Bash – How Does `cat <> file` Work

bashio-redirectionshell

cat < file prints the contents of file to stdout.

cat > file reads stdin until Ctrl+D is detected and the input text is written to file.

cat <> file, at least in my version of Bash, prints the contents of file happily (without error), but doesn't modify the file nor does it update the modification timestamp.

How does the Bash standard justify the seemingly ignored > in the third statement – and, more importantly, is it doing anything?

Best Answer

Bash uses <> to create a read-write file descriptor:

The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.

cat <> file opens file read-write and binds it to descriptor 0 (standard input). It's essentially equivalent to < file for any sensibly-written program, since nobody's likely to try writing to standard input ordinarily, but if one did it'd be able to.

You can write a simple C program to test that out directly - write(0, "hello", 6) will write hello into file via standard input.

<> should also work in any other POSIX-compliant shell with the same effect.

Related Solutions

Bash – Inconsistencies between redirecting input from file, here docs, and here strings

$(<servers.txt) is a special shortcut in bash for $(cat <servers.txt), as documented under “Command substitution”. It is an exception to the regular syntax where the redirection would be applied to an empty command and thus do nothing except signal an error if the file doesn't exist. Bash doesn't extend this exception to here documents or here strings: you need to include cat explicitly.

This feature was added in bash 2.02 (it's mentioned in the changelog). In the source code, it's implemented by the logic around the call to cat_file in parse_and_execute in builtins/evalstring.c.

Python Shell – Write Python Stdout to File Immediately

This is happening because normally when process STDOUT is redirected to something other than a terminal, then the output is buffered into some OS-specific-sized buffer (perhaps 4k or 8k in many cases). Conversely, when outputting to a terminal, STDOUT will be line-buffered or not buffered at all, so you'll see output after each \n or for each character.

You can generally change the STDOUT buffering with the stdbuf utility:

stdbuf -oL python script.py > log

Now if you tail -F log, you should see each line output immediately as it is generated.

Alternatively explicit flushing of the output stream after each print should achieve the same. It looks like sys.stdout.flush() should achieve this in Python. If you are using Python 3.3 or newer, the print function also has a flush keyword that does this: print('hello', flush=True).

Best Answer

Related Solutions

Bash – Inconsistencies between redirecting input from file, here docs, and here strings

Python Shell – Write Python Stdout to File Immediately

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