Linux wc command & file descriptor

Question

in linux terminal,

$ echo hi > foo
$ wc foo > foo
$ cat foo
0 0 0 foo (Output)
$

But if

$ echo hi >foo
$ wc foo
1 1 3 foo (Output)
$

Why these outputs are different?

Because `>` truncates the file before anything else happens. — Benjamin W., Mar 23 '17 at 17:57

score 1 · Accepted Answer · answered Mar 23 '17 at 17:57

1

When you redirect output to a file, the file is first emptied. So the file doesn't contain anything in it when wc tries to read it.

In general, you shouldn't try to use the same file as input and output, it will almost never work.

answered Mar 23 '17 at 17:57

Barmar

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Gajesh · Answer 2 · 2018-01-18T07:06:18.240

1

As Barmar said in his answer the ' > ' Symbol empties the file before writing the new data to the file to which output is being redirected. so what you are essentially doing is using wc on an emptied file.

Instead of truncating the output you can append (' >> ') the output using the following code :

$ wc foo >> foo
$ cat foo
hi
1 1 3 foo

To get a more deep understanding on I/O Redirection in Linux follow the following article which covers the topic in detail.

An Introduction to Linux I/O Redirection

edited Jan 18 '18 at 07:06

answered Mar 23 '17 at 18:16

Gajesh

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1

Thanks it's working for me... – Harshal Khodifad Aug 15 '17 at 10:38

Linux wc command & file descriptor

2 Answers2