Wednesday, May 2, 2012

Different ways to print first few characters of a string in Linux



  In this article, we will see the different ways in which we can extract and print the first 3 characters of every line in a file. Assume a file with sample contents as below:
$ cat file
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1.  This is the best of all since its purely internal.  The file is read in the file loop and the first 3 characters are retrieved using the shell.
$ while read line
> do
>  echo ${line:0:3}
> done < file
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   ${x:0:3} means to extract 3 characters from position 0 from the variable x. In this way, the shell can also be used to extract sub-string from a variable. This is one of the most important features of the shell.

2. Using cut, we can cut the first 3 characters.
$ cut -c -3 file
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   The same can also be achieved by having 1-3 in place of -3.

3.  grep is used to print the contents of a file matching a specific content. grep prints an entire line by default. -o option of grep allows to print only the pattern matched. The dot(.) matches a single character. By giving 3 dots, it matches 3 characters and the control(^) character makes it to match from the beginning.
$ grep -o '^...' file
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4. In case of lesser characters, we can provide that many dots. What if it is some 20 characters? Giving that many dots will look clumsy. Regular expressions provide an option {n} which means the preceeding character should match n times. Here, a single character is matched using the dot(.), and the number 3 tells it to match it 3 times. The backslash(\) is to to prevent the grep from interpreting the '{' as literal.
$ grep -o '^.\{3\}' file
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5. The substr function of awk does it easily. The substr syntax is: substr(string,starting position, offset). The string is the entire line which is $0. 0 is the starting position, 3 is the number of characters to extract.
$ awk '{print substr($0,0,3);}' file
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6. The sed solution is almost same as the grep earlier which we did. The entire line is broken into two parts; first 3 characters and the rest. By giving \1, the first 3 characters are printed which are sub-grouped earlier.
$ sed 's/\(.\{3\}\).*/\1/' file
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7. The perl option also has the substr function. $_ represents the line read in perl.
$ perl -lne 'print substr($_,0,3);' file
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