Pass grep output to variable and use that variable as input to sed in shell script

Question

I'm looking to write a shell script that grabs a line number from a file using grep, and use that line numbers as head and tail for sed command to cut the file.

My script looks something like this:

head=$(grep -n -i -B 1 "^\s\+abcd" <sourcefilename> | head -n 1 | cut -d: -f1)
tail=$(grep -n -i  -B 1 " efgh" <sourcefilename>  | tail -n 1| cut -d: -f1)

if($head!=NULL)
then
        sed -n "$head,$tailp" <sourcefile>.txt > <newfile>.txt
fi

My goal, is to use first grep, and get the head line number when it matches the pattern, then use the second grep to get tail line number when it matches the pattern, and use those to as inputs for sed with -n switch and create a file that only has line numbers from head to tail.

If I execute it individually against the file, like

grep -n -i "^\s\+abcd" <filename> | head -n 1 | cut -d: -f1 , it gives me 11 and 
grep -n -i  " efgh" <filename>  | tail -n 1| cut -d: -f1 gives me 106.

Then I used these numbers as inputs and do

sed -n 11,106 <sourcefile>.txt > <newfile>.txt

it works perfectly. I'm trying to automate the process to have a script that can run against multiple files at once.

Also, the if statement with NULL means when grep doesn't return anything, just don't run the loop, which seems to also error out.

com.new.security.JXSecurityException: No certificates encoded in supported ways were found at com.tibco.security.CertUtils.streamToCerts(CertUtils.java:634) at com.tibco.ae.tools.palettes.generalpalette.ImportTrustedCertCommand.buttonPressed(ImportTrustedCertAction.java:133) at com.tibco.ae.designerapi.forms.ConfigForm.actionPerformed(ConfigForm.java:1326) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995) at com.tibco.security.CertUtils.streamToCerts(CertUtils.java:626) ... 86 more — Tk_93
For example there is this above stack trace. Use: grep -n -i "^\s\+at" sample.txt | head -n 1 | cut -d: -f1 grep -n -i " more" sample.txt | tail -n 1| cut -d: -f1 — Tk_93
$tailp is trying to expand a variable named tailp. If you want the value in the variable tail followed by a p, use ${tail}p — William Pursell

James Brown James Brown · Accepted Answer · 2017-07-11T23:23:32

You could probably solve that with awk only. First some data:

$ cat file
1
2
3
4
5
$ awk '/2/,/4/' file
2
3
4

You could replace the 2 and 4 with your proper regexen for the head and the tail.

Edit: an example of grep -B 1:

$ awk '/2/{f=1;print p} f{print} /4/{f=""} {p=$0}' file
1
2
3
4

Pass grep output to variable and use that variable as input to sed in shell script

3 Answers