Arguments tell a program what to work on. Some arguments tell it how to do the work. These are flags. A flag starts with a dash, -.

wc: count the lines and words in a file

wc counts what is in a file. With no flag, it writes three numbers:

learner@tidepool:~$ wc notes.txt
 4  8 49 notes.txt

A byte is a unit of size. In a text file, each letter, space and line end is one byte.

Add a flag

With a flag, wc writes only what you ask for:

learner@tidepool:~$ wc -l notes.txt
4 notes.txt
learner@tidepool:~$ wc -w notes.txt
8 notes.txt

Short and long flags

Many flags have two forms. A short flag has one dash and one letter. A long flag has two dashes and a word. The two forms do the same thing.

Short flags are fast to type. Long flags are easier to read.

Put short flags together

Short flags can share one dash. These two lines do the same thing:

wc -l -w notes.txt
wc -lw notes.txt

A dash makes the flag

Only an argument that starts with - is a flag. Without the dash, wc reads lw as the name of a file:

learner@tidepool:~$ wc lw
wc: lw: No such file or directory

Each program has its own flags

cat -n puts a number before each line:

learner@tidepool:~$ cat -n notes.txt
     1  deploy started
     2  build ok
     3  tests ok
     4  deploy finished

The letter n means "number the lines" for cat. In another program, -n can mean something different. The next lesson shows one.