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Tuesday, January 17, 2012

Keeping data and code in one shell script file

In the past I have written scripts which required a lot of data. I could have the script read a second file with all the data in it but occasionally I'd lose the data file and still have the script. Or I'd have dozens of scripts and data files but no idea which data went with which script.

Solution: put the script and the data into one file. The script could then read the data from itself. Here is an example script:


for entry in $(sed -n -e '/^exit/,$ p' $0 | grep -v exit | sed -e '/^$/d' | sed -e '/^#.*/d' | sed -e 's/ /_/g'); do
        entry=$(echo $entry | sed -e 's/_/ /g')
        firstname=$(echo $entry | awk -F, '{print $1}')
        lastname=$(echo $entry | awk -F, '{print $2}')
        number=$(echo $entry | awk -F, '{print $3}')
        echo "$firstname $lastname's phone number is $number"

Darrell,Grainger,(416) 555-1212
John,Doe,(323) 555-1234
Jessica,Alba,(909) 555-9999

In this example, the data is a list of comma separated fields. Let's examine the list in the for statement. The $0 is the file currently executing, i.e. the file with this script and the data.

The sed command prints everything from the line which starts with exit to the end of file. The grep command gets rid of the line which starts with exit. The next sed command discards all blank lines. The third sed command discards all lines which start with #. This allows us to start a line with # if we want to add comments or comment out a line of data.

The final sed command on the for statement replaces all spaces with underscores. The reason for this is because if I have a line with a space, the for statement will process it as two separate records. I don't want that. I want to read one line as one record.

Inside the body of the for loop, the first line converts all the underscores back to spaces. If you want to have underscore in your data, this will not work. The solution is to pick a character which is not part of your data set. You can pick anything so long as the character you pick is the same in the for statement and the first line of the for loop body. The g in the sed statement is important in case there is more than one space.

The next three lines show how to break the line apart by commas. If you need to use commas in your data then pick a different character to separate the fields. The -F switch in the awk statement sets the field separator. So if you use exclamation mark as a field separator, you need to change the awk statement to -F!.

The echo statement is just an example of using the data.

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