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How to check if there is only one row in the file?

 
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Top Forums UNIX for Dummies Questions & Answers How to check if there is only one row in the file?
# 8  
Old 10-06-2010
I suppose the ultimate in bulletproof robust is to allow that the second line could be massive, so get just the first line and if the ls -l file size is > (first_line_length + 1), send the message! Since the response is email, we know the speed is not a big concern, but resource use might be. Most tools including sed read ahead, sometimes just the amount of a FILE* default buffer (5120 or 8192 are typical) but sometimes a full line. Luckily, 'line' does not! I assume the file length is correct while the file is open to be actively being extended.

Code:
if (( $( line <file_in|wc -c ) < $( ls -ld file_in |sed 's/  */        /g' |cut -f 5 ) ))
then
 echo ... | mailx ....
fi

Isn't truss so educational? And you can decide you need it after the program starts running and without source code! Here is 'line' in action. How many know dynamic libs are mmap'd so all share those pages but pages are never read if lib entry is not called? Notice that it never hits EOF after the newline:

Code:
$ truss line <<!
> hello
> !
execve("/usr/bin/line", 0x7b117940, 0x7b117948) .......... = 0 [32-bit]
utssys(0x7b119aa0, 0, 0) ................................. = 0
open("/usr/lib/dld.sl", O_RDONLY, 074444) ................ = 4
read(4, "02\v010e0512@ \0E 1d0214\0\0\0\0".., 128) ....... = 128
lseek(4, 128, SEEK_SET) .................................. = 128
read(4, "10\0\004\0\0\0( \0033 94\0\0\0\0".., 48) ........ = 48
mmap(NULL, 209812, PROT_READ|PROT_EXEC, MAP_SHARED|MAP_SHLIB|MAP_STATICPREDICTION, 4, 40960) = 0xc0010000
mmap(NULL, 17752, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_SHLIB, 4, 253952) = 0x7b0eb000
close(4) ................................................. = 0
sysconf(_SC_CPU_VERSION) ................................. = 532
utssys(0x7b119ea0, 0, 0) ................................. = 0
getuid() ................................................. = 3080 (3080)
getuid() ................................................. = 3080 (3080)
getgid() ................................................. = 6900 (6900)
getgid() ................................................. = 6900 (6900)
mmap(NULL, 8192, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7b0e9000
open("/opt/graphics/OpenGL/lib/libogltls.sl", O_RDONLY, 0200) ERR#2 ENOENT
open("/usr/lib/libc.2", O_RDONLY, 0200) .................. = 4
fstat(4, 0x7b11a748) ..................................... = 0
read(4, "0210010e0512@ \0\0\0\0\0\0\0\0\0".., 128) ....... = 128
lseek(4, 128, SEEK_SET) .................................. = 128
read(4, "10\0\004\0\0\0( \012e3< \0\010\0".., 48) ........ = 48
read(4, "80\0\0\v\0\0\004\0\0\0\0", 12) .................. = 12
mmap(NULL, 1241088, PROT_READ|PROT_EXEC, MAP_SHARED|MAP_SHLIB, 4, 290816) = 0xc0100000
mmap(NULL, 57344, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS|MAP_SHLIB, -1, 0) = 0x7b0db000
mmap(0x7b0d2000, 36864, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_SHLIB, 4, 1531904) = 0x7b0d2000
mmap(NULL, 16384, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7b0ce000
close(4) ................................................. = 0
open("/usr/lib/libdld.2", O_RDONLY, 0200) ................ = 4
fstat(4, 0x7b11a948) ..................................... = 0
read(4, "02\v010e0512@ \0E 1d0217\0\0\0\0".., 128) ....... = 128
lseek(4, 128, SEEK_SET) .................................. = 128
read(4, "10\0\004\0\0\0( \0\0) \ \0\010\0".., 48) ........ = 48
read(4, "80\0\0\v\0\0\004\0\0\0\0", 12) .................. = 12
mmap(NULL, 12288, PROT_READ|PROT_EXEC, MAP_SHARED|MAP_SHLIB, 4, 8192) = 0xc0003000
mmap(NULL, 4096, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_SHLIB, 4, 20480) = 0x7b0cd000
close(4) ................................................. = 0
mmap(NULL, 16384, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7b0c9000
mmap(NULL, 88, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7b0c8000
sigsetreturn(0x7b0cfd56, 0x6211988, 1392) ................ = 0
read(0, "h ", 1) ......................................... = 1
read(0, "e ", 1) ......................................... = 1
read(0, "l ", 1) ......................................... = 1
read(0, "l ", 1) ......................................... = 1
read(0, "o ", 1) ......................................... = 1
read(0, "\n", 1) ......................................... = 1
alarm(0) ................................................. = 0
hellowrite(1, "h e l l o ", 5) ................................ = 5

write(1, "\n", 1) ........................................ = 1
exit(0) .................................................. WIFEXITED(0)
$

Last edited by DGPickett; 10-06-2010 at 02:14 PM..
 
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