Sed command to delete "\" which causes "*** multiple target patterns. Stop." error - sed

Question

In a file, I'm having the lines like this -
a.lo a.o: abc/util.c \
/usr/lib/def.h
b.lo b.o: hash/imp.h \
/usr/lib/toy.c \
c.lo c.o: high/scan.c \
high/scan_f.c
Here you can see one extra \ (back slash) at the end of line number 4 (/usr/lib/toy.c ). How can I use sed command to remove this / (back slash)? Because of this I'm getting "*** multiple target patterns. Stop." error.
P.S. - I'm having this extra \ (back slash) at multiple places in my file. So using sed to delete it by line number won't be feasible. Need something which can check for .lo .o and check a line before, if it finds a \ (back slash) remove it.

Answer 1

Maybe not the simplest but this should work:
sed -nE '${s/\\$//;p;};N;s/\\([^\\]*:)/\1/;P;D' input_file
The main idea is to concatenate input lines in the pattern space (a sed internal text buffer), such that it always contains 2 consecutive lines, separated by a newline character. We then just delete the last \ before a :, if any, print the first of the 2 lines and remove it from the pattern space before continuing with the next line.
sed commands are separated by semi-columns (;) and grouped with curly braces ({...}). They are optionally preceded by a line(s) specification, for instance $ that stands for the last line of the input. So, in our case, ${s/\\$//;p;} applies only to the last line while the rest (N;s/\\([^\\]*:)/\1/;P;D) applies to all lines.
The -n option suppresses the default output. We need this to control the output ourselves with the p (print) command.
The -E option enables the use of extended regular expressions.
Let's first explain the tricky part: N;s/\\([^\\]*:)/\1/;P;D. It is a list of 4 commands that are run for each line of the input because there is no line(s) specification before the commands.
When sed starts processing the input the pattern space already contains the first line (a.lo a.o: abc/util.c \ in your example). This is how sed works: by default it puts the current line in the pattern space, applies the commands and restarts with the next line.
N appends the next input line (/usr/lib/def.h) to the pattern space with a newline character as separator. The pattern space now contains:
a.lo a.o: abc/util.c \
/usr/lib/def.h
N also increments the current line number which becomes 2.
s/\\([^\\]*:)/\1/ deletes the last \ before the first : in the pattern space, if there is one. In our example the only \ is after the first :. The pattern space is not modified.
P prints the first part of the pattern space, up to the first newline character. In our example what is printed is:
a.lo a.o: abc/util.c \
D deletes the first part of the pattern space, up to the first newline character (what has just been printed). The pattern space contains:
/usr/lib/def.h
D also starts a new cycle but different from the normal sed processing, it does not read the next line and leaves the pattern space and current line number unmodified. So when restarting the pattern space contains line number 2 and the the current line number is still 2.
By induction we see that, each time sed restarts executing the list of commands, the pattern space contains the current line, as normal. When processing line number 4 of your example it contains:
/usr/lib/toy.c \
After N it contains:
/usr/lib/toy.c \
c.lo c.o: high/scan.c \
And there, the substitution command (s/\\([^\\]*:)/\1/) matches and deletes the first \:
/usr/lib/toy.c
c.lo c.o: high/scan.c \
It is thus:
/usr/lib/toy.c
that is printed and removed from the pattern space. Exactly what you want.
The last line needs a special treatment. When we start processing it the pattern space contains:
high/scan_f.c
If we don't do anything special N does not change it (there is no next line to concatenate) and terminates the processing. The last line is never printed.
This is why another list of commands is needed, just for the last line: ${s/\\$//;p;}. It applies only to the last line because it is preceded by a line(s) specification ($ for last line). The first command in the list (substitute s/\\$//) removes a trailing \, if there is one. The second (p) prints the pattern space.
Note: if you know that the last line does not end with a trailing backslash you can simplify a bit:
sed -nE '$p;N;s/\\([^\\]*:)/\1/;P;D' input_file

Answer 2

I agree with #G.M. in general, but this will work.
sed captures text before trailing "\" (if present) on lines starting with "\" and prints only that text on those lines. All other text is also printed, of course
sed -e 's/\(.* \)\\$/\1/' input_file

Answer 3

The question is a bit unclear about how to identify the lines from which a trailing backslash should be removed, but inasmuch as the input looks like set of a makefile-format prerequisite lists from which some lines have been removed, I take the objective to be to remove backslashes where they appear after the last (remaining) prerequisite in a list. That requires looking ahead to the next line, so it will be helpful to make use of sed's hold space to store data while you look ahead at the next line to figure out what to do with it.
This would be a pretty robust solution for that problem:
sed -nE 's/\s*(\\){0,1}$/ \\/; :a; /:/ { x; s/\s*\\$//; p; d; }; H; $ { s/.*/:/; b a }' input
That builds up each prerequisite list in the hold space, with backslashes and newlines embedded, then dumps it when the next target list or the end of the input arrives.
Details:
the -n option turns off automatically printing the pattern space after each line
the -E option turns on extended regular expressions
the sed expression contains several sub-expressions, joined by semicolons:
s/\s*(\\){0,1}$/ \\/ : ensure that the current line in the pattern space ends with a space and backslash, without adding a second backslash to lines that already have one
:a : labels that point in the script 'a'
/:/ { x; s/\s*\\$//; p; d; } : on lines that contain a colon, swap the pattern and hold spaces, remove the trailing backslash from (the new contents of) the pattern space, print the result, then start the next cycle
H : (if control reaches this point) append a newline and the contents of the pattern space to the hold space
$ { s/.*/:/; b a } : on the last line of input trigger dumping the hold space by putting a colon in the pattern space and jumping to label 'a'
[end of expression] : read the next line into the pattern space and start over
Alternatively, it would more exactly follow your request, and avoid introducing a leading blank line, to do this:
sed -n ':a; /\\$/! { p; d; }; h; :b; $ { x; s/\\//; p; }; n; /:/ { x; s/\\$//; p; x; b a; }; H; /\\$/ b b; s/.*//; x; p' input
That also assembles pieces in the hold space before ultimately printing them, but it goes about it in a different way:
it starts (at label a) by checking whether the line in the pattern space ends with a backslash. If not (/\\$/!), then it prints the pattern space and starts the next cycle.
otherwise, it replaces the current contents of the hold space with the contents of the pattern space (which must already end with a backslash), then
(at label b) if the current line is the last then it retrieves the contents of the hold space, strips the trailing newline, and prints the result ($ { x; s/\\//; p; }). Either way,
it attempts to read the next input line, and terminates if there are no more (n).
if that results in the pattern space containing a colon within, then the contents of the hold space are printed, less trailing backslash, and control is sent back to label a to process the colon-containing line as a new first line (/:/ { x; s/\\$//; p; x; b a; }).
otherwise, a newline and the contents of the pattern space are appended to the hold space (H).
if the pattern space ends with a backslash then control branches back to label b to consider reading another line (/\\$/ b b).
otherwise, the hold space is printed and cleared (s/.*//; x; p), and
if there are any more lines then the next is read and a new cycle started.
That makes fewer assumptions about the nature of the input, but it is a bit more complicated.