Linux -- head/tail

本文介绍Linux中常用的文件操作命令,如cat、more、less等,用于显示文件内容;head、tail用于提取文件的前几行或后几行,以及如何组合使用这些命令来实现特定功能。

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1)文件内容查阅

要查阅一个文件的内容时,最常使用的显示文件内容的命令可以说是 cat 与 more 及 less 了!

此外,如果我们要查看一个很大型的文件 (好几百MB时),但是我们只需要后端的几行字而已,那么该如何是好?

tail 呀,此外, tac这个命令也可以达到!

    ---- cat  由第一行开始显示文件内容

    ---- tac  从最后一行开始显示,可以看出 tac 是 cat 的倒写!

    ---- nl   显示的时候,顺道输出行号!

    ---- more 一页一页的显示文件内容

    ---- less 与 more 类似,但是比 more 更好的是,他可以往前翻页!

    ---- head 只看头几行

    ---- tail 只看尾巴几行

    ---- od   以二进位的方式读取文件内容!

参考网址:http://vbird.dic.ksu.edu.tw/linux_basic/0220filemanager_3.php

2)数据撷取---head/tail

我们可以将输出的数据作一个最简单的撷取,那就是取出前面 (head)取出后面 (tail)文字的功能。

不过,要注意的是, head 与 tail 都是以『』为单位来进行数据撷取的喔!还有|grep 也可以用。

head 的英文意思就是『头』啦,那么这个东西的用法自然就是显示出一个文件的前几行了。

若没有加上 -n 这个选项时,默认只显示十行,若只要一行呢?那就加入『 head -n 1 filename 』即可!

另外那个 -n 选项后面的参数较有趣,如果接的是负数,例如上面范例的-n -100时,代表列前的所有行数, 但不包括后面100行。

举例来说,/etc/manpath.config共有141行,则上述的命令『head -n -100 /etc/manpath.config』 就会列出前面41行,后面100行不会列印出来了。

相当于列出前面(所有的行数-100)

这个 tail 的用法跟 head 的用法差不多类似,只是显示的是后面几行就是了!默认也是显示十行,若要显示非十行,就加 -n number 的选项即可。

其实与head -n -xx有异曲同工之妙。当下达『tail -n +120 /etc/man.config』 代表该文件从120行以后都会被列出来.

同样的,在manpath.config共有131行,因此第120~131行就会被列出来啦! 前面的119行都不会被显示出来.

tail -f /var/log/messages

由于/var/log/messages随时会有数据写入,你想要让该文件有数据写入时就立刻显示到萤幕上, 就利用 -f 这个选项,它可以一直侦测/var/log/messages这个文件。

新加入的数据都会被显示到萤幕上。 直到你按下[crtl]+c才会离开tail的侦测。

例题:假如我想要显示 /etc/man.config 的第 11 到第 20 行呢?

答:在第 11 到第 20 行,那么我取前 20 行,再取后十行,所以结果就是:

     『 head -n 20 /etc/man.config | tail -n 10 』,就可以得到11-20的内容了!

使用上面代码编译出现以下问题:make -C ../../ /root/bpf/linux-4.19.90/samples/bpf/ BPF_SAMPLES_PATH=/root/bpf/linux-4.19.90/samples/bpf make[1]: Entering directory '/root/bpf/linux-4.19.90' CALL scripts/checksyscalls.sh DESCEND objtool make -C /root/bpf/linux-4.19.90/samples/bpf/../../tools/lib/bpf/ RM='rm -rf' LDFLAGS= srctree=/root/bpf/linux-4.19.90/samples/bpf/../../ O= Warning: Kernel ABI header at 'tools/include/uapi/linux/bpf.h' differs from latest version at 'include/uapi/linux/bpf.h' HOSTCC /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.o /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c: In function ‘print_violation’: /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:53:26: warning: implicit declaration of function ‘localtime’; did you mean ‘st_atime’? [-Wimplicit-function-declaration] struct tm *tm_info = localtime(&ts); ^~~~~~~~~ st_atime /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:53:26: warning: initialization makes pointer from integer without a cast [-Wint-conversion] /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:55:5: warning: implicit declaration of function ‘strftime’ [-Wimplicit-function-declaration] strftime(time_buf, sizeof(time_buf), "%Y-%m-%d %H:%M:%S", tm_info); ^~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:55:5: warning: incompatible implicit declaration of built-in function ‘strftime’ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:55:5: note: include ‘<time.h>’ or provide a declaration of ‘strftime’ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c: At top level: /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:69:13: warning: function declaration isn’t a prototype [-Wstrict-prototypes] static void read_violations() { ^~~~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c: In function ‘read_violations’: /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:77:12: error: variable ‘attr’ has initializer but incomplete type struct perf_event_attr attr = { ^~~~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:78:10: error: ‘struct perf_event_attr’ has no member named ‘sample_type’ .sample_type = PERF_SAMPLE_RAW, ^~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:78:24: error: ‘PERF_SAMPLE_RAW’ undeclared (first use in this function) .sample_type = PERF_SAMPLE_RAW, ^~~~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:78:24: note: each undeclared identifier is reported only once for each function it appears in /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:78:24: warning: excess elements in struct initializer /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:78:24: note: (near initialization for ‘attr’) /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:79:10: error: ‘struct perf_event_attr’ has no member named ‘type’ .type = PERF_TYPE_SOFTWARE, ^~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:79:17: error: ‘PERF_TYPE_SOFTWARE’ undeclared (first use in this function) .type = PERF_TYPE_SOFTWARE, ^~~~~~~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:79:17: warning: excess elements in struct initializer /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:79:17: note: (near initialization for ‘attr’) /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:80:10: error: ‘struct perf_event_attr’ has no member named ‘config’ .config = PERF_COUNT_SW_BPF_OUTPUT, ^~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:80:19: error: ‘PERF_COUNT_SW_BPF_OUTPUT’ undeclared (first use in this function) .config = PERF_COUNT_SW_BPF_OUTPUT, ^~~~~~~~~~~~~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:80:19: warning: excess elements in struct initializer /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:80:19: note: (near initialization for ‘attr’) /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:81:10: error: ‘struct perf_event_attr’ has no member named ‘size’ .size = sizeof(attr), ^~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:81:23: error: invalid application of ‘sizeof’ to incomplete type ‘struct perf_event_attr’ .size = sizeof(attr), ^ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:81:17: warning: excess elements in struct initializer .size = sizeof(attr), ^~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:81:17: note: (near initialization for ‘attr’) /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:82:10: error: ‘struct perf_event_attr’ has no member named ‘wakeup_events’ .wakeup_events = 1, ^~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:82:26: warning: excess elements in struct initializer .wakeup_events = 1, ^ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:82:26: note: (near initialization for ‘attr’) /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:77:28: error: storage size of ‘attr’ isn’t known struct perf_event_attr attr = { ^~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:85:23: error: ‘__NR_perf_event_open’ undeclared (first use in this function); did you mean ‘bpf_perf_event_ret’? perf_fd = syscall(__NR_perf_event_open, &attr, -1, 0, -1, 0); ^~~~~~~~~~~~~~~~~~~~ bpf_perf_event_ret /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:100:11: warning: implicit declaration of function ‘mmap’ [-Wimplicit-function-declaration] buf = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, perf_fd, 0); ^~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:100:33: error: ‘PROT_READ’ undeclared (first use in this function); did you mean ‘IPPROTO_RAW’? buf = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, perf_fd, 0); ^~~~~~~~~ IPPROTO_RAW /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:100:45: error: ‘PROT_WRITE’ undeclared (first use in this function); did you mean ‘PROT_READ’? buf = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, perf_fd, 0); ^~~~~~~~~~ PROT_READ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:100:57: error: ‘MAP_SHARED’ undeclared (first use in this function) buf = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, perf_fd, 0); ^~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:101:16: error: ‘MAP_FAILED’ undeclared (first use in this function); did you mean ‘MAP_SHARED’? if (buf == MAP_FAILED) { ^~~~~~~~~~ MAP_SHARED /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:113:55: error: dereferencing pointer to incomplete type ‘struct perf_event_mmap_page’ __u64 data_head = __sync_fetch_and_add(&header->data_head, 0); ^~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:131:5: warning: implicit declaration of function ‘munmap’ [-Wimplicit-function-declaration] munmap(buf, mmap_size); ^~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c: In function ‘load_xdp’: /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:194:15: warning: implicit declaration of function ‘if_nametoindex’ [-Wimplicit-function-declaration] ifindex = if_nametoindex(iface); ^~~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:214:31: warning: implicit declaration of function ‘bpf_object__find_program_by_name’; did you mean ‘bpf_object__find_program_by_title’? [-Wimplicit-function-declaration] prog_fd = bpf_program__fd(bpf_object__find_program_by_name(obj, "xdp_whitelist")); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ bpf_object__find_program_by_title /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:214:31: warning: passing argument 1 of ‘bpf_program__fd’ makes pointer from integer without a cast [-Wint-conversion] In file included from /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:11:0: ./tools/lib/bpf/libbpf.h:128:5: note: expected ‘struct bpf_program *’ but argument is of type ‘int’ int bpf_program__fd(struct bpf_program *prog); ^~~~~~~~~~~~~~~ /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c: At top level: /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.c:238:13: warning: function declaration isn’t a prototype [-Wstrict-prototypes] static void unload_xdp() { ^~~~~~~~~~ make[2]: *** [scripts/Makefile.host:107: /root/bpf/linux-4.19.90/samples/bpf/xdp_whitelist_user.o] Error 1 make[1]: *** [Makefile:1695: /root/bpf/linux-4.19.90/samples/bpf/] Error 2 make[1]: Leaving directory '/root/bpf/linux-4.19.90' make: *** [Makefile:224: all] Error 2
最新发布
07-22
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