Linux shell reads the file line by line,

Linux shell reads the file line by line,
Read File lines

• For
  
  • Command replacement
  • Code block redirection
• While
  
  • Pipeline operator
  • Code block redirection

For

IFS=$'\n'for line in `ls -l`do    (( count++ ))doneecho $count

Note:
When reading for, the system automatically uses spaces as the delimiter. Therefore, you need to define IFS as separated by line breaks

ls -l > forout.logmaxlength=$(wc -l < forout.log)for i in `seq $maxlength`do    (( count++ ))done < forout.logecho $count

Note:
Wc requires redirection. Otherwise, the output format is "number of rows file name ".

While

while read line     do        (( count++ ))    done < readFile.log    echo $count

Note:
If the while condition expression does not write read, line is output, but the final result is the same.

    cat readFile.log | while read line    do        #(( count++ ))        echo "$line"    done    #echo $count

Note:
The final output count is a null value. It feels that a pipeline is a sub-process, and the sub-process variables cannot return the parent process.

Time

Ime commandPrint the execution time of a command or program
The time Command prints the time, system time, and time used to execute a command in seconds.Standard Error.
Usage: time [-p] Command [Argument... ]
The time Command result consists of three lines: real, user, and sys. Here we use real values, and the CPU is divided into user and sys.

• The real value indicates the time consumed from the beginning of the program to the end of the program execution, including the CPU time.
• The user value indicates the time used by the program and the Child routines in the library it calls.
• Sys is the time when a system call is executed by a program directly or indirectly.

Test the above four methods
1.
Real 0m0. 022 s
User 0m0. 000 s
Sys 0m0. 004 s

2.
Real 0m0. 066 s
User 0m0. 004 s
Sys 0m0. 016 s

3.
Real 0m0. 001 s
User 0m0. 000 s
Sys 0m0. 000 s

4.
Real 0m0. 015 s
User 0m0. 000 s
Sys 0m0. 008 s

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