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Alias HEAD for automatic smart output

Terminal - Alias HEAD for automatic smart output
alias head='head -n $((${LINES:-`tput lines 2>/dev/null||echo -n 12`} - 2))'
2010-04-08 22:37:06
User: AskApache
Functions: alias echo
26
Alias HEAD for automatic smart output

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. ( http://www.askapache.com/linux-unix/bash-power-prompt.html )

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.

Alternatives

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Terminal - Alternatives

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What others think

This is great. Also the -n option still works.

Comment by houghi 267 weeks and 4 days ago

You could also use this with "tail".

Comment by jmcantrell 267 weeks and 3 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))'
Comment by dennisw 267 weeks and 1 day 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.

Comment by dennisw 267 weeks and 1 day ago

@dennisw Are you head-ing something that is wrapping lines?

Comment by vgm64 267 weeks and 1 day ago

Bingo! +10 internets for you, -40 for me! (I'll return now to my tiddlywinks.)

Comment by dennisw 267 weeks and 1 day 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!

Comment by AskApache 267 weeks ago

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