杭州网站改版公司电话,网站栏目内链怎么做,手机软件开发应用,外贸公司网站建设需要注意目录 一、truncate功能概述实例#xff08;可用于删除文件末尾指定大小的内容#xff09; 二、head功能概述实例#xff08;可用于删除文件末尾指定大小的内容#xff09; 三、tail功能概述#xff1a;实例#xff08;可用于删除文件开头指定大小的内容#xff09; 四、… 目录 一、truncate功能概述实例可用于删除文件末尾指定大小的内容 二、head功能概述实例可用于删除文件末尾指定大小的内容 三、tail功能概述实例可用于删除文件开头指定大小的内容 四、dd概述 五、应用1. 清空文件内容 一、truncate
truncate --help
Usage: truncate OPTION... FILE...
Shrink or extend the size of each FILE to the specified sizeA FILE argument that does not exist is created.If a FILE is larger than the specified size, the extra data is lost.
If a FILE is shorter, it is extended and the extended part (hole)
reads as zero bytes.Mandatory arguments to long options are mandatory for short options too.-c, --no-create do not create any files-o, --io-blocks treat SIZE as number of IO blocks instead of bytes-r, --referenceRFILE base size on RFILE-s, --sizeSIZE set or adjust the file size by SIZE bytes--help display this help and exit--version output version information and exitSIZE is an integer and optional unit (example: 10M is 10*1024*1024). Units
are K, M, G, T, P, E, Z, Y (powers of 1024) or KB, MB, ... (powers of 1000).SIZE may also be prefixed by one of the following modifying characters:extend by, - reduce by, at most, at least,
/ round down to multiple of, % round up to multiple of.功能概述
缩小或扩展文件到指定的大小默认文件不存在会被创建如果文件大于指定的大小额外的末尾的数据将被丢弃如果文件小于指定的大小则对其进行扩展并且扩展部分读取为零字节
实例可用于删除文件末尾指定大小的内容 -s 参数设置或调整文件到指定的大小。 -s, --sizeSIZE set or adjust the file size by SIZE bytes$ echo 123456789 test_truncate
$ hexdump -C test_truncate
00000000 31 32 33 34 35 36 37 38 39 0a |123456789.|
# 文件大于指定的大小额外的末尾的数据将被丢弃
$ truncate -s 5 test_truncate
$ hexdump -C test_truncate
00000000 31 32 33 34 35 |12345|
# 如果文件小于指定的大小则对其进行扩展并且扩展部分读取为零字节
truncate -s 10 test_truncate
$ hexdump -C test_truncate
00000000 31 32 33 34 35 00 00 00 00 00 |12345.....|-r 参数将文件设置为于参考文件相同的大小。 -r, --referenceRFILE base size on RFILE$ ls -l test_truncate*
-rw-rw-r-- 1 guest guest 10 Nov 30 01:52 test_truncate
-rw-rw-r-- 1 guest guest 21 Nov 30 02:00 test_truncate_ref
$ hexdump -C test_truncate
00000000 31 32 33 34 35 00 00 00 00 00 |12345.....|
0000000a
# 将test_truncate文件的大小设置为与test_truncate_ref文件相同
$ truncate -r test_truncate_ref test_truncate
$ ls -l test_truncate*
-rw-rw-r-- 1 guest guest 21 Nov 30 02:01 test_truncate
-rw-rw-r-- 1 guest guest 21 Nov 30 02:00 test_truncate_ref
$ hexdump -C test_truncate
00000000 31 32 33 34 35 00 00 00 00 00 00 00 00 00 00 00 |12345...........|
00000010 00 00 00 00 00 |.....|-c 参数文件不存在不创建。 -c, --no-create do not create any files# 带-c参数不会创建不存在的文件
ls -l test_truncate
ls: cannot access test_truncate: No such file or directory
$ truncate -s 100 -c test_truncate
$ ls -l test_truncate
ls: cannot access test_truncate: No such file or directory# 不带-c参数文件不存在则创建内容都是0
$ truncate -s 100 test_truncate
$ ls -l test_truncate
-rw-rw-r-- 1 guest guest 100 Nov 30 01:07 test_truncate
$ hexdump -C test_truncate
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000060 00 00 00 00 |....|
00000064二、head
head --help
Usage: head [OPTION]... [FILE]...
Print the first 10 lines of each FILE to standard output.
With more than one FILE, precede each with a header giving the file name.
With no FILE, or when FILE is -, read standard input.Mandatory arguments to long options are mandatory for short options too.-c, --bytes[-]K print the first K bytes of each file;with the leading -, print all but the lastK bytes of each file-n, --lines[-]K print the first K lines instead of the first 10;with the leading -, print all but the lastK lines of each file-q, --quiet, --silent never print headers giving file names-v, --verbose always print headers giving file names--help display this help and exit--version output version information and exitK may have a multiplier suffix:
b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024,
GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.功能概述
将每个文件的前10行打印到标准输出如果有多个文件则先打印文件名
$ head test_head test_truncatetest_head
1234567890 test_truncate
123451234567890实例可用于删除文件末尾指定大小的内容 -c 参数打印文件的前K个字节如果是-K则去除末尾K字节打印前面所有字节。 -c, --bytes[-]K print the first K bytes of each file;with the leading -, print all but the lastK bytes of each file# 末尾有个回车符
$ cat test_head
1234567890
$ head -c 3 test_head
123$
$ head -c -3 test_head
12345678$ -n 参数打印文件的前K行如果是-K则去除末尾K行打印前面所有行。 -n, --lines[-]K print the first K lines instead of the first 10;with the leading -, print all but the lastK lines of each file$ cat test_head
1234567890
234567890
34567890
4567890
567890
$ head -n 2 test_head
1234567890
234567890
$ head -n -2 test_head
1234567890
234567890
34567890三、tail
tail --help
Usage: tail [OPTION]... [FILE]...
Print the last 10 lines of each FILE to standard output.
With more than one FILE, precede each with a header giving the file name.
With no FILE, or when FILE is -, read standard input.Mandatory arguments to long options are mandatory for short options too.-c, --bytesK output the last K bytes; or use -c K to outputbytes starting with the Kth of each file-f, --follow[{name|descriptor}]output appended data as the file grows;an absent option argument means descriptor-F same as --followname --retry-n, --linesK output the last K lines, instead of the last 10;or use -n K to output starting with the Kth--max-unchanged-statsNwith --followname, reopen a FILE which has notchanged size after N (default 5) iterationsto see if it has been unlinked or renamed(this is the usual case of rotated log files);with inotify, this option is rarely useful--pidPID with -f, terminate after process ID, PID dies-q, --quiet, --silent never output headers giving file names--retry keep trying to open a file if it is inaccessible-s, --sleep-intervalN with -f, sleep for approximately N seconds(default 1.0) between iterations;with inotify and --pidP, check process P atleast once every N seconds-v, --verbose always output headers giving file names--help display this help and exit--version output version information and exitIf the first character of K (the number of bytes or lines) is a ,
print beginning with the Kth item from the start of each file, otherwise,
print the last K items in the file. K may have a multiplier suffix:
b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024,
GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.With --follow (-f), tail defaults to following the file descriptor, which
means that even if a tailed file is renamed, tail will continue to track
its end. This default behavior is not desirable when you really want to
track the actual name of the file, not the file descriptor (e.g., log
rotation). Use --followname in that case. That causes tail to track the
named file in a way that accommodates renaming, removal and creation.功能概述
-n显示文件的最后 n 行默认为 10 行。-f实时追踪文件的变化并输出新增的内容。-q不显示文件名。-s设置输出的间隔时间秒。-c以字节为单位显示指定范围的内容。
实例可用于删除文件开头指定大小的内容 -c 参数打印文件的后K个字节如果是K则从第K个字节开始打印所有后面的内容。 -c, --bytesK output the last K bytes; or use -c K to outputbytes starting with the Kth of each file重点 经验证0和1都表示从第一个字节开始和cat功能一样。可用于删除文件前面的内容输出到新文件 如需删除文件前面的K字节则参数为(K1) $ cat test_tail
1234567890
$ tail -c 3 test_tail
90
$ tail -c 3 test_tail
34567890
$ tail -c 0 test_tail
1234567890
$ tail -c 1 test_tail
1234567890-f 参数实时追踪文件的变化并输出新增的内容。可指定显示几行文件中的内容。 -f, --follow[{name|descriptor}]output appended data as the file grows;an absent option argument means descriptor$ cat test_tail
1234567890
234567890
34567890
4567890
567890
$ tail -2f test_tail
4567890
567890 -n 参数显示文件的最后 n 行默认为 10 行。 -n, --linesK output the last K lines, instead of the last 10;or use -n K to output starting with the Kth$ cat test_tail
1234567890
234567890
34567890
4567890
567890
$ tail -n 3 test_tail
34567890
4567890
567890四、dd
dd --help
Usage: dd [OPERAND]...or: dd OPTION
Copy a file, converting and formatting according to the operands.bsBYTES read and write up to BYTES bytes at a timecbsBYTES convert BYTES bytes at a timeconvCONVS convert the file as per the comma separated symbol listcountN copy only N input blocksibsBYTES read up to BYTES bytes at a time (default: 512)ifFILE read from FILE instead of stdiniflagFLAGS read as per the comma separated symbol listobsBYTES write BYTES bytes at a time (default: 512)ofFILE write to FILE instead of stdoutoflagFLAGS write as per the comma separated symbol listseekN skip N obs-sized blocks at start of outputskipN skip N ibs-sized blocks at start of inputstatusLEVEL The LEVEL of information to print to stderr;none suppresses everything but error messages,noxfer suppresses the final transfer statistics,progress shows periodic transfer statisticsN and BYTES may be followed by the following multiplicative suffixes:
c 1, w 2, b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024, xM M
GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.概述
if 表示输入文件of 表示输出文件options 是一些可选参数。下面是一些常用的参数
bs设置每次读取和写入的块大小单位为字节或者是可以添加的后缀如b、k、m等默认为512字节。count设置要复制的块数。iflag设置输入选项常用的选项有direct绕过缓存直接读取和sync同步数据到磁盘。oflag设置输出选项常用的选项有direct绕过缓存直接写入和sync同步数据到磁盘。
五、应用
1. 清空文件内容
echo 文件名
echo 文件名
cat /dev/null 文件名
truncate -s 0 文件名
dd if/dev/null of文件名
