Alias HEAD for automatic smart output

alias head='head -n $((${LINES:-`tput lines 2>/dev/null||echo -n 12`} - 2))'
Run the alias command, then issue ps aux | head and resize your terminal window (putty/console/hyperterm/xterm/etc) then issue the same command and you'll understand. ${LINES:-`tput lines 2>/dev/null||echo -n 12`} Insructs the shell that if LINES is not set or null to use the output from `tput lines` ( ncurses based terminal access ) to get the number of lines in your terminal. But furthermore, in case that doesn't work either, it will default to using the deafault of 12 (-2 = 10). The default for HEAD is to output the first 10 lines, this alias changes the default to output the first x lines instead, where x is the number of lines currently displayed on your terminal - 2. The -2 is there so that the top line displayed is the command you ran that used HEAD, ie the prompt. Depending on whether your PS1 and/or PROMPT_COMMAND output more than 1 line (mine is 3) you will want to increase from -2. So with my prompt being the following, I need -7, or - 5 if I only want to display the commandline at the top. ( https://www.askapache.com/linux/bash-power-prompt/ ) 275MB/748MB [7995:7993 - 0:186] 06:26:49 Thu Apr 08 [askapache@n1-backbone5:/dev/pts/0 +1] ~ In most shells the LINES variable is created automatically at login and updated when the terminal is resized (28 linux, 23/20 others for SIGWINCH) to contain the number of vertical lines that can fit in your terminal window. Because the alias doesn't hard-code the current LINES but relys on the $LINES variable, this is a dynamic alias that will always work on a tty device.
Sample Output
[7995:7993 - 0:2497] [askapache@n1-backbone5:pts/0 +1] ~ $ lsof -a -d 1 -d 0-4 -w|head
COMMAND   PID USER   FD   TYPE DEVICE  SIZE       NODE NAME
bash     7995 askapache    0u   CHR  136,0                2 /dev/pts/0
bash     7995 askapache    1u   CHR  136,0                2 /dev/pts/0
bash     7995 askapache    2u   CHR  136,0                2 /dev/pts/0
[7995:7993 - 0:2497] [askapache@n1-backbone5:pts/0 +1] ~ $

[7995:7993 - 0:2497] [askapache@n1-backbone5:pts/0 +1] ~ $ lsof -a -d 1 -d 0-4 -w|head
COMMAND   PID USER   FD   TYPE DEVICE  SIZE       NODE NAME
bash     7995 askapache    0u   CHR  136,0                2 /dev/pts/0
bash     7995 askapache    1u   CHR  136,0                2 /dev/pts/0
bash     7995 askapache    2u   CHR  136,0                2 /dev/pts/0
bash     9305 askapache    0u   CHR  136,2                4 /dev/pts/2
bash     9305 askapache    1u   CHR  136,2                4 /dev/pts/2
bash     9305 askapache    2u   CHR  136,2                4 /dev/pts/2
lsof    17475 askapache    0u   CHR  136,0                2 /dev/pts/0
lsof    17475 askapache    1w  FIFO    0,6       1056477120 pipe
lsof    17475 askapache    2u   CHR  136,0                2 /dev/pts/0
lsof    17475 askapache    3r   DIR   0,96     0          1 /proc
lsof    17475 askapache    4r   DIR   0,96     0 1078132745 /proc/17475/fd
[7995:7993 - 0:2497] [askapache@n1-backbone5:pts/0 +1] ~ $

What Others Think

This is great. Also the -n option still works.
houghi · 732 weeks and 5 days ago
You could also use this with "tail".
jmcantrell · 732 weeks and 4 days ago
In some cases, LINES might not be set so it's a good idea to provide a default. This will make it the same as the non-alias version: alias head='head -n $((${LINES:-12}-2))'
dennisw · 732 weeks and 2 days ago
Also, for some reason even with just a one-line prompt, I need -4 for head and -5 for tail in order to leave the previous prompt at the top of the screen.
dennisw · 732 weeks and 2 days ago
@dennisw Are you head-ing something that is wrapping lines?
vgm64 · 732 weeks and 2 days ago
Bingo! +10 internets for you, -40 for me! (I'll return now to my tiddlywinks.)
dennisw · 732 weeks and 2 days ago
Thats a great idea dennisw, I've since updated the code to reflect your 'default reversion' tip.. plugs 1 more attempt to find the lines using tput first... Yes this works exactly the same way for tail, I loves it!
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projack · 12 weeks and 4 days ago

What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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