" %2.6f\n",($3+$4)/2)}'

using " $ " we can get any we wish and do things with that, however, there is a way to address rows also. For that there is a key work " NR ". This following piece of code will not print the third column of the file, it will print line (row) numbers also

> cat file.txt | awk '{printf(" %d %s\n ",NR,$3)}'

sometime we can do great things by conditional use of awk. For example, the following piece of code prints the square toot of the third column when it is positive

> cat file.txt | awk '{if($3 > 0.0) printf(" %2.6f \n ",sqrt($3))}'

between single quotes we can embed a piece of c and can get input for that by using " $ "

One interesting thing to note is that using pipe " | " we can join codes together. For example, the following piece of code uses both sed and awk.

> cat file.txt | sed -n s/hot/cool/p | awk '{printf(" %s\n",$3)}'

perl

perl is a very flexible scripting language. It can be used to manipulate strings, accommodates many features of c programming, and by mixing linux commands and using external executable can do amazing things. The " hello world " of perl is as follows:

#!/usr/bin/perl

print "Hello World ! \n";

write the above program in a file and name it something.pl (hello.pl) and run it as follows:

> perl hello.pl

if it prints " Hello World ! " on terminal you are successful otherwise check that weather perl is properly installed on your computer or not.

Every perl program must start with " #!/usr/bin/perl & . Apart from the above way, we can run a perl program by making it executable.

>  chmod a+x hello.pl 

> ./hello.pl