How do I search for a particular string in Linux?

Question

I want to search for a value in the VASP OUTCAR file i.e the format of the line should be

 Volume of Typ   1:     98.5 %

What command line should I type in OUTCAR to go for a specific line or word?

Answer 1

The OUTCAR is a text file. Therefore, you can use the grep command to grab what you want to search for.

• Print the current line:

  grep  "key_information_string" OUTCAR
  

• Print more lines below the current line:

  grep  "key_information_string" OUTCAR -A n
  

• Print more lines above the current line:

  grep  "key_information_string" OUTCAR -B n
  

Here $n$ represents the number of lines you want to print, such as 5.

Answer 2

Use rg

If you have a significant size file or file(s) to search, do yourself a favor and choose one of the many significantly faster alternatives to grep, the de facto fastest being ripgrep.

Specific values

# literal string
rg <options> 'Volume of Typ   1:     98.5 %'     path/to/file/or/directory

# any value for the key
rg <options> 'Volume of Typ   1:'                path/to/file/or/directory

# any numeric value
rg <options> 'Volume of Typ   1:\s*\d+\.?\d*'    path/to/file/or/directory

# any numeric percentage
rg <options> 'Volume of Typ   1:\s*\d+\.?\d*\s%' path/to/file/or/directory

Where <options> are any additional parameters you want to pass (may be omitted).
The path argument may be omitted as well, it defaults to the current working directory.
Obviously, you could make the search parameter for the key a pattern too.

N.B. The syntax is the same for grep (or ag, or the like).

Selection

If you wanted to select the values only (as in strip the key and unit / suffix), sed is an option:

cat path/to/file | sed -en 's!^Volume of Typ\s*1:\s*\([0-9]*\.[0-9]*\).*$!\1!gp'

At this point, you've got a list of line-terminated, matched values on your screen.
Neato, no more noise!

Next

Other than looking at the output on screen, you could:

• store it in another file <previous_command> > path/to/values.csv
• pipe it to the next command (e.g. to visualize the values):
  <previous_command> | awk '{print NR " " $0}' | gnuplot -p -e "plot '<cat'"
  (Exercise left to the avid reader: The extraction of the Typ n type number to plot on the x-axis, instead of the running index NR.)

Narrower conditions are obviously possible.

Tips

• Single quoted ' strings make regexes in the shell significantly easier to type.
• Use a better delimiter for sed than the often cited /, ! for instance.

References

• man rg
• man sed
• regular expressions
• online regex debugger

Bonus: sed breakdown

cat path/to/file | sed -en 's!^Volume of Typ\s*1:\s*\([0-9]*\.[0-9]*\).*$!\1!gp'

• cat¹ reads the file from disk and prints its contents to stdout
• | is a pipe
• sed is a stream editor:
  • the -e flag isn't strictly necessary, it's just there to allow for possibly multiple "script" inputs at once
  • -n suppresses output (get rid of non-matching lines)
  • the script argument decodes as s!<search_pattern>!<replacement>!<flags>
    • wherein <search_pattern>
      • ^ beginning of the line
      • Volume of Typ literal string to be matched
      • \s* any number of whitespaces
      • : literal string to be matched
      • \s* any number of whitespaces
      • (...) group that may be referenced (no effect on matching)
      • [0-9]* any number of digits (character class with a * quantifier)
      • \. an escaped, literal period (. has a logical meaning in regular expressions)
    • wherein <replacement> is \1, a backreference to the group (...) above
    • wherein the <flags> are:
      • g apply globally (you want more than one match)
      • p print matches to stdout (we had negated all output with -n earlier to get rid of non-matching lines)

¹ For the sake of completeness: cat's actual purpose is text (or file) concatenation and it takes any number of arguments. Most of its invocations in its lifetime have certainly been with one argument to read a file.

Answer 3

To search for all the strings in all of the files in current folder you would use simply:

grep string *

Or in subfolder:

grep string subfolder/*

Unfortunately recursion is not possible so you can only search one folder at the time.

Answer 4

If you open with vim, you can find it by following command line.

vim OUTCAR and input '?Volume of Type' or '/Volume of Type', press n or N to find previous one next one with string 'Volume of Type'.