During a pairing session on Ruby, we had an issue with what file the API was stuck on. Luckily, there is a command lsof which stands for list open files.

Usage

When lsof is ran without parameters, it will show all the files opened by any process. It can get a bit messy, so I recommend greping through to find the file/process you want.

To find out what files Ruby has opened you can use lsof -c. Here is what it looks like when running rails server:

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$ lsof -c ruby
ruby    55222 nano   10w   REG    1,1   1214625 34875337 ~/Developer/Projects/contact-manager/log/development.log
ruby    55222 nano   11u   REG    1,1     49152 34875460 ~/Developer/Projects/contact-manager/db/development.sqlite3

To find out what is accessing any file in a directory, use the lsof +D /usr/directory

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$ lsof +D /Users/nano/Developer/Projects/contact-manager
vim     44957 nano  cwd    DIR    1,1      748 34873022 ~/Developer/Projects/contact-manager

You can list all opened internet sockets using lsof -i.

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$ lsof -i :62658
Rdio      63484 nano   23u  IPv4 0x2674679fdf954c9b      0t0  TCP 10.0.1.14:62658->8.18.203.81:sunproxyadmin (ESTABLISHED)
# We can find out the process id for Unicorn running on port 8080
$ lsof -i :8080
COMMAND   PID USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
ruby    54171 nano    9u  IPv4 0x2674679fe1b36243      0t0  TCP *:http-alt (LISTEN)
ruby    54172 nano    9u  IPv4 0x2674679fe1b36243      0t0  TCP *:http-alt (LISTEN)

After finding the process id, you can find out what that process is accessing using lsof +p PID

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$ lsof +p 79766
COMMAND   PID USER   FD   TYPE             DEVICE  SIZE/OFF     NODE NAME
ruby    54172 nano  cwd    DIR                1,1       748 34873022 ~/Developer/Projects/contact-manager
ruby    54172 nano  txt    REG                1,1      9184 17640590 ~/.rvm/rubies/ruby-2.0.0-p247/bin/ruby
ruby    54172 nano  txt    REG                1,1   2793108 17640592 ~/.rvm/rubies/ruby-2.0.0-p247/lib/libruby.2.0.0.dylib
ruby    54172 nano  txt    REG                1,1    118244   615195 /usr/local/Cellar/libyaml/0.1.4/lib/libyaml-0.2.dylib
ruby    54172 nano  txt    REG                1,1    378152   797957 /usr/local/Cellar/openssl/1.0.1e/lib/libssl.1.0.0.dylib
ruby    54172 nano  txt    REG                1,1   1663200   797956 /usr/local/Cellar/openssl/1.0.1e/lib/libcrypto.1.0.0.dylib

Using several commands leads you to any files that may be stuck in processing. I have found that it is an essential tool in debugging a multitude of issues.

References