Linuxexec and file descriptor tips

When I recently read "proficient in unix shell script programming", I saw exec <$1 exec 1> $ OUTFILE. After reading it online for a long time, I finally got it done. The record is as follows.

For Linux, all operations on devices and files are performed using file descriptors.

The file descriptor is a non-negative integer, which is an index value and points to the record table for each process in the kernel to open the file.

When an existing file is opened or a new file is created, the kernel returns a file descriptor to the process. When you need to read and write files, you also need to pass the file descriptor as a parameter to the corresponding function.

Generally, when a process starts, three files are opened: standard input, standard output, and standard error handling. These three files correspond to the file descriptors 0, 1, and 2 respectively, that is, STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO are replaced by macros.

View and set the number of LINUX file descriptors

Ulimit-n

Ulimit-n1024

Instance 1: allocate file descriptors through exec

Exec 3 <> hello.txt # bind to file descriptor "3" in read/write mode"

Echo "hello exec"> & 3 # write "hello exec". If there is content before, it will overwrite it from the beginning of the file.

Echo "hello world"> & 3 # write "hello world", a new row!

Exec 3> &-# disable writing and prohibit writing. However, it actually cannot be read ~

# If it is exec 3 <&-, disable reading, and it cannot write ~

In the example, the hello.txt file is bound to descriptor 3.

Example 2: Redirect standard output

Exec 1> hello.txt # redirects the output to the hello.txt file. From here, the output in this script will be written into the hello.txt file.

Echo "hello exec"

Echo "hello world"

Instance 3: Restore redirection

Exec 100> & 1 # connect file descriptor 100 to standard output

# Since it will be output to the terminal later, we have to use a temporary descriptor to save it!

Exec 1> hello.txt # redirects the output to the hello.txt file. From here, the output in this script will be written into the hello.txt file.

Echo "hello exec"

Echo "hello world"

Exec 1> & 100 100> &-# connect the standard output to 100, which is the previously saved standard output.

# Shut down the descriptor 100 for a hundred times. Because the standard output has been restored, it is unnecessary to keep it.

Echo "oh, my god! "# This sentence will be displayed on the terminal

Example 4: input redirection

Exec 100 <& 0

Exec

Read line1

Echo $ line1

Read line2

Echo $ line2

Exec 0 <& 100 100> &-

Read custom

Example 5: read and write files

#! /Bin/bash

LANG = C

Echo "begin"

OUTFILE = "hello2.txt"

INFILE = "hello.txt"

Function file1

{

> $ OUTFILE # zero out the file, which is equivalent to clearing the file

Exec 4> & 1

Exec 1> $ OUTFILE

While read LINE

Do

Echo "$ LINE"

:

Done <$ INFILE

Exec 1> & 4

Exec 4> &-

}

File1

Echo "end"

Appendix: find and exec

(1) In the current directory (including sub-Directories), find all txt files and find the rows containing the string "bin"

Find./-name "*. txt"-exec grep "bin "{}\;

(2) Delete All txt files in the current directory (including subdirectories)

Find./-name "*. txt"-exec rm {}\;