head和tail命令詳解

   

基礎命令學習目錄首頁

 

原文鏈接：https://www.cnblogs.com/amosli/p/3496027.html

當要查看上千行的大文件時，我們可不會用cat命令把整個文件內容給打印出來，相反，我們可能只需要看文件的一小部分地內容（例如文件的前十行和后十行）,我們也有可能需要打印出來前n行或后n行,也有可能打印除了前n行或后n行之外的所有行，也有可能需要實時監控log日志的更新，那么怎么實現呢？下面一起來看一下linux下使用率極高的head ,tail兩個命令。

一、head命令詳解

首先，輸入head --help查看幫助信息:

amosli@amosli-pc:~/learn/fd$ 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 last
                             K bytes of each file
  -n, --lines=[-]K         print the first K lines instead of the first 10;
                             with the leading `-', print all but the last
                             K 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 exit

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.

head命令的語法格式為:

head [OPTION]... [FILE]...

接下來將在實例中講解各參數含義及用法：

實例：

1.使用head命令查看文件內容前十行

新建test.txt,共14行.

amosli@amosli-pc:~/learn/fd$ cat -n test.txt 
     1    a
     2    b
     3    c
     4    d
     5    e
     6    f
     7    g
     8    h
     9    i
    10    j
    11    k
    12    l
    13    m
    14    n

使用head命令查看前十行,head命令默認顯示文件前十行

amosli@amosli-pc:~/learn/fd$ head test.txt 
a
b
c
d
e
f
g
h
i
j

2.-n參數，顯示test.txt文件的前3行

amosli@amosli-pc:~/learn/fd$ head -n 3 test.txt 
a
b
c

英文提示信息：

  -n, --lines=[-]K         print the first K lines instead of the first 10;                   

 

3.-n參數顯示除了文件最后3行外的所有內容

amosli@amosli-pc:~/learn/fd$ head -n -3 test.txt 
a
b
c
d
e
f
g
h
i
j
k

英文提示信息：

  -n, --lines=[-]K         print the first K lines instead of the first 10;
                             with the leading `-', print all but the last
                             K lines of each file

加上'-',打印所有內容除了最后的K行。

4.-c參數，按文件內容大小來打印，打印前2個字節的內容

amosli@amosli-pc:~/learn/fd$ head -c 2 test.txt 
a

2個字節就是一個“a”字母。

英文提示信息：

  -c, --bytes=[-]K         print the first K bytes of each file;

5.-c參數，打印除了最后2個字節的文件內容

amosli@amosli-pc:~/learn/fd$ head -c -2 test.txt 
a
b
c
d
e
f
g
h
i
j
k
l
m

英文提示信息：

  -c, --bytes=[-]K         print the first K bytes of each file;
                             with the leading `-', print all but the last
                             K bytes of each file

6.-q參數，打印時不顯示文件名稱

amosli@amosli-pc:~/learn/fd$ head -q test.txt 
a
b
c
d
e
f
g
h
i
j

英文提示信息：

  -q, --quiet, --silent    never print headers giving file names

其實后面跟上--quiet,--silent都是一樣的，都不會顯示文件名稱，和默認打印是一樣的效果。

7.-v參數，打印是顯示文件名稱

amosli@amosli-pc:~/learn/fd$ head -v test.txt 
==> test.txt <==
a
b
c
d
e
f
g
h
i
j

英文提示信息：

  -v, --verbose            always print headers giving file names

其中，用--verbose和-v顯示的是一樣的效果

amosli@amosli-pc:~/learn/fd$ head --verbose test.txt 
==> test.txt <==
a
b
c
d
e
f
g
h
i
j

8.打印多個文件的內容

amosli@amosli-pc:~/learn/fd$ head -n 3 test.txt test2.txt 
==> test.txt <==
a
b
c

==> test2.txt <==
c
d
e

 

二、tail命令詳解

tail命令和head 命令非常相似，只不過它是打印文件的尾部內容的，當然也有一些特色之處,下面一起來看看吧。

首先，輸入tail --help看一下提示信息

amosli@amosli-pc:~/learn/fd$ 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, --bytes=K            output the last K bytes; alternatively, use -c +K
                           to output bytes starting with the Kth of each file
  -f, --follow[={name|descriptor}]
                           output appended data as the file grows;
                           -f, --follow, and --follow=descriptor are
                           equivalent
  -F                       same as --follow=name --retry
  -n, --lines=K            output the last K lines, instead of the last 10;
                           or use -n +K to output lines starting with the Kth
      --max-unchanged-stats=N
                           with --follow=name, reopen a FILE which has not
                           changed size after N (default 5) iterations
                           to see if it has been unlinked or renamed
                           (this is the usual case of rotated log files).
                           With inotify, this option is rarely useful.
      --pid=PID            with -f, terminate after process ID, PID dies
  -q, --quiet, --silent    never output headers giving file names
      --retry              keep trying to open a file even when it is or
                             becomes inaccessible; useful when following by
                             name, i.e., with --follow=name
  -s, --sleep-interval=N   with -f, sleep for approximately N seconds
                             (default 1.0) between iterations.
                             With inotify and --pid=P, check process P at
                             least once every N seconds.
  -v, --verbose            always output headers giving file names
      --help     display this help and exit
      --version  output version information and exit

If 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 tail'ed 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 --follow=name in that case.  That causes tail to track the
named file in a way that accommodates renaming, removal and creation.

語法格式：

tail [OPTION]... [FILE]...

這里由於head和tail實在比較像，這里為了節省篇幅，對相似之處將盡量簡潔

實例：

test2.txt，共有12行內容為從c-n

amosli@amosli-pc:~/learn/fd$ cat -n test2.txt 
     1    c
     2    d
     3    e
     4    f
     5    g
     6    h
     7    i
     8    j
     9    k
    10    l
    11    m
    12    n

 

1.-c 參數，根據文件字節進行輸出打印

  -c, --bytes=K            output the last K bytes; alternatively, use -c +K
                           to output bytes starting with the Kth of each file

打印test2.txt中的最后4 bytes，如下：

amosli@amosli-pc:~/learn/fd$ tail -c 4 test2.txt 
m
n

tail -c +4 test2.txt 加上一個‘+’會是什么效果呢？

amosli@amosli-pc:~/learn/fd$ tail -c +4 test2.txt 

e
f
g
h
i
j
k
l
m
n

少打印了c d 兩個字母，那么 -c +K的意思也就很明了了，即打印文件的所有內容除了前面的K個字節

2、-n參數，根據文件行數進行打印

看一下提示信息：

 -n, --lines=K            output the last K lines, instead of the last 10;
                           or use -n +K to output lines starting with the Kth
      --max-unchanged-stats=N
                           with --follow=name, reopen a FILE which has not
                           changed size after N (default 5) iterations
                           to see if it has been unlinked or renamed
                           (this is the usual case of rotated log files).
                           With inotify, this option is rarely useful.
      --pid=PID            with -f, terminate after process ID, PID dies

打印test2.txt最后的3行內容:

amosli@amosli-pc:~/learn/fd$ tail -n 3 test2.txt 
l
m
n

從第3行開始輸出test2.txt的所有內容：

amosli@amosli-pc:~/learn/fd$ tail -n +3 test2.txt 
e
f
g
h
i
j
k
l
m
n

3.-q參數，-v參數

 -q, --quiet, --silent    never output headers giving file names
      --retry              keep trying to open a file even when it is or
                             becomes inaccessible; useful when following by
                             name, i.e., with --follow=name

不打印文件名稱信息：

amosli@amosli-pc:~/learn/fd$ tail -q  test2.txt 
e
f
g
h
i
j
k
l
m
n

打印文件名稱信息：

amosli@amosli-pc:~/learn/fd$ tail -v test2.txt 
==> test2.txt <==
e
f
g
h
i
j
k
l
m
n

４、-f參數

tail 命令的一個很重要的用法是從一個內容不斷增加的文件中讀取數據。新增加的內容部民被添加到文件的尾部，因此當新內容被寫入文件的時候，可以用tail將其顯示出來。只是簡單的使用tail的話，它會讀取文件的最后10行，然后退出，這樣就不能做到實時監控，加入-f參數就可以做到實時監控文件的更新內容了。

amosli@amosli-pc:~/learn/fd$ tail -f test2.txt 
g
h
i
j
k
l
m
n
o
p

ctrl+alt+t新開一個終端，然后執行下面的命令:

amosli@amosli-pc:~/learn/fd$ echo  'xyz' >> test2.txt

這個時候你就可以看到前一個終端在里出現了‘xyz’

amosli@amosli-pc:~/learn/fd$ tail -f test2.txt 
g
h
i
j
k
l
m
n
o
p
xyz

這樣就能實時監控項目里的log日志了。

如果想設定一個間隔時間去查看文件的更新應該怎么做呢？請看-s參數

5、-s參數

英文提示信息：

-s, --sleep-interval=N   with -f, sleep for approximately N seconds
                             (default 1.0) between iterations.
                             With inotify and --pid=P, check process P at
                             least once every N seconds.

如每隔５秒查看一次test2.txt的內容是否更新

amosli@amosli-pc:~/learn/fd$ tail -f -s 5 test2.txt
j
k
l
m
n
o
p
xyz

 默認是１秒更新一次。

6.--pid參數

 tail 具有一個很意思的特性，當某個給定進程結束之后，tail也會隨之終結.

假如我們正在讀取一個不斷增長的文件，進程Foo一直在向該文件追加數據，那么tail就會一直執行，直到進程Foo的結束.

$PID=$(pidof Foo)
$tail -f file --pid $PID
#當進程Foo結束之后，tail也會跟着結束

例如用gedit打開test2.txt，不斷加入數據,然后在終端里使用tail進行監聽，具體為：

amosli@amosli-pc:~/learn/fd$ PID=$(pidof gedit)
amosli@amosli-pc:~/learn/fd$ tail -f -s 2 test2.txt --pid $PID
h
i
j
k
l
m
n
o
p
xyz

然后不斷在gedit中追加入‘yyy’后保存，按理說終端里應該會更新，但我的終端不知為何沒有更新數據，這里就不帖出來了。

關閉gedit后，tail監控也關閉掉了。

 

head和tail取文件第m行到第n行

[root@xiaoma /root/mcw] test!
#vim test.txt

[root@xiaoma /root/mcw] test!
#cat test.txt
1 ma
2 chang
3 wei
4 mo
5 jiang

[root@xiaoma /root/mcw] test!
#cat test.txt |head -4|tail -2 >test1.txt

[root@xiaoma /root/mcw] test!
#cat test1.txt
3 wei
4 mo