How to use `cat` to see the top of a very long file?
Use less
, which will show the file from the top, allowing you to scroll through it, just like man
:
less my-verylong-file
- Press q to exit.
If you're just interested in seeing the n first lines of a file, head
may be an alternative:
head -n 25 filename
will show the 25 first lines of the file.
Same thing for the n last lines of a file with tail
:
tail -n 25 filename
man
uses less(1)
as a pager by default. Use it instead of cat
. See also @vidarlo's answer.
How to customize less
to make it even better:
I like to alias m=less
, so it's just a single-letter command, because I type it all the time. Putting a |m
at the end of anything pipes it into a pager.
You could put options like -iMRj5X
in the alias (e.g. alias m='less -iMRX
), but I do that with my ~/.lesskey
file. (See lesskey(1)
).
-i
: searches are case-insensitive (unless you use any capital letters)-M
: longer status line, showing line number and file-percentage-R
: allow some control-codes through, so you can pipe colorized commands into less.-X
: don't switch to the terminal emulator's "alternate" screen, so whatever you were looking at will still be there when youq
uit out ofless
. (great for man pages after you find the option you want, and want to look at it while typing it.)-j5
: searches put the target line at row 5 instead of the top of the screen. So you can see context on both sides of your search result. (Sometimes I change this interactively, by typing-j40
or something insideless
, if it's most useful to see context before a search hit).
I also bind .
to next-file
, and ,
to prev-file
, because the default bindings are two separate characters which are much slower to type: :n
and :p
.
This is my .lesskey
:
$ cat .lesskey
. next-file
, prev-file
#env
LESS = iMRj5X
Run lesskey
to "compile" it into a ~/.less
.
This probably mattered more 20 years ago, but less
reads that binary file instead of parsing a text config file every time it starts.