So just to see if I could, I wrote a version of cat using pure bash. Pure bash is a bash script which uses nothing but bash builtins to accomplish it’s goal. To determine if a particular command is a builtin, you can use the command type -t “command” (the command type, is itself a builtin). Some notable commands which are builtins include echo, read, exec, return. Some notable commands which are not builtins include cat and grep. As follows is my implementation of cat in pure bash.
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#!/bin/bash
INPUTS=( "${@:-"-"}" )
for i in "${INPUTS[@]}"; do
if [[ "$i" != "-" ]]; then
exec 3< "$i" || exit 1
else
exec 3<&0
fi
while read -ru 3; do
echo -E "$REPLY"
done
done
Now, keep reading if you want a small lesson in advanced bash. I’ll go line by line to explain what this is doing.
#!/bin/bashLine 1 is the shebang
INPUTS=( "${@:-"-"}" )Line 2 assigns the array variable INPUTS either the arguments provided on the
command line if they exist, or the single character -. The way this happens is
as follows: $@ is the variable to reference the positional parameters (the
arguments to your program). If you have not heard of $*, read
this.
The way I reference the positional parameters is like ${@}. That’s because the
brackets allow me to add a default value to the variable. A default value is
the value that the variable will seem to have if the variable is not set. The
way to use a default value is with the :-, like so: ${@:-"hello"}. So if
$@ is not set, it will seem to have the value "hello". You will then notice
that is all enclosed in (). That makes an array out of the positional
parameters (the first argument to the program becomes the first element in the
array, the second argument becomes the second element, etc.).
for i in "${INPUTS[@]}"; doLine 3 begins a for loop which will assign to i each value stored in the
array INPUTS which was discussed earlier. The @ index used is the same for
arrays as $@ is for the positional parameters.
Maybe I’ll explain more when I’m less lazy.