Append file to file in bash without cat

Question

It's easy to append an existing file to another file with cat: cat file1 >> file2

Sometimes I have read what people wrote, "do not use cat unless you are concatenating two files." This is sensible. One does not really need cat if it is taking only one argument: cat file | xargs program is worse than xargs program < file.

So how do I append a file to another file without cat? Neither < file1 >> file2 nor >> file2 < file1 work. If I must call a program to do this, what is the orthodox way?

Answer 1

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.

Answer 2

# Redirecting Input 'from' file to Command Substitution as "echo" argument
# "echo" stdout write 'to' file
echo "$(<from)" > to

# Redirecting Input 'from' file to Command Substitution as "printf" argument
# "printf" stdout write 'to' file
printf "%s" "$(<from)" > to

# Redirecting Input 'from' file to "sed" and stdout write 'to' file
sed -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' file
export buf="$(<from)" && envsubst < template > to