1

I have a VM running CentOs 7.2.1511. It includes ncurses 5.9. I use both MobaXterm and PuTTY.

Whenever I run the clear command, the scrollback buffers are cleared automatically as well.

This behavior is not observed in ncurses 5.7 (on another CentOs VM CentOS release 6.5 (Final)).

Reading ncurses changelog, a change was made so that ncurses takes into account "the E3 extended capability to clear the terminal's scrollback buffer"

The man page also mentions terminfo:

It looks in the environment for the terminal type and then in the terminfo database to figure out how to clear the screen.

So terminfo has data that clear uses to determine whether it should clear the terminal's scrollback buffer. I found a utility, infocmp, that can print out terminfo descriptions.

infocmp

NAME

   infocmp - compare or print out terminfo descriptions

I have placed the output below under details.

The "clear" entry in terminfo shows this "clear=\E[H\E[2J,".

When I run strace clear on the CentOS7 VM I see this (full strace is below, under details)

write(1, "\33[3;J\33[H\33[2J", 12
)      = 12

When I run strace clear on the CentOS6.5 VM I see this

write(1, "\33[H\33[2J", 7
)

I think the new version of clear is picking up something from somewhere that's telling it to prepend "\33[3;J". (That is probably the change the changelog talks about)

But as the man page stated; that information is gathered from terminfo. My terminfo clear entry does not have \33[3;J\33[H\33[2J; rather it has \33[H\33[2J.

So; if I am missing something and this is not a bug in ncurses 5.9;

How do I change the behavior of clear on Centos7 to NOT clear the scrollback buffer?

I am not looking at modifying the code and compile, nor replace 5.9 with 5.7.

Thank you all


Details:

O/S: CentOS Linux release 7.2.1511 (Core) ncurses: ncurses version 5.9 (patch 20130511) [retrieved from 'man clear']

Output of infocmp on CentOS 7:

# infocmp
#       Reconstructed via infocmp from file: /usr/share/terminfo/x/xterm
xterm|xterm terminal emulator (X Window System),
        am, bce, km, mc5i, mir, msgr, npc, xenl,
        colors#8, cols#80, it#8, lines#24, pairs#64,
        acsc=``aaffggiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
        bel=^G, blink=\E[5m, bold=\E[1m, cbt=\E[Z, civis=\E[?25l,
        clear=\E[H\E[2J, cnorm=\E[?12l\E[?25h, cr=^M,
        csr=\E[%i%p1%d;%p2%dr, cub=\E[%p1%dD, cub1=^H,
        cud=\E[%p1%dB, cud1=^J, cuf=\E[%p1%dC, cuf1=\E[C,
        cup=\E[%i%p1%d;%p2%dH, cuu=\E[%p1%dA, cuu1=\E[A,
        cvvis=\E[?12;25h, dch=\E[%p1%dP, dch1=\E[P, dl=\E[%p1%dM,
        dl1=\E[M, ech=\E[%p1%dX, ed=\E[J, el=\E[K, el1=\E[1K,
        flash=\E[?5h$<100/>\E[?5l, home=\E[H, hpa=\E[%i%p1%dG,
        ht=^I, hts=\EH, ich=\E[%p1%d@, il=\E[%p1%dL, il1=\E[L,
        ind=^J, indn=\E[%p1%dS, invis=\E[8m,
        is2=\E[!p\E[?3;4l\E[4l\E>, kDC=\E[3;2~, kEND=\E[1;2F,
        kHOM=\E[1;2H, kIC=\E[2;2~, kLFT=\E[1;2D, kNXT=\E[6;2~,
        kPRV=\E[5;2~, kRIT=\E[1;2C, kb2=\EOE, kbs=\177, kcbt=\E[Z,
        kcub1=\EOD, kcud1=\EOB, kcuf1=\EOC, kcuu1=\EOA,
        kdch1=\E[3~, kend=\EOF, kent=\EOM, kf1=\EOP, kf10=\E[21~,
        kf11=\E[23~, kf12=\E[24~, kf13=\E[1;2P, kf14=\E[1;2Q,
        kf15=\E[1;2R, kf16=\E[1;2S, kf17=\E[15;2~, kf18=\E[17;2~,
        kf19=\E[18;2~, kf2=\EOQ, kf20=\E[19;2~, kf21=\E[20;2~,
        kf22=\E[21;2~, kf23=\E[23;2~, kf24=\E[24;2~,
        kf25=\E[1;5P, kf26=\E[1;5Q, kf27=\E[1;5R, kf28=\E[1;5S,
        kf29=\E[15;5~, kf3=\EOR, kf30=\E[17;5~, kf31=\E[18;5~,
        kf32=\E[19;5~, kf33=\E[20;5~, kf34=\E[21;5~,
        kf35=\E[23;5~, kf36=\E[24;5~, kf37=\E[1;6P, kf38=\E[1;6Q,
        kf39=\E[1;6R, kf4=\EOS, kf40=\E[1;6S, kf41=\E[15;6~,
        kf42=\E[17;6~, kf43=\E[18;6~, kf44=\E[19;6~,
        kf45=\E[20;6~, kf46=\E[21;6~, kf47=\E[23;6~,
        kf48=\E[24;6~, kf49=\E[1;3P, kf5=\E[15~, kf50=\E[1;3Q,
        kf51=\E[1;3R, kf52=\E[1;3S, kf53=\E[15;3~, kf54=\E[17;3~,
        kf55=\E[18;3~, kf56=\E[19;3~, kf57=\E[20;3~,
        kf58=\E[21;3~, kf59=\E[23;3~, kf6=\E[17~, kf60=\E[24;3~,
        kf61=\E[1;4P, kf62=\E[1;4Q, kf63=\E[1;4R, kf7=\E[18~,
        kf8=\E[19~, kf9=\E[20~, khome=\EOH, kich1=\E[2~,
        kind=\E[1;2B, kmous=\E[M, knp=\E[6~, kpp=\E[5~,
        kri=\E[1;2A, mc0=\E[i, mc4=\E[4i, mc5=\E[5i, meml=\El,
        memu=\Em, op=\E[39;49m, rc=\E8, rev=\E[7m, ri=\EM,
        rin=\E[%p1%dT, rmacs=\E(B, rmam=\E[?7l, rmcup=\E[?1049l,
        rmir=\E[4l, rmkx=\E[?1l\E>, rmm=\E[?1034l, rmso=\E[27m,
        rmul=\E[24m, rs1=\Ec, rs2=\E[!p\E[?3;4l\E[4l\E>, sc=\E7,
        setab=\E[4%p1%dm, setaf=\E[3%p1%dm,
        setb=\E[4%?%p1%{1}%=%t4%e%p1%{3}%=%t6%e%p1%{4}%=%t1%e%p1%{6}%=%t3%e%p1%d%;m,
        setf=\E[3%?%p1%{1}%=%t4%e%p1%{3}%=%t6%e%p1%{4}%=%t1%e%p1%{6}%=%t3%e%p1%d%;m,
        sgr=%?%p9%t\E(0%e\E(B%;\E[0%?%p6%t;1%;%?%p2%t;4%;%?%p1%p3%|%t;7%;%?%p4%t;5%;%?%p7%t;8%;m,
        sgr0=\E(B\E[m, smacs=\E(0, smam=\E[?7h, smcup=\E[?1049h,
        smir=\E[4h, smkx=\E[?1h\E=, smm=\E[?1034h, smso=\E[7m,
        smul=\E[4m, tbc=\E[3g, u6=\E[%i%d;%dR, u7=\E[6n,
        u8=\E[?1;2c, u9=\E[c, vpa=\E[%i%p1%dd,

strace clear: (CentOS7)

# strace clear
execve("/bin/clear", ["clear"], [/* 36 vars */]) = 0
brk(0)                                  = 0xf60000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd26198c000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=41031, ...}) = 0
mmap(NULL, 41031, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd261975000
close(3)                                = 0
open("/lib64/libtinfo.so.5", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0@\316\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=174520, ...}) = 0
mmap(NULL, 2268928, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fd261535000
mprotect(0x7fd26155a000, 2097152, PROT_NONE) = 0
mmap(0x7fd26175a000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x25000) = 0x7fd261750
close(3)                                = 0
open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0 \34\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2107816, ...}) = 0
mmap(NULL, 3932736, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fd26116d000
mprotect(0x7fd261323000, 2097152, PROT_NONE) = 0
mmap(0x7fd261523000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b6000) = 0x7fd26150
mmap(0x7fd261529000, 16960, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fd261529000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd26198b000
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd261989000
arch_prctl(ARCH_SET_FS, 0x7fd261989740) = 0
mprotect(0x7fd261523000, 16384, PROT_READ) = 0
mprotect(0x7fd26175a000, 16384, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ)     = 0
mprotect(0x7fd261986000, 4096, PROT_READ) = 0
munmap(0x7fd261975000, 41031)           = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) =0
brk(0)                                  = 0xf60000
brk(0xf81000)                           = 0xf81000
brk(0)                                  = 0xf81000
stat("/root/.terminfo", 0xf601c0)       = -1 ENOENT (No such file or directory)
stat("/etc/terminfo", {st_mode=S_IFDIR|0755, st_size=6, ...}) = 0
stat("/usr/share/terminfo", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
access("/etc/terminfo/x/xterm", R_OK)   = -1 ENOENT (No such file or directory)
access("/usr/share/terminfo/x/xterm", R_OK) = 0
open("/usr/share/terminfo/x/xterm", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=3371, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd261985000
read(3, "\32\0010\0&\0\17\0\235\1l\5xterm|xterm terminal"..., 4096) = 3371
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7fd261985000, 4096)            = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) =0
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) =0
ioctl(1, TIOCGWINSZ, {ws_row=62, ws_col=112, ws_xpixel=896, ws_ypixel=930}) = 0
fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 0), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd261985000
write(1, "\33[3;J\33[H\33[2J", 12
)      = 12
exit_group(0)                           = ?
+++ exited with 0 +++
0

The place to look is in the extended capabilities, shown using the -x option of infocmp, e.g.,

infocmp -1x |grep E3

would show

    E3=\E[3J,

(I used the -1 option to format the output as a single column). You can as usual do

infocmp -x  >foo
vi foo # remove/adjust whatever
sudo tic -x foo

Both xterm and linux (Linux console) support this feature.

Further reading:

  • Thank you very much! I edited the file and changed E3=\E[3J, to E3=,. However I had to remove linux from the infocmp arguments as the output didn't include E3. – halbeik Sep 21 '16 at 22:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.