Getting head to display all but the last line of a file: command substitution and standard I/O redirection

head -n -1 will give you all except the last line of its input.

head is the wrong tool. If you want to see all but the last line, use:

sed \$d

The reason that

# Sample of incorrect code:
echo "hello" | head -n $(wc -l | sed -E -e 's/\s//g')

fails is that wc consumes all of the input and there is nothing left for head to see. wc inherits its stdin from the subshell in which it is running, which is reading from the output of the echo. Once it consumes the input, it returns and then head tries to read the data...but it is all gone. If you want to read the input twice, the data will have to be saved somewhere.

Using sed:

sed '$d' filename

will delete the last line of the file.

$ seq 1 10 | sed '$d' 
1
2
3
4
5
6
7
8
9

For Mac OS X specifically, I found an answer from a comment to this Q&A.

Assuming you are using Homebrew, run brew install coreutils then use the ghead command:

cat myfile.txt | ghead -n -1

Or, equivalently:

ghead -n -1 myfile.txt

Lastly, see brew info coreutils if you'd like to use the commands without the g prefix (e.g., head instead of ghead).

cat myfile.txt | echo $(($(wc -l)-1))

This works. It's overly complicated: you could just write echo $(($(wc -l)-1)) <myfile.txt or echo $(($(wc -l <myfile.txt)-1)). The problem is the way you're using it.

cat myfile.txt | head -n $(wc -l | sed -E -e 's/\s//g')

wc consumes all the input as it's counting the lines. So there is no data left to read in the pipe by the time head is started.

If your input comes from a file, you can redirect both wc and head from that file.

head -n $(($(wc -l <myfile.txt) - 1)) <myfile.txt

If your data may come from a pipe, you need to duplicate it. The usual tool to duplicate a stream is tee, but that isn't enough here, because the two outputs from tee are produced at the same rate, whereas here wc needs to fully consume its output before head can start. So instead, you'll need to use a single tool that can detect the last line, which is a more efficient approach anyway.

Conveniently, sed offers a way of matching the last line. Either printing all lines but the last, or suppressing the last output line, will work:

sed -n '$! p'
sed '$ d'