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Using awk to find the offset of the pattern, and dd to truncate the file at that point and append the new trailer:

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("echo \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           exit
       }
       {l+=length+1}
    ' {} \;
}

With an awk implementation which supports nextfile (gawk, bwk, some versions of mawk), that could be done more efficiently by passing batches of files to awk:

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("echo \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           l=0; nextfile
       }
       {l+=length+1}
    ' {} +
}

$ truncatoo '/END DATA/' 'NEW END' file.txt
$ truncatoo '/END DATA/' 'NEW END' . -type f -name '*.txt'

$ file="a'b\$q  * r"; seq 1 100 >"$file"
$ truncatoo /7/ EOT "$file"; cat "$file"
1
2
3
4
5
6
EOT

Instead of the icky 2>/dev/null status=noxfer could be used with dd implementations which support it.

The quoting kludge and environment variables passing is a mess, it could use some improvement.

Using awk to find the offset of the pattern, and dd to truncate the file at that point and append the new trailer:

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("printf \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           exit
       }
       {l+=length()+1}
    ' {} \;
}

truncatoo '/END DATA/' 'NEW END\n' file.txt
truncatoo '/END DATA/' 'NEW END\n' . -type f -name '*.txt'

With an awk implementation which supports nextfile (gawk, bwk, some versions of mawk [1]), that could be done more efficiently by passing batches of files to awk:

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("printf \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           l=0; nextfile
       }
       {l+=length()+1}
    ' {} +
}

$ file="a'b\$q  * r"; seq 1 100 >"$file"
$ truncatoo /7/ 'CUT\n' "$file"; cat "$file"
1
2
3
4
5
6
CUT

Instead of the icky 2>/dev/null status=noxfer could be used with dd implementations which support it.

The quoting kludge and environment variables passing is a mess, it could use some improvement.

[1]: according to the GNU awk manual, it should be supported in mawk too. However, the older version of mawk from Debian 10 does not support it.

With standard awk, and using dd to truncate the file, since awk doesn't support seek or truncate.

Instead of dd, truncate could be used on Linux, and instead of the icky 2>/dev/null status=noxfer could be used with dd implementations which support it.

The quoting kludge and environment variables passing is a mess, it could use some improvement.

TODO: nextfile doesn't work right in Debian's 10 mawk

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("echo \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           l=0; nextfile
       }
       {l+=length+1}
    ' {} +
}
 
truncatoo '/END DATA/' 'NEW END' file.txt
truncatoo '/END DATA/' 'NEW END' . -type f -name '*.txt'

 $ file="a'b\$q  * r"; seq 1 100 >"$file"
$ truncatoo /7/ EOT "$file"; cat "$file"
1
2
3
4
5
6
EOT

Using awk to find the offset of the pattern, and dd to truncate the file at that point and append the new trailer:

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("echo \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           exit
       }
       {l+=length+1}
    ' {} \;
}

With an awk implementation which supports nextfile (gawk, bwk, some versions of mawk), that could be done more efficiently by passing batches of files to awk:

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("echo \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           l=0; nextfile
       }
       {l+=length+1}
    ' {} +
}

$ truncatoo '/END DATA/' 'NEW END' file.txt
$ truncatoo '/END DATA/' 'NEW END' . -type f -name '*.txt'
 
$ file="a'b\$q  * r"; seq 1 100 >"$file"
$ truncatoo /7/ EOT "$file"; cat "$file"
1
2
3
4
5
6
EOT

Instead of the icky 2>/dev/null status=noxfer could be used with dd implementations which support it.

The quoting kludge and environment variables passing is a mess, it could use some improvement.

With standard awk, and using dd to truncate the file, since awk doesn't support seek or truncate.

Instead of dd, truncate could be used on Linux, and instead of the icky 2>/dev/null status=noxfer could be used with dd implementations which support it.

The quoting kludge and environment variables passing is a mess, it could use some improvement.

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("echo \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           l=0; nextfile
       }
       {l+=length+1}
    ' {} +
}

truncatoo '/END DATA/' 'NEW END' file.txt
truncatoo '/END DATA/' 'NEW END' . -type f -name '*.txt'

$ file="a'b\$q  * r"; seq 1 100 >"$file"
$ truncatoo /7/ EOT "$file"; cat "$file"
1
2
3
4
5
6
EOT

With standard awk, and using dd to truncate the file, since awk doesn't support seek or truncate.

Instead of dd, truncate could be used on Linux, and instead of the icky 2>/dev/null status=noxfer could be used with dd implementations which support it.

The quoting kludge and environment variables passing is a mess, it could use some improvement.

TODO: nextfile doesn't work right in Debian's 10 mawk

# usage truncatoo pattern new_end find_args ...
truncatoo(){
    pat=$1; shift; tail=$1; shift
    LC_CTYPE=C TAIL=$tail find "$@" -exec awk -v q="'" "$pat"'{
           gsub(q,q"\\"q q,FILENAME);
           system("echo \"$TAIL\" | dd bs="l" seek=1 of="q FILENAME q" 2>/dev/null");
           l=0; nextfile
       }
       {l+=length+1}
    ' {} +
}

truncatoo '/END DATA/' 'NEW END' file.txt
truncatoo '/END DATA/' 'NEW END' . -type f -name '*.txt'

$ file="a'b\$q  * r"; seq 1 100 >"$file"
$ truncatoo /7/ EOT "$file"; cat "$file"
1
2
3
4
5
6
EOT