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babag

09-17-2008 12:19 PM

how to replace with variable using sed?

i have the following:

Code:

variable=$(sed -n '3p' truefalse.txt)



echo $variable



if [ "$variable" = "True" ] ; then

    

    sed -i '3 cFalse' truefalse.txt

else

    

    sed -i '3 cTrue' truefalse.txt

fi



variable=$(sed -n '3p' truefalse.txt)



echo $variable

this reads line three of a simple text file and
changes True to False or False to True depending
on what the line says.

how would i replace the '3' with a variable?
what is the syntax for that?

i tried a couple of things but was just guessing
and it didn't work.

thanks,
BabaG

w3bd3vil

09-17-2008 12:27 PM

Here is how you do that.

Quote:

bash-3.1# TEST=3
bash-3.1# export TEST
bash-3.1# cat test.txt
1
2
3
4
5
bash-3.1# sed -n $TEST'p' test.txt
3

jan61

09-17-2008 01:41 PM

Moin,

here's a simple example:

Code:

jan@jack:~/tmp/tf> cat data

True

False

False

True

True

True

False

jan@jack:~/tmp/tf> LINE=3

jan@jack:~/tmp/tf> sed "$LINE{s/True/_/;s/False/True/;s/_/False/}" data

True

False

True

True

True

True

False

jan@jack:~/tmp/tf> LINE=5

jan@jack:~/tmp/tf> sed "$LINE{s/True/_/;s/False/True/;s/_/False/}" data

True

False

False

True

False

True

False

If you want to use variables within a sed, you may not use '' around the sed script - no shell substitutions are done (this is a shell feature, not limited to sed). You must use "" instead or you place the variable outside the '' - for example:

Code:

sed 's/'"$VAR"'/something/' file

Jan

babag

09-17-2008 03:16 PM

thanks to you both! exactly what i was looking for.

BabaG

jschiwal

09-17-2008 03:28 PM

If a part of the sed command is provided by a variable, you need to be wary of whether the variable may contain a character such as a forward slash, which you would normally escape. If the variable contains a pathname for example, consider using another character such as "#" instead of a slash.

There is a trick you can try to test if the arguments are being passed as you believe. This may help with debugging.

set sed "$VAR"'s#'$var1'#old-'"$var1"'#p' file
echo $2

The arguments for set will be in $0, $1, etc. So you can echo them and see how the variable expansion went. The "sed" command will be in $1. The arguments to sed start with $2.

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