look - Linux


The look command in Linux is used for finding lines in a sorted file that begin with a given string. It is particularly useful for quick searches in large text files that are sorted alphabetically. The command can be effectively used in dictionary checks, autocomplete features, or searching indexed data in sorted files.


The basic syntax of the look command is as follows:

look [options] string [file]
  • string: The text string to search for in the file.
  • file: Optional parameter specifying the file to search. If not provided, look uses the file /usr/share/dict/words.


look supports several options to modify its behavior:

  • -a: Use the alternate dictionary, /usr/share/dict/web2.
  • -d: Consider only alphanumeric characters and spaces in comparisons.
  • -f: Ignore the case of alphabetic characters (case insensitive).
  • -t <char>: Terminate comparisons upon reaching the character <char>.
  • -b (GNU Extension): Matches sequences anywhere in lines, not just at the beginning.

By default, look performs a binary search, comparing lines case-sensitively from the beginning up to the end of the input string unless modified by options.


  1. Simple Search:

    look hap /usr/share/dict/words

    This searches for lines starting with “hap” in the default dictionary file.

  2. Case Insensitive Search:

    look -f python

    Finds lines starting with “python”, “Python”, “PYTHON”, etc.

  3. Using Alternative Dictionary:

    look -a design

    Search for “design” in the alternate dictionary.

  4. Specifying Termination Character:

    look -t n lo /usr/share/dict/words

    This stops comparing strings at the first occurrence of ‘n’.

  5. Ignore Non-Alphabetic Characters:

    look -d 123abc

    Searches for “123abc”, but only alphanumeric characters and spaces are considered.

Common Issues

  • File Not Sorted: If the file isn’t sorted, look may fail to find existing matches. Ensure that the file is sorted using the sort command if uncertain.

  • Encoding Issues: Non-ASCII characters might not be handled as expected depending on locale and terminal settings. Use locale settings to match file encoding.

  • Performance: On very large files, performance can be impacted. Consider preprocessing or indexing large datasets to improve efficiency.


look can be integrated with other commands for complex tasks:

  • Combine with grep:

    look hello /usr/share/dict/words | grep -v "ly$"

    This finds words starting with “hello” that do not end with “ly”.

  • Using in Scripts:

    for word in $(look $prefix /path/to/sorted/file); do
        # process each matched line
  • grep: Search files for lines matching a pattern.
  • sort: Sorts lines of text files.
  • awk: Pattern scanning and processing language.

Additional information and updates about the look command can be found at official Linux documentation resources or the man page (man look).