Bash – Use `cat` or `less` Depending on Line Count

catless

When looking at the content of files, I'd like to automatically cat short files but less those that exceed the screen size. I could use something with wc -l, but is there a better way that maybe also considers the window size / current amount of lines available?

Best Answer

To give you the formula which involves the wc-based check:

(($(wc -l<input_file)<=$(tput lines))) && echo 'will fit' || echo 'not enough'

There is a $LINES shell variable which can also be used:

(($(wc -l<input_file)<=LINES)) && echo 'will fit' || echo 'not enough'

But $LINES is updated only when at the command prompt. To understand what I mean, run this and resize the terminal window during the sleep:

( sleep 3; echo $LINES; tput lines )

Related Solutions

Cat Command – Display Headers and Line Numbers

A very rough awk implementation:

BEGIN{
    OLDFILENAME="";
}
FNR==1{
    if (OLDFILENAME != "") {
        printf("#### Processed (chars: %s - lines: %s)\n", FWC, FLC);
    }
    printf("#### Processing: %s\n", FILENAME);
    OLDFILENAME=FILENAME;
    FWC=0;
    FLC=0;
}
{
    printf("%04d - %s\n", FNR, $0);
    FWC = FWC + length($0);
    FLC = FLC + 1;
}
END{
    if (OLDFILENAME != "") {
        printf("#### Processed (chars: %s - lines: %s)\n", FWC, FLC);
    }
}

Execute awk -f AWKFILE trapetz simpson to get:

#### Processing: trapetz
0001 - x = 0:0.0001:7pi
0002 - plot(x, sin(x).*cos(x))
0003 - Area = trapz(x, sin(x).*cos(x))
#### Processed (chars: 70 - lines: 3)
#### Processing: simpson
0001 - f = inline(sin(x).*cos(x));
0002 - Area2 = quad(f, 0, 7pi, 1e-16)
#### Processed (chars: 57 - lines: 2)

Less Command – Exit ‘Less’ Without Clearing the Screen

This is actually a function of the terminal emulator you are using (xterm, gnome-terminal, konsole, screen). An alternate screen, or altscreen, gets launched when programs such as less or vim are invoked. This altscreen has no history buffer and exits immediately when you quit the program, switching back to the original screen which restores the previous window content history and placement.

You can prevent less from launch in an altscreen by passing the argument "-X".

less -X /path/to/some/file

You can also pass "-X" as an environment variable. So if you are using bash, place this in ~/.bashrc:

export LESS="-X"

However, this disbles the termcap (terminal capability) initialization and deinitialization, so other views when you use less may appear off.

Another option would be to use screen and set the option altscreen off in your ~/.screenrc. less will not clear the screen and should preserve color formatting. Presumably tmux will have the same option.

This blog entry describes the problem and offers some different solutions specific to gnome-terminal with varying success.

Best Answer

Related Solutions

Cat Command – Display Headers and Line Numbers

Less Command – Exit ‘Less’ Without Clearing the Screen

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