passing variable to sed command

Question

i need to pass a variable value to sed command. here is the description, what i am doing.

i am assigning a value to variable :

export ALSIZE=14420 $ echo $ALSIZE 14420

now i am using that value in sed command to read file from $ALSIZE line to end of the file. and i got the error

$  sed -n '$ALSIZE,$p' /db1/u04/oradata/GG11/ggserr.log
Unrecognized command: $ALSIZE,$p ====== >> 

used the variable value in "" (double quotes) still got error.

$ sed -n '"$ALSIZE",$p' /db1/u04/oradata/GG11/ggserr.log
Unrecognized command: "$ALSIZE",$p  ===== >>> 

i am getting response back

$ sed -n '14420,$p' /db1/u04/oradata/GG11/ggserr.log

2013-12-26 06:36:17  INFO    OGG-01026  Oracle GoldenGate Capture for Oracle, dpsbprd.prm:  Rolling over remote file ./dirdat/siebel/r1003911.

2013-12-26 06:43:31  INFO    OGG-01026  Oracle GoldenGate Capture for Oracle, dpsbprd.prm:  Rolling over remote file ./dirdat/siebel/r1003912.
2013-12-26 11:07:47  INFO    OGG-01026  Oracle GoldenGate Capture for Oracle, dpsbprd.prm:  Rolling over remote file ./dirdat/siebel/r1003913.

what is the mistake i am doing. could you please advice ?

Answer 1

The single quotes means that the '$' in $ALSIZE is expanded as a literal "\$ALSIZE". What happens if you rewrite it to:

sed -n $ALSIZE',$p' /db1/u04/oradata/GG11/ggserr.log

(leaving the single quotes until after the $ALSIZE variable)?

Answer 2

use double quotes:

kent$  s=5

kent$  seq 10 |sed -n "$s,$ p"
5
6
7
8
9
10

Answer 3

All of these would work:

sed -n $ALSIZE',$p' /db1/u04/oradata/GG11/ggserr.log
sed -n "$ALSIZE"',$p' /db1/u04/oradata/GG11/ggserr.log
sed -n "$ALSIZE,\$p" /db1/u04/oradata/GG11/ggserr.log

The key is understanding how quoting works:

• You can quote part of a string or all of it: hello, hel'lo', 'hello', 'hel''lo' are all the same to the shell
• Shell variables within double quotes are expanded to their value: "$ALSIZE" is expanded to "14420", which doesn't need to be quoted, so you could write simply $ALSIZE without quoting
• Shell variables within single quotes are not expanded, so '$ALSIZE' will be used a the literal text "$ALSIZE" which is not what you want
• In the "$ALSIZE,\$p" example the second $ has to be escaped to prevent the shell from expanding the $p variable, which probably has no value