Echo

echo -e "\n" >> filename will add new line (that’s why -e to the filename). You can use single quotes so you do not need to write -e and \n in multiline text but you need to escape other '. The best was to write multiline text is with cat > filename << HERE_DOC ... some lines with ' or " ... HERE_DOC. First \HERE_DOC or 'HERE_DOC' when no parametar expanded. Remember to use double << not single < redirection. Inside HERE_DOC there should not be backtick `` since it will be evaluated as shell command.

cat > my.txt << 'HERE_DOC'
here can be ' " \ / anything
HERE_DOR

Sed

sed -i '/haus/a home' filename will inplace (-i) search for haus and append home after that line (beside insert before i, this could be a append, c change matched line, d delete matched line). Multiple lines need to have “" at the end of line because only first line will be used otherwise. When using double quotes you also need \n (last line should not put \n\ because you probably do not want to insert blank new line). With single quotes multiline echo ' ...' does not need that new line character (last line should ends with ' because if you put on next line you will insert blank line). Double backslash at beggining is needed so first line is properly indented. Remember that no char (even space) could be after last \ .

# double quotes need \n, can close quote in new line, need double \\ at begging
sed -i config/routes.rb -e "/^end$/i \\
  # root page\n\
  root 'pages#home'\
"

# single quotes, no need for \n
# I do not like this because I need to use double quotes in ruby or ugly escape
sed -i config/routes.rb -e '/^end$/i \
  # root page\
  root '"'"'pages#home'"'"'\
  root "pages#home"'

Most common usage is to add before some line, for example line begins with test. If you need single quote ' for example 'a' than you need to use prefix $ and escape quote with single backslash ` \ . You could also use string concatenation: single with double quote string ’“‘a’”’. If you need " than put \”` so backslash survive bash command.

sed -i config/secrets.yml -e $'/^test:/i \
  # aws s3\
  aws_bucket_name: <%= ENV[\'AWS_BUCKET_NAME\'] %>'

Instead of search, you can append on line numbers 3,6aTEXT or at the last line sed '$aTEXT_AT_END (last line is $ or "\$" if " is used) or at first line

sed -i app/assets/stylesheets/application.scss -e '1i\
/*\
 *= require rails_bootstrap_forms\
 */'

sed 's/find/replace/g will replace word find with replace.

Brackets need to be escaped like \(, \1 means first group, & means matched text… tutorial Adding , and new text after match could be with sed 's/\(match.*\)/\1,\ntext/

Sed has something different regular expressions so follow this link

When you want to change chars (and not whole line) than you can use s/me/you/g, or several regexp in same command for example

sed src/app/index.module.coffee \
  -e 's/, /,\n  /g' \
  -e 's/  \[/\[\n  /g' \
  -e 's/\]/,\n\]/'

If you have a lot regexp, than its better to use here-doc and read from standard input - link

sed src/app/index.module.coffee -f - <<HERE_DOC
s/, /,\n  /g
s/  \[/\[\n  /g
s/\]/,\n\]/
HERE_DOC

Note that all commands one per line. As with sed '' you need backslash for each line of multiline command.

When you really need to add multiline template and don’t want to escape ' and to add “" to the end of each line, than you can try following command inspiration which will replace new lines with ћ (N means multiline match, a label… multiline match :a;N;$!ba;s/\n/ћ/g, note that $ needs to be escaped if inside "") and than return back new line.

Quote in 'HERE_DOC' will not substitute params so $ / or \ will remain in here doc. Instead of << you can use <<- and closing HERE_DOC can be indented with tab character. For regexp we need to escape / and ** with \/ and \\ (s:\\:\\\\:g;s:/:\\/:g)

sed src/app/index.module.coffee -e "s/client/$(sed ':a;N;$!ba;s/\n/ћ/g;s:\\:\\\\:g;s:/:\\/:g;' <<'HERE_DOC'
    I'm "$just"
    some long /'\ multiline <\template>.
HERE_DOC
)/g;s/ћ/\n/g"

Sometimes we need to add/replace line with template (not replace regexp) than we need to move replacing s/#/\n/g outside of sed since it will not be applied to new template. Note than we can’t use inline replacement but we can move tmp file.

sed src/app/index.module.coffee -e "/PLACEHOLDER/a $(sed ':a;N;$!ba;s/\n/ћ/g;s:\\:\\\\:g;s:/:\\/:g;' <<'HERE_DOC'
    I'm "$just"
    some long /'\ multiline <\template>.
HERE_DOC
)" | sed "s/ћ/\n/g" > tmp && mv tmp src/app/index.module.coffee

Here is another solution which replace the PLACEHOLDER line link

cat > /tmp/template <<\HEREDOC
    I'm "$just"
    some long /'\ multiline <\template>.
HERE_DOC
sed -i myfile.rb -e '/PLACEHOLDER/ {
  r /tmp/template
  d
}'
rm /tmp/template

If you need sudo than you can use tee for example

cat << HERE_DOC | sudo tee -a /etc/systemd/system/mongodb.service
HERE_DOC

Grep

Just a few command line options with grep

  • grep pattern filename so if you omit filename than standard input will be used, which is nice to test your big regex, for example asd << (exit with Ctrl+d)
    • grep "asd <<"
  • grep -o 111 will output only matched parts (not whole possible big lines)
  • grep -l asd output only filenames
  • grep only specific file type is with grep asd --include \*.yml or with a find command
    • find . -name *.yml -exec grep asd {} \;
  • rename could be done with simple regex:
    • rename 's/\.in.html.erb/\.html.erb/' app/views/*/*
    • rename 's/\.in.html.erb/\.html.erb/' app/views/*/*/* for nested folders
  • if you want to output only matching group than it is better to use sed
    • sed -n 's/^.*[^0-9]\([0-9][0-9]*\).*/\1/p' get only numbers
    • echo "asd123" | sed -n 's/asd/***/p' replace asd with ***
  • exclude mathing with grep -v that is invert match. This is usefull when you wan to exclude current grep for example ps aux | grep -v grep | grep my-process

Regex

https://regex101.com/ enable multiline flag if you are using $ end of line https://github.com/tom-lord/regexp-examples#usage to show matching examples

  • OR is with (|). But you need to escape | for example: \(asd\|qwe\) lines that contains asd or qwe. We need parentheses because alternator operator | has the lowest precedence of all, so usually you want world boundaries like ^([0-9]|[1-9][0-9])$ (0..99, but not 0asd, or asd9
  • contains asd but not qwe is asd((?!qwe).)*$. Oposite not containing.
  • contains asd and qwe but not zxc in between asd((?!qwe).)*zxc
  • include end of line for multiline search, use matcher \_. finds any character including end-of-line. Use \n for new line characterm for example asd(.|\n)*<div>. \{-} stopping at first occurence (* is too greedy and would stop at last occurence). If you want ruby regex ignore new line you can use modifier m, like match all dl s.match /dl.*dl/m
  • to stop at first match use non greedy match, just add ? after ? or * or +, like registration_email.html_part.decoded.match(/(http:.*?confirmation.*?)"/)[1] to grab inside first next ".
  • to include matching delimiter when spliting in ruby, instead of content.split(/[?.!]/) we can use a positive lookbehind regular expression (i.e. ?<=) inside a parenthesis capture group to keep the delimiter at the end of each string content.split(/(?<=[?.!])/) https://stackoverflow.com/questions/18089562/how-do-i-keep-the-delimiters-when-splitting-a-ruby-string
  • match start of line with lowercase ^[a-z] (but one string could have multiple lines). Better is to check start of string with lowercase \A[a-z] (in ruby example) "123\ntest".match /^[a-z]/ will return t… (you can use alternative string.start_with? /^[a-z]/
  • repetative match is with {number} like for example for numbers 000…999 use ^[0-9]{3}$. To find two or more spaces you can use \s{2,} so 2 is minimum count of matching chars.
  • whitespace \s, word \w character
  • \A start and \z end of string. That is better than ^ (start of line) and $ (end of line) since that will match until a new line asd@asd.asd\n<script>alert('danger')</script>
  • replace groups of text with another text that use matching text, for example add space if there is no space before MAC, 'aMAC'.gsub /([^\s])(MAC)/, '\1 \2'

When using grep, you can enable Per regexp PCRE with -P or --per-regexp. That is needed for negative lookahead. When using -P than no need to escape.

In bash you need to escape *()|* (for both single and double quotes). In vim you also need to escape again (for both single and double quotes): grep '\(asd\\|qwe\)' is the same as in vim :grep '\\(asd\\\|qwe\\)'. Note When using -P than no need one escape (vim escape is still required).

In vim vim pattern search inside buffer.

AWK

  • split by one char: echo "1:3" | awk '{split($0,a,":");print a[1] a[2]}' # 13
  • split by multiple char: echo "1:3x5" | awk -F':|x' '{print $1 $2 $3}' # 135