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I am new to shell scripting and I'm very confused between awk -FS and awk -f commands used. I've tried reading multiple pages on the difference between these two but was not able to understand clearly. Kindly help. Here is an example: Lets consider that a text file say, data.txt has the below details.

S.No Product Qty Price
1-Pen-2-10
2-Pencil-1-5
3-Eraser-1-2

Now, when i try to use the following command:

$ awk -f'-' '{print $1,$2} data.txt

I get the below output:

1 Pen
2 Pencil
3 Eraser

But when i use the command:

$ awk -FS'-' '{print $1,$2} data.txt

the output is:

1-Pen-2-10
2-Pencil-1-5
3-Eraser-1-2

I don't understand the difference it does using the -FS command. Could somebody help me out on what exactly happens between these two commands. Thanks!

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2 Answers 2

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7

You are more confused than you think. There is no -FS.

FS is a variable that contains the field separator.

-F is an option that sets FS to it's argument.

-f is an option whose argument is the name of a file that contains the script to execute.

The scripts you posted would have produced syntax errors, not the output you say they produced, so idk what to tell you...

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4 Comments

Thank you! i think i understood this concept very badly. my bad. thanks for the clarification. Will work on it.
Technically -FS'-' is the same as -F "S-". So, echo "QRS-TUV" | awk -FS'-' '{print $1, $2}' outputs QR TUV
this was what my doubt was too. but when i try to use echo "ABC-DEF" | awk -FS'-' '{print $1,$2}' , I get ABC-DEF only
@Dev that would be the expected output since S- (as stored in FS) does not occur anywhere in your input so it only has 1 field so $1 is ABC-DEF. If you check the output you file a blank char added to the end of the line - that's the output field separator (OFS) before the null string represented by $2. btw wrt your incorrect use of the word doubt - the correct English would be thought or expectation or similar. doubt is when you think negatively about something.
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-FS is not an argument to awk. -F is, as is -f.

The -F argument tells awk what value to use for FS (the field separator).

The -f argument tells awk to use its argument as the script file to run.

This command (I fixed your quoting):

awk -f'-' '{print $1,$2}' data.txt

tells awk to use standard input (that's what - means) for its argument. This should hang when run in a terminal. And should be an error after that as awk then tries to use '{print $1,$2}' as a filename to read from.

This command:

awk -FS'-' '{print $1,$2}' data.txt

tells awk to use S- as the value of FS. Which you can see by running this command:

awk -FS'-' 'BEGIN {print "["FS"]"}'
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Thank you! my bad. need to work on this concept more. thanks for the clarification. Will work on it.

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