File Tools

file_tools.bsh

Tools that deal with opening file descriptors and the like.

find_open_fd

Arguments:

[name] - Variable name to return available file descriptor. Default: fd
{START_FD} - The first fd to start checking. Default: 10

Output:

{name} - Value of next unused file descriptor

Find the first open file descriptor

Usage

This is for backwards compatibility. In bash 4.1 or newer, this isn’t necessary, as:

exec {fd}<>"${some_buffer}"
echo "hi" >&"${fd}"
read "-u${fd}" some_var
exec {fd}>&-

is better and ~100x faster.

Example

mkfifo /tmp/test
find_open_fd
eval "exec ${fd}<>"/tmp/test"
echo hi >&${fd}
read -u ${fd} -t 1 line
eval "exec ${fd}>&-"

Bugs

Probably not thread safe

See also

open_fd, close_fd

open_fd

Arguments:

All options are optional
-p - Pipe mode (use mkfifo)
-f - File mode (use touch)
-n {name} - Name of buffer used
-s - Use the same fd for read and write
-r {#} - Use this specific fd for reading
-w {#} - Use this specific fd for writing
-R {name} - Name of variable to store read fd in. Default: fd_r
-W {name} - Name of variable to store write fd in. Default: fd_w
Variables names starting with _ should be avoided as they may clash with variables used in this function

Output:

Read and write variables will contain the corresponding file descriptors
filename - Name of file/pipe that was opened.

Find available file descriptor and open a file/pipe for you

Example

# Make a pipe
pipe="$(mktemp -u)"

# Open the pipe
open_fd -p -s -n -W fd "${pipe}"

# Write to the pipe - does not block as long as buffer not filled
echo "hi" >&${fd}

# Blocking read from pipe. Use timeout command to add a timeout
timeout 1 cat - <&${fd}

# Read from the pipe and save to a variable without a subshell
IFS='' read -u "${fd}" -t 0.1 -r my_variable

# Close and cleanup
close_fd "${fd}"
rm "${pipe}"

Do NOT
  output=$(open_fd)
Because open_fd will be executed in a subshell, and thus the opened fd will
be useless. This is the design decision behind storing the file descriptor
in the fd variable.

Note

Opening a pipe without the same fd for read and write doesn’t work very well. It hangs on most tested systems. Workarounds are unpredictable at best. So just don’t do that.