Basic sed questions and answers

  1. What is sed? – sed is stream editor, a Unix tool for working with streams of text data. See the awful truth about sed.
  2. How do you substitute strings with sed? – Use ’s/old/new’ command, so sed ’s/hello/goodbye/’ would substitute the occurrence of the word hello to goodbye.
  3. How do you inject text with sed? – & in the substitution string defines the pattern found in the search string. As an example, here’s us trying to find a word ‘hello’ and replacing it with ‘hello and how are you’:

    echo ‘hello there’ | sed ’s/^hello/& and how are you/’

  4. Can I find several patterns and refer to them in the replacement string? – Yes, use (pattern) and then refer to your patterns as \1, \2, \3 and so on.
  5. If the string is ‘old old old’ and I run ’s/old/new’, I get ‘new old old’ as the result. I need ‘new new new‘. – You forgot the global modifier, which would replace every occurrence of the pattern with the substitution. ’s/old/new/g‘ will work.
  6. But I want ‘old old new’ from the previous example. – Just use the numeric modifier saying you want the third occurrence to be replaced. ’s/old/new/3‘ will work.
  7. I wrote a rather complex sed script. How do I save and run it? – Assuming that your file is named myscript1.sed, you can invoke sed -f myscript1.sed.
  8. How do I delete trailing whitespaces from each line? – sed ’s/[ \t]*$//’ Here we’re replacing any occurrence of a space or a tab with nothing. Check sed one-liners for more examples.
  9. How do you print just a few first lines of the file? – sed 1q will give you just the first line, sed 10q the first 10 lines.
  10. How do you replace a pattern only if it’s found, so that it’s executed faster? – Nest the replacement statement: sed ‘/old/ s/old/new/g’ file.txt

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