Command-line interface on Linux/UNIX

This is not a UNIX guide, but just a loose list of tips, hints, etc., that may come in handy when working in a command-line environment.

To use the output of one command as the input for the next command, use back quotes ("inverted commas") around the first command. This is called command substitution.

To combine files side-by-side into a single file use
```
paste file1 file2 ... > output
```
If file1, file2, ... contain the same number of lines, output will contain their contents as columns.
You may have many entries in your PATH variable, or there may be multiple copies of an executable on a file system. To determine which version of program will be executed, use
```
which program
```
To find out which libraries are needed/used by a dynamically linked executable, use
```
ldd /path/program
```
Note that ldd will not use PATH to search for program; just must specify it via an explicit path. However, you can take advantage of which as follows
```
ldd `which program`
```
Note the back quotes. Side note: On OS X, if you have xcode installed the ldd command can be mimicked via otool -L

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