I am trying to remove lines from a text file from a Bash Script using command sed.
Here is how this function works.
User enters record number
Program Searches for record number
Program deletes record
Here is my code:
r=$(grep -h "$record" student_records.txt|cut -d"," -f1) #find the record that needs to be deleted
echo $line -->> This proves that previous command works
sed -i '/^$r/d' student_records.txt -->> this does not work
Any ideas?
To remove a line containing $record from the file:
grep -v "$record" student_records.txt >delete.me && mv -f delete.me student_records.txt
In the above, $record is treated as a regular expression. This means, for example, that a period is a wildcard. If that is unwanted, add the -F option to grep to specify that $record is to be treated as a fixed string.
Consider these two line:
r=$(grep -h "$record" student_records.txt|cut -d"," -f1) #find the record that needs to be deleted
echo $line -->> This proves that previous command works
The first line defines a shell variable r. The second line prints the shell variable line, a variable which was unaffected by the previous command. Consequently, the second line is not a successful test on the first.
sed -i '/^$r/d' student_records.txt -->> this does not work
Obseve that the expression $r appears inside single-quotes. The shell does not alter any inside single quotes. Consequently, $r will remain a dollar sign followed by an r. Since a dollar sign matches the end of a line, this expression will match nothing. The following would work better:
sed -i "/^$r/d" student_records.txt
Unlike the grep command, however, the above sed command is potentially dangerous. It would be easy to construct a value of r that would cause sed to do surprising things. So, don't use this approach unless you trust the process by which you obtained r.
record?If there is more than one line that matches record, then the following would generate an unterminated address regex error from sed:
r=$(grep -h "$record" student_records.txt|cut -d"," -f1)
sed -i "/^$r/d" student_records.txt
This error is an example of the surprising results that can happen when a shell variable is expanded into a sed command.
By contrast, this approach would remove all matching lines:
grep -v "$record" student_records.txt >delete.me && mv -f delete.me student_records.txt
sed -i "/^$record,/d" student_records.txt
grep to obtain $r from $record is rather roundabout, still.$record and delete that line via grep. This makes unnecessary the creation of r. (The same direct approach could be done with sed -i.)