"How do I find the 100th line in a file" - a common question for newcomers to coding. The short answer is to open the file, and loop through to read lines until the one that you want. Although most languages have a seek command or function, that works by bytes and with a typical text / ascii file, line lengths vary so that you don't know exactly where to seek if you try to use it.

As an alternative, in some circumstances you may wish to read the whole file into a list / array and then look by line number - in Perl:

  open FH,"rails_routes";
  @lines = <FH>;
  print $lines[39];

or read the whole file into a string and then turn it into a list - in Tcl:

  set fp [open rails_routes]
  set stuff [read $fp]
  set lines [split $stuff "\n"]
  puts [lindex $lines 39]

but in such cases you need to be careful if the file's potentially huge.

Where a large file is accessed a lot by record number, and doesn't change often, you might consider setting up an index or (say) every thousandth line, so that you can seek most of the way to what you want - or perhaps you should be using a database rather than a plain file if you have that requirement. Isn't it wonderful how technologys go together!
(written 2011-06-09)

 T209 - Tcl/Tk - File and Directory Handling
  [779] The fragility of pancakes - and better structures - (2006-06-26)
  [785] Running external processes in Tcl and Tcl/Tk - (2006-06-29)
  [1407] Reading from another process in Tcl (pipes and sockets) - (2007-10-26)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [2467] Tcl - catching an error before your program crashes - (2009-10-22)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [3617] The fileutil package and a list of file system commands in Tcl - (2012-02-18)
  [4461] Reading from a URL, and reading Json, from your Tcl script - (2015-03-12)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4524] Tcl - a new example for data reformatting - (2015-10-10)

P215 - Perl - More about Files
  [1225] Perl - functions for directory handling - (2007-06-09)
  [1709] There is more that one way - Perl - (2008-07-14)
  [1832] Processing all files in a directory - Perl - (2008-10-11)
  [2405] But I am reading from a file - no need to prompt (Perl) - (2009-09-14)
  [2964] An introduction to file handling in programs - buffering, standard in and out, and file handles - (2010-09-21)
  [3412] Handling binary data in Perl is easy! - (2011-08-30)
  [3839] Spraying data from one incoming to series of outgoing files in Perl - (2012-08-15)

Back to Moving on - a task for the hotel staff!

Previous and next or Horse's mouth home

Forward to Travelling around

Rabbit Shelter
Is Melksham town centre dying?
How much has Perl (and other languages) changed?
Travelling around
Reading the nth line from a file (Perl and Tcl examples)
Moving on - a task for the hotel staff!
Summer Sunday Trains - outings from Swindon, Chippenham, Melksham, Trowbridge and Westbury
Self Portrait (in words)
Twitter Phishing Trips ... and a great new alert service
Friday - Electrician, Food Festival, C++ Course, Rail Group Meeting