Shell - saving contents of file to variable then outputting the variable

Question

First off, I'm really bad at shell, as you'll notice :)

Now then, I have the following task: The script gets two arguments (fileName, N). If the number of lines in the file is greater then N, then I need to cut the last N lines, then overwrite the contents of the file with it.

I thought of saving the contents of the file into a variable, then just cat-ing that to the file. However for some reason it's not working.

I have problems with saving the last N lines to a variable.

This is how I tried doing it:

lastNLines=`tail -$2 $1`
cat $lastNLines > $1

Maybe I misunderstood, but wouldn't tail -n N inFile > outFile be good enough? — pfnuesel
– pfnuesel, Commented Nov 16, 2013 at 20:47
@pfnuesel the problem is the OP wants to overwrite the same file he's reading from. — shx2
– shx2, Commented Nov 16, 2013 at 20:49
@pfnuesel By the looks of it, he's trying to write to the same file he's reading from. When you end something in >$1, Bash will open it for writing, thus emptying the file, before executing tail. In other words, tail would just find an empty file. — RoliSoft
– RoliSoft, Commented Nov 16, 2013 at 20:50

RoliSoft · Accepted Answer · 2013-11-16 20:55:52Z

2

Your lastNLines is not a filename. cat takes filenames. You also cannot open the input file for writing, because the shell truncates it before tail can get to it, which is why you need to use a temporary file.

However, if you insist on not using a temporary file, here's a non-portable solution:

tail -n$2 $1 | sponge $1

You may need to install moreutils for sponge.

answered Nov 16, 2013 at 20:55

RoliSoft

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shx2 · Accepted Answer · 2013-11-16 20:54:13Z

1

The arguments cat takes are file names, not the content.

Instead, you can use a temp file, like this:

tail -$2 $1 > $1._tmp
mv $1._tmp $1

To save the content to a variable, you can do what you already included in your question, or:

lastNLines=`cat $1`

(after the mv command, of course)

edited Nov 16, 2013 at 20:54

answered Nov 16, 2013 at 20:47

shx2

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1 Comment

Lord Zsolt Over a year ago

Okay, and how about saving the contents of the file to a variable.

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