CODEWITHSUNDEEP
shellunix
user3179105
user3179105

Reputation: 33

Unix shell script - purpose of colon

I have come across a shell script with a line which reads as follows

Pfile=/params/tech1.dat:$Pfile;export Pfile

The purpose is to create and export a variable called Pfile containing value "/params/tech1.dat"

But what is the `:$Pfile`` doing ? Specifically what purpose is the colon serving ?

Have trawled lots of Unix info sources and manuals but cannot find an example which helps explain the above.

Upvotes: 3

Views: 7314

Answers (3)

damienfrancois
damienfrancois

Reputation: 59110

The colon serves as a delimiter on many variables that list paths. It is just a character that is chosen by convention for that purpose.

Here, the piece of code assigns to Pfile the string that is obtained by evaluating the right hand site, which consists in a constant string /params/tech1.dat: and a variable $Pfile. It would perhaps appear more clearly if it were written Pfile="/params/tech1.dat:$Pfile";export Pfile.

In your specific example, /params/tech1.dat is prepended to $Pfile so that, assuming the value of $Pfile is /other/path it then becomes /params/tech1.dat:/other/path. This is understood by many programs as meaning: look in /params/tech1.dat then in other/path.

Common examples: PATH, LD_LIBRARY_PATH, LIBRARY_PATH, CPATH, PYTHONPATH, etc.

If $Pfile is previously unset or empty, it ends up with a trailing colon: /params/tech1.dat: which may, or may not, depending on your program, be understood as the working directory (it is the case for the above-listed examples).

Note that : is a valid character in a path name in many file systems so in the unlikely event that some path contains one, it should probably be escaped.

Finally, note that in other contexts, : is a Bash function that does nothing.

Upvotes: 3

unxnut
unxnut

Reputation: 8839

In this case, you are appending the existing value of $Pfile to /params/tech1.dat. If there is no existing value, you will get /params/tech1.dat: as the value assigned to $Pfile. : acts as a delimiter between the two values.

Typically, : is used as a delimiter for directories in shell variables such as PATH and LD_LIBRARY_PATH. Don't know if you have a certain reason to use : in your variable.

Upvotes: 0

lurker
lurker

Reputation: 58244

That prepends the path /params/tech1.dat to whatever Pfile might already have in it. So Pfile might already be:

echo $Pfile
/foo/bar:/blah/bleh

Then you do execute the statement:

Pfile=/params/tech1.dat:$Pfile;export Pfile

and you get:

echo $Pfile
/params/tech1.dat:/foo/bar:/blah/bleh

As was mentioned by others, the colon (:) is typically used as a path or field delimiter.

Upvotes: 2

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