count lines in a file

The standard way is with wc, which takes arguments to specify what it should count (bytes, chars, words, etc.); -l is for lines:

$ wc -l file.txt
1020 file.txt

Steven D forgot GNU sed:

sed -n '$=' file.txt

Also, if you want the count without outputting the filename and you're using wc:

wc -l < file.txt

Just for the heck of it:

cat -n file.txt | tail -n 1 | cut -f1

As Michael said, wc -l is the way to go. But, just in case you inexplicably have bash, perl, or awk but not wc, here are a few more solutions:

Bash-only

$ LINECT=0; while read -r LINE; do (( LINECT++ )); done < file.txt; echo $LINECT

Perl Solutions

$ perl -lne 'END { print $. }' file.txt

and the far less readable:

$ perl -lne '}{ print $.' file.txt

Awk Solution

$  awk 'END {print NR}' file.txt