Shell – How to POSIX-ly count the number of lines in a string variable

posixshell-scriptstringvariable

I know I can do this in Bash:

wc -l <<< "${string_variable}"

Basically, everything I found involved <<< Bash operator.

But in POSIX shell, <<< is undefined, and I have been unable to find an alternative approach for hours. I am quite sure there is a simple solution to this, but unfortunately, I didn't find it so far.

Best Answer

The simple answer is that wc -l <<< "${string_variable}" is a ksh/bash/zsh shortcut for printf "%s\n" "${string_variable}" | wc -l.

There are actually differences in the way <<< and a pipe work: <<< creates a temporary file that is passed as input to the command, whereas | creates a pipe. In bash and pdksh/mksh (but not in ksh93 or zsh), the command on right-hand side of the pipe runs in a subshell. But these differences don't matter in this particular case.

Note that in terms of counting lines, this assumes that the variable is not empty and does not end with a newline. Not ending with a newline is the case when the variable is the result of a command substitution, so you'll get the right result in most cases, but you'll get 1 for the empty string.

There are two differences between var=$(somecommand); wc -l <<<"$var" and somecommand | wc -l: using a command substitution and a temporary variable strips away blank lines at the end, forgets whether the last line of output ended in a newline or not (it always does if the command outputs a valid nonempty text file), and overcounts by one if the output is empty. If you want to both preserve the result and count lines, you can do it by appending some known text and stripping it off at the end:

output=$(somecommand; echo .)
line_count=$(($(printf "%s\n" "$output" | wc -l) - 1))
printf "The exact output is:\n%s" "${output%.}"
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