Append file to file in bash without cat

This is an orthodox use of cat. The "useless use of cat" involves using cat to read the contents of a single file and pipe them to another program which could just as easily read directly from the file using input redirection. Here, cat is doing all the reading and writing; there isn't anything simpler you could replace it with, since bash does not provide a built-in that reads from standard input and writes to standard output.

# Redirecting Input 'from' file to Command Substitution as "echo" argument# "echo" stdout write 'to' fileecho "$(<from)" > to# Redirecting Input 'from' file to Command Substitution as "printf" argument# "printf" stdout write 'to' fileprintf "%s" "$(<from)" > to# Redirecting Input 'from' file to "sed" and stdout write 'to' filesed -n '/.*/p' <from > to

envsubst
In normal operation mode, standard input is copied to standard
output, with references to environment variables of the form ‘$VARIABLE’`
or ‘${VARIABLE}’ being replaced with the corresponding values.

# create file with name 'template'echo '$buf' > template# Redirecting Input 'from' file to Command Substitution # Assign to 'buf' variable and export # envsubst standard input from 'template' file copied 'to' fileexport buf="$(<from)" && envsubst < template > to 

you can do

while read linedo    echo $linedone < file1 >> file2