How create a temporary file in shell script?

If you're on system which has mktemp, you should use it as other answers.

With POSIX toolchest:

umask 0177
tmpfile=/tmp/"$0"."$$"."$(awk 'BEGIN {srand();printf "%d\n", rand() * 10^10}')"
trap 'rm -f -- "$tmpfile"' INT TERM HUP EXIT
: > "$tmpfile"

Some shells have the feature built-in.

zsh

zsh's =(...) form of process substitution uses a temporary file. For instance =(echo test) expands to the path of a temporary file that contains test\n.

$ {cat $file; ls -l /dev/fd/3; echo test2 >&3; cat $file} 3<> ${file::==(echo test)}
test
lrwx------ 1 stephane stephane 64 Jan 30 11:19 /dev/fd/3 -> /tmp/zshMLbER0
test2

That file is automatically removed, once the command has finished.

bash/zsh on Linux.

Here-files or here-strings in bash and zsh are implemented as deleted temporary files.

So if you do:

exec 3<<< test

The file descriptor 3 is connected to a deleted temporary file that contains test\n.

You can get its content with:

cat <&3

If on Linux, you can also read or write to that file via /dev/fd/3

$ exec 3<<< test
$ cat <&3
test
$ echo foo > /dev/fd/3
$ cat /dev/fd/3
foo

(some other shells use pipes, or may use /dev/null if the here doc is empty).

Use mktemp to create a temporary file or directory:

temp_file=$(mktemp)

Or for a direcotry:

temp_dir=$(mktemp -d)

At the end of the script you have to delete the temporary file/dir:

rm ${temp_file}
rm -R ${temp_dir}

mktemp creates file in the /tmp directory or in the drectory given with the --tmpdir argument.