Count the number of lines with at least one occurrence of the y character:

$ cat test.txt
asd  dd :; >
y YYYyy  yyy
 .

asdkjlyes
    kjkjhkjhy

$ grep -o '.*y.*' ./test.txt | wc -l
3

Text replacement:

$ sed 's/string to replace/replacement string/g' original-file.txt > new-file.txt

Dynamic, in-place replacement of unreleased text with today’s date:

$ sed -i "s/unreleased/`date +'%Y-%m-%d'`/" ./changelog.md

Update the release date in citation.cff file:

$ perl -pi -e "s/date-released: \d+-\d+-\d+/date-released: $(date +'%Y-%m-%d')/" ./citation.cff

Replace all occurrences of str1 by str2 in all files below the /folder path:

$ find /folder -type f -print -exec sed -i 's/str1/str2/g' "{}" \;

Same as above but ignore all content of .svn folders and .zip files:

$ find /folder -type f -not -regex ".*\/\.svn\/.*" -not -iname "*\.zip" -print -exec sed -i 's/str1/str2/g' "{}" \;

Remove trailing spaces and tabs in every XML files:

$ find /folder -iname "*.xml" -exec sed -i 's/[ \t]*$//' "{}" \;

Place a new --- line at the start of each .markdown files ( see result  ):

$ find ./folder -iname "*.markdown" -exec sed -i '1s/^/---\n/' "{}" \;

Place a new --- line before the first empty line of each .markdown files ( see result  ):

$ find ./folder -iname "*.markdown" -exec sed -i '0,/^$/s//---\n/' "{}" \;

Remove lines starting with prefix1: or prefix2: in all .markdown files:

$ find /folder -iname "*.markdown" -exec perl -p -i -e 's/(prefix1|prefix2): .*\n//sg' "{}" \;

Strip time from date: prefixed lines and quote the value. I.e. replace all occurrences of date: 2012-01-01 00:00:00 +0000 by date: "2012-01-01" :

$ find /folder -iname "*.markdown" -exec perl -p -i -e 's/date: (\d+-\d+-\d+) .*\n/date: "$1"\n/sg' "{}" \;

Remove lines matching a regex (encoding particular markdown TOC entries ), save the result in place and save a backup of the original content in a .bak file:

$ gawk -i inplace -v INPLACE_SUFFIX=.bak '!/^- \[(Contribute|Contributing|Licence|License)\]\(#.+\)$/{print}' ./readme.md

Use sed address ranges to spot, in a Markdown file, all blocks led by a ::: directive, and terminated by a blank line. Then replace in each of these matched blocks the a letter by XXX . Notice how a occurrences outside the blocks are not replaced by XXX :

$ cat ./example.md

This is a code block:

:::shell-session
→ apache
→ java
→ python

This is another block:

:::shell-session
→ rust
→ haskell
→ javascript

This is a random sentence.

$ sed "/^:::/,/^$/ s/a/XXX/g" ./example.md

This is a code block:

:::shell-session
→ XXXpXXXche
→ jXXXvXXX
→ python

This is another block:

:::shell-session
→ rust
→ hXXXskell
→ jXXXvXXXscript

This is a random sentence.

In the same spirit as above but this time to spot indented blocks starting with ::: , then wrap them into triple-backticks fences:

$ cat ./example.md

This is a code block:

    :::shell-session
    → apache
    → java
    → python

This is another block:

    :::shell-session
    → rust
    → haskell
    → javascript

This is a random sentence.

$ find ./folder -iname "*.md" \
> -exec sed -i "/^    :::/,/^$/ s/^$/    \`\`\`\n/" "{}" \; \
> -exec sed -i "/^    :::/,/^$/ s/:::/\`\`\`/"      "{}" \;

$ cat ./example.md

This is a code block:

    ```shell-session
    → apache
    → java
    → python
    ```

This is another block:

    ```shell-session
    → rust
    → haskell
    → javascript
    ```

This is a random sentence.

Strip in-place the block of text starting with XXX and ending with an empty line:

$ cat ./example.md

This is a code block:

XXX{shell-session}
→ apache
→ java
→ python

This is a random sentence.

$ perl -i -ne "print if not /XXX/ .. /^$/" ./example.md

$ cat ./example.md

This is a code block:

This is a random sentence.

Same as above, but with sed :

$ sed -i "/^XXX/,/^$/ d" ./example.md

Python one-liner to delete the first occurrence of a block of text delimited by triple-backticks fences. Contrary to methods above, this one is not distracted by blank lines within the text block:

$ python -c 'import re; from pathlib import Path; file = Path("./example.md"); file.write_text(re.sub(r"^\`\`\`.*?\`\`\`\n\n", "", file.read_text(), count=1, flags=re.MULTILINE | re.DOTALL))'

Append the content of the addendum.txt file to all .markdown files:

$ find ./folder -iname "*.markdown" -print -exec bash -c 'cat ./addendum.txt >> "{}"' \;

Replace all accentuated characters by their non-accentuated variants (thanks Matthieu for the tip):

$ echo "éÈça-$" | iconv -t ASCII//translit

Get the date of last week:

$ date +"%Y-%m-%d" -d last-week

Get the current date in english:

$ env LC_TIME=en date +"%a %b %d %Y"

Get the number of seconds since epoch  :

$ date +%s

Convert back epoch time to human-readable date:

$ date --date=@1234567890

In place charset transcoding:

$ recode utf-8..latin-1 utf8text.txt