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Shell.md

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Shell

Basics

  • Unzip:

    • Definition:

      unzip {filename}

    • Example:

      unzip logfile.zip

  • Copy the output of a command to clipboard (only for macos):

    • Definition:

      {command-with-output} | pbcopy

    • Example:

      cat input.txt | pbcopy

  • Calculate the number of lines of an ouput:

    • Definition:

      # you can pipe the output to word count with the -l flag for number of lines
{some-command} | wc -l

    • Usage:

      grep "user" logfile0001.log | wc -l

Grep

  • Print search results:

    • Definition:

      # Method 0: Searching for a string inside a file
grep {search-string} {filename}

# Method 1: Pass all the file names
grep {search-string} {filename1} {filename2}
  
# Method 2: You may use a double dash with no name, it means no more flags can be defined
grep {search-string} -- {filename1} {filename2}

    • Example:

      # Method 0
grep username logfile0001.log

# Method 1
grep username logfile0001.log logfile0002.log

# Method 2
grep username -- logfile0001.log logfile0002.log

  • Copy the contents to a file:

    • Definition:

      grep {search-string} {filename} >{new_filename}

    • Example:

      grep username logfile0001.log >searched_logs.log

  • Using the regex for digits in grep:

    • Definition:

      # Method 0: You'll have to use the -E flag to activate extended regex and then use the \d+ identifier
grep -E "some string \d+" {filename}

# Method 2: Invoke egrep instead of grep
egrep "some string \d+" {filename}

    • Usage:

      # Method 0
grep -E "Writing \d+ lines to the console." logfile0001.log

# Method 1
egrep "Writing \d+ lines to the console." logfile0001.log

Sed

  • Normally sed is invoked like this:

    • Definition:

      sed {script} {filename}

    • Example:

      # Method 0: to replace _first_ occurrence of ‘hello’ to ‘world’ in the file input.txt and print the changes
sed 's/hello/world/' input.txt

# Method 1: to replace _all_ occurences, you'll have to specify the g flag at the end of the script
sed 's/hello/world/g' input.txt

# Method 2: To paste modified changes of input.txt to output.txt
sed 's/hello/world/g' input.txt > output.txt

# Method 3: To edit files in place 
sed -i 's/hello/world/g' file.txt

  • Creating and using a sed script:

    • Creating the script:

      echo 's/hello/world/' > myscript.sed

    • Passing the script through sed:

      # Method 1
sed -f myscript.sed input.txt > output.txt

# Method 2
sed --file=myscript.sed input.txt > output.txt

  • Replace the contents of [] to something else:

    • Definition:

      # For anything inside the brackets we can use: \[.*\]

# Content that should be allowed inside brackets:
#   Match anything that's not a closing bracket: ^]
#   It may appear 0 or more times: [^]]*

# So the find pattern will be: \[[^]]*\] 
sed -i "s/\[[^]]*\]/$key/g" {filename}

    • Usage:

      # As an example it'll replace [23, 48, 51] to [...]
sed -i "s/\[[^]]*\]/[...]/g" {filename}

NVM

  • Change Default version of NVM

    nvm alias default v10.24.0

  • Use Default

    nvm use default

Regex

  • Special characters in regex that needs to be escaped { } [ ] / \ + * . $^ | ? using the \.

  • These type of characters are accepted for \w => a, A, 1, _

  • These type of characters are accepted for \d => 1

  • These type of characters are accepted for \s => whitespace

  • Regex sees all text as one line. But we use the multiline flag to handle each line separately.

  • Regex does a greedy match by default. This means that the matchmaking will be as long as possible. If you'll use .*r, it'll match the whole line ber beer beeer beeeer. Although, if you'll use ? like this .*?r then it'll match ber.