1.Tenth Line
How would you print just the 10th line of a file?
For example, assume that file.txt has the following content:
Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10
Your script should output the tenth line, which is:
Line 10
-------------------
# Read from the file file.txt and output the tenth line to stdout. #Solution One: #head -n 10 file.txt | tail -n +10 #Solution Two: #awk 'NR==10' file.txt #Solution Three: sed -n 10p file.txt
涉及知识点：
－>head 用来显示档案的开头至标准输出中，默认head命令打印其相应文件的开头10行。
语法格式：head [参数]... [文件]...
命令参数：
-q 隐藏文件名
-v 显示文件名
-c<字节> 显示字节数
-n<行数> 显示的行数
－>tail命令用于显示指定文件末尾内容，不指定文件时，作为输入信息进行处理。常用查看日志文件。
命令参数：
-f 循环读取
-q 不显示处理信息
-v 显示详细的处理信息
-c<数目> 显示的字节数
-n<行数> 显示行数
--pid=PID 与-f合用,表示在进程ID,PID死掉之后结束.
-q, --quiet, --silent 从不输出给出文件名的首部
-s, --sleep-interval=S 与-f合用,表示在每次反复的间隔休眠S秒
可参考：我使用过的Linux命令之tail - 输出文件尾部/动态监视文件尾部
->awk是一个强大的文本分析工具，相对于grep的查找，sed的编辑，awk在其对数据分析并生成报告时，显得尤为强大。简单来说awk就是把文件逐行的读入，以空格为默认分隔符将每行切片，切开的部分再进行各种分析处理。
语法格式：
awk '{pattern + action}' {filenames}
pattern 表示 AWK 在数据中查找的内容，而 action 是在找到匹配内容时所执行的一系列命令
可参考：linux awk命令详解
->sed 是一种在线编辑器，它一次处理一行内容。处理时，把当前处理的行存储在临时缓冲区中，称为“模式空间”（pattern space），接着用sed命令处理缓冲区中的内容，处理完成后，把缓冲区的内容送往屏幕。接着处理下一行，这样不断重复，直到文件末尾。
语法格式：
sed [-hnV][-e<script>][-f<script文件>][文本文件]
2.Transpose File
Given a text file file.txt, transpose its content.
You may assume that each row has the same number of columns and each field is separated by the ' ' character.
For example, if file.txt has the following content:
name age
alice 21
ryan 30
Output the following:
name alice ryan
age 21 30
－－－－－－－－－
# Read from the file file.txt and print its transposed content to stdout.
# using awk for this purpose
awk '
{
for(i=1; i<=NF; i++)
{
if(line[i] == "")
{
line[i] = $i
}
else
{
line[i] = line[i]" "$i
}
}
}
END{
for(i=1; i<=NF; i++)
{
print line[i]
}
}
' file.txt
如果The number of columns is two.则可以用以下方法：
test2
name age alice 21 ryan 30
solution：
MindeMacBook-Pro:闲杂笔记 minzhu$ cut -d " " -f1 test2 |xargs name alice ryan MindeMacBook-Pro:闲杂笔记 minzhu$ cut -d " " -f2 test2 |xargs age 21 30
3.Valid Phone Numbers
Given a text file file.txt that contains list of phone numbers (one per line), write a one liner bash script to print all valid phone numbers.
You may assume that a valid phone number must appear in one of the following two formats: (xxx) xxx-xxxx or xxx-xxx-xxxx. (x means a digit)
You may also assume each line in the text file must not contain leading or trailing white spaces.
For example, assume that file.txt has the following content:
987-123-4567 123 456 7890 (123) 456-7890
Your script should output the following valid phone numbers:
987-123-4567 (123) 456-7890
------------
file.txt
987-123-4567 123 456 7890 (123) 456-7890
solution1:
grep -e '(^[0-9]{3}-[0-9]{3}-[0-9]{4}$)' -e '(^([0-9]{3})[ ]{1}[0-9]{3}-([0-9]{4})$)' file.txt
explanation:
- In Bash, we use
to escape next one trailing character; ^is used to denote the beginning of a line$is used to denote the end of a line{M}is used to denote to match exactlyMtimes of the previous occurence/regex(...)is used to group pattern/regex together
Back to this problem: it requires us to match two patterns, for better readability, I used -e and separate the two patterns into two regexes, the first one matches this case: xxx-xxx-xxxx and the second one matches this case: (xxx) xxx-xxxx
solution2:
awk < file.txt '/^[0-9][0-9][0-9]-[0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]$/ || /^([0-9][0-9][0-9]) [0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]$/ {print}'
The format for 'awk':
awk < file 'pattern {action}'
or
awk 'pattern {action}' file
Note: 'print' action without any arguments means print out the whole line.
4.Word Frequency
Write a bash script to calculate the frequency of each word in a text file words.txt.
For simplicity sake, you may assume:
words.txtcontains only lowercase characters and space' 'characters.- Each word must consist of lowercase characters only.
- Words are separated by one or more whitespace characters.
For example, assume that words.txt has the following content:
the day is sunny the the the sunny is is
Your script should output the following, sorted by descending frequency:
the 4 is 3 sunny 2 day 1
-----------------
words.txt
the day is sunny the the the sunny is is
solution1:
awk '{for(i=1;i<=NF;i++) a[$i]++} END {for(k in a) print k,a[k]}' words.txt | sort -k2 -nr
solution2:
sed 's/^s+//g; s/s+/ /g; s/s+$//g' words.txt | tr ' ' '
' | sort | uniq -c | sort -nr | awk -F' ' '{print $2" "$1}'
- use sed to strip head & tail spaces，and change inline spaces to one space
- use tr to trans space to return (these two steps also can be done cat words.txt | tr -s ' ' ' ')
- sort the words
- uniq to count words
- sort the stats result，-n for numeric sort，-r for reverse
- use awk to format the output
参考：leetcode