wc -l file to a variable

Question

Given a filepath, I would like to find out how many lines there are and store them into a variable. For example:

/three_little_pigs.csv

straw
wood
bricks

In a file, I would like to have the number (or string 3) stored. Tried the following commands:

export P="three_little_pigs.csv"
NUM_LINES=(wc -l < "${P}")

but I'm always getting this error:

bash: house: line 12: syntax error near unexpected token `<'

It's not NUM_LINES=(wc -l < "${P}") but rather NUM_LINES=$(wc -l < "${P}") Also no need to capitalize the var name and just simply "$P" — Valentin Bajrami, Jul 24 '17 at 07:20
Also, you don’t need the export for the code you’ve shown. — G-Man Says 'Reinstate Monica', Jun 23 '20 at 22:24

score 8 · Accepted Answer · edited Jun 23 '20 at 22:21

8

To run a subshell use $(...) in bash. And you don't need to redirect the input, but simply name the input file. So omit the <:

NUM_LINES=$(wc -l "$P")

Output:

3 three_little_pigs.csv

To get rid of the filename, one possibility (there are many others) is to use awk:

NUM_LINES=$(wc -l "$P" | awk '{print $1}')

EDIT 1:

OK, using redirection (<) will omit the filename at all ... sorry ;)

NUM_LINES=$(wc -l < "$P")

edited Jun 23 '20 at 22:21

answered Jul 24 '17 at 07:20

ChristophS

2

Note that with some wc implementations, after NUM_LINES=$(wc -l < "$P"), $NUM_LINES will contain leading blanks. To discard them, a common trick is to use arithmetic expansion: NUM_LINES=$(($(wc -l "$P"))). That would also convert an empty output (like when the file can't be open) to 0. – Stéphane Chazelas Jul 24 '17 at 07:41
With some shells, like AT&T ksh or zsh (and assuming the default value of $IFS, one can also do wc -l < "$P" | read num_lines instead to get rid of those blanks. – Stéphane Chazelas Jul 24 '17 at 07:44
1

Another advantage of wc -l < "$P" over wc -l "$P" is that it would work for files whose name starts with -. With some wc implementations like GNU wc, it's also better than wc -l -- "$P" as it works for a file called -. – Stéphane Chazelas Jul 24 '17 at 07:48
Great! Didn't know about arithmetic expansion! Helpful!! Will continue reading about ... – ChristophS Jul 24 '17 at 07:49
@StéphaneChazelas But won't the num_lines shell variable be lost in the pipe-created subshell once read operation is complete? I mean, how does one get at the value in the $num_lines shell var. – Jul 24 '17 at 12:02
@RakeshSharma, not in AT&T ksh or zsh that don't run that read in a subshell. See also the lastpipe option in bash (with restrictions) and yash. – Stéphane Chazelas Jul 24 '17 at 12:03

1 Answers1