bash - read or write to specific line in text file?
 

i have a text file that is to be modified by one script
and read by another. the text file is simple and short,
consisting of 16 true/false lines as follows:

true
false
true
true
true
false
false
false
false
false
false
false
false
false
false
false

how do i tell the first script to write to a specific line
in the textfile to modify it.

for example, line five is currently 'true' in the above
listing. how would i tell the script to overwrite that to
false?

the second script will be running a loop. depending on
how many times the loop has run, the script needs to
read a different line in the text file. how do i specify
to only read, say, line five of the above example?

the file is short enough that i probably could read the
whole thing into an array as i've been shown previousy
how to do that, but i thought this might be more efficient
and good to learn.

thanks,
BabaG

Like this, for example

sed -i '5s/true/false/' truefalse.txt

Well, for example, you can do it like this

sed -n '3p' truefalse.txt

prints the third line.

thanks uncle theodore!

so, how would i turn :

sed -n '3p' truefalse.txt

into something useful, like:

what would be the syntax for something like that?

thanks again,
BabaG

If you don't care what line 5 is, use this to unconditionally replace the line:ta0kira

i can get this to print out the third line:

but how do i get that third line into a variable?

tried:

but that didn't do it.

thanks,
BabaG

To get the output of a command, put it inside "$()".

thanks david the h.

this also seems to work (search is a good thing):

is there any advantage of one solution over the other?

thanks again,
BabaG

By the way, you can also use the same pattern directly in an if statement:

There are several good tutorials that explain the syntax details of the various scripting functions, such as http://www.linuxcommand.org

They both do the same thing, but most people find the $() syntax more readable and less mistake-prone, so it's usually recommended. I seem to recall that there may also be a few unusual situations where the backticks won't work, but I don't know enough about scripting myself to say for sure.

trying this but getting an error for unexpected 'else':

ok. two things.

first, found the problem with the unexpected 'else'.
i'd left out a ; between the ']' and 'then'.
thanks again david the h for the link. that's where i
found the missing ;.

secondly, i couldn't get either of the examples above
to replace the text in the truefalse.txt document. i
did, however, by playing, find that a sort of hybrid
did make this all work:
thanks everyone,
BabaG

Whoops! I missed that in my post too; so if you got it from me, I'm sorry. I've gone back and fixed it in my post.

PS: Don't forget that the test is case-sensitive.