How to form a sed expression containing escaped characters

sed

Given a sed expression (and GNU sed 4.2.2 on ArchLinux)

/match/i\tline1\n\tline2

which should insert two tab-indented lines above the match, I find that the escaping of the first character (in the example, \t) is ignored but all other escaped characters are treated correctly.

Testing this like this:

echo match | sed -e '/match/i\tline1\n\tline2'

results in

tline1
    line2
match

It doesn't matter what the initial escaped character is (e.g. tab or newline) the result is the same. What is the correct way to consrtuct the expression so that the first character is treated correctly?

Best Answer

Check the gnu sed manual (http://www.gnu.org/software/sed/manual/html_node/Other-Commands.html#Other-Commands) -- the i command is actually the i\ command, so you just need an extra backslash

echo match | sed -e '/match/i\\tline1\n\tline2'
# ---------------------------^

Related Solutions

Bash – Left and right square brackets treated differently by sed/bash

[ introduces a character class and ] closes it.

If sed sees only ] (i.e. the closing command), it will be OK, sed assumes this is not special command.

Using only [ (without a closing ]) confuses sed, so it must be escaped.

On a side note, there is no need for

g (global) flag (only one substitution per line)
escape [ in replacement.

I managed your change with

sed -e "s/]/']/" -e "s/\[/['/"

sed – Extra Backslash Required in First Appended Newline

It's not exactly documented behaviour, but then the entire thing is an extension. All three commands that were extended similarly (a, i and c) behave the same way: leading whitespace is ignored, and if the text begins with \, then it is discarded. Essentially, it doesn't matter to sed which of these forms are used:

i foo
i \foo
i \
foo

They're all identical. (I know I used i here, but check the the descriptions of a, i and c - it's surreal.)

And the code shows as much:

1111         case 'a':
1112         case 'i':
1113         case 'c':
1114           ch = in_nonblank ();
1115 
1116         read_text_to_slash:
1117           if (ch == EOF)
1118             bad_prog (_(EXPECTED_SLASH));
1119 
1120           if (ch == '\\')
1121             ch = inchar ();

Best Answer

Related Solutions

Bash – Left and right square brackets treated differently by sed/bash

sed – Extra Backslash Required in First Appended Newline

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