Example of bash read and write files in Linux

#!/bin/bash

#读取文件内容到变量中

Filecontent= ' Cat ~/file.txt ' is the TAB key on that
Echo $filecontent

#取得文件内容的每一行

For Fileline in $filecontent
Todo
Echo $fileline
Done

#写内容到文件中

echo $filecontent >> test.txt

Bash built-in UNIX command

.: Executes the program in the current process environment. With source.
. The File:dotunix command reads the commands from file files and executes them.
: null operation, return exit status 0.

Alias: Displays and creates an alias for an existing UNIX command.
BG: Put your homework in the background.
Bind: Displays the binding of the current keyword to the function, or binds the keyword to the ReadLine function or macro.
Break: Jump out of the loop from the most inner layer.
Builtin [Sh-builtin [args]]: runs a built-in Shellunix command and transmits parameters, returning exit status 0. This UNIX command is useful when a function has the same name as a built-in UNIX command.
CD [ARG]: Change the directory, if not with parameters, back to the home directory, with parameters switch to the directory referred to by the parameter.
Command COMAND [ARG]: Executes the UNIX command even if there is a function of the same name. In other words, skip function lookup.
DECLARE [VAR]: Displays all variables, or declares variables with optional attributes.
Dirs: Displays the directory of the current record (the result of the pushd).
Disown: Deletes an active job from the job table.
echo [args]: Displays the args and wraps the line.
Enable: Enables or disables the Unix command that is built into the shell.
eval [args]: Read the args into the shell and execute the generated UNIX command.
EXEC command: Run UNIX command to replace the current shell.
Exit [n]: Exit shell with State N.
Export [var]: Make variables available for quilt identification.
FC: Historical Modification of UNIX commands, for Editing historical UNIX commands.
FG: Put the backstage work on the front desk.
getopts: Resolves and processes Unix command-line options.
Hash: Controls the internal hash table used to speed up Unix command lookups.
Help [command]: Displays useful information about the built-in UNIX commands. If a UNIX command is specified, details of the UNIX command are displayed.
History: Displays a list of Unix command histories with line numbers.
Jobs: Show jobs in the background.
Kill [-signal Process]: sends a signal to a process specified by the PID number or job number. Enter Kill-l to view the list of signals.
Let: used to compute the value of an arithmetic expression and assign the result of an arithmetic operation to a variable.
Local: Use in a function to limit the scope of a variable to within a function.
Logout: Exit login shell.
POPD: Deletes an item from the catalog stack.
PUSHD: Adds an entry to the catalog stack.
PWD: Print out the current working directory.
Read [VAR]: reads a row from the standard input and saves it to the variable var.
ReadOnly [var]: Sets the variable var to read-only and does not allow resetting of the variable.
return [n]: Exit from function, n is the exit state value assigned to the Returnunix command.
Set: Sets options and positional parameters.
Shift [n]: shifts the position parameter to the left N times.
Stop PID: Pauses the operation of the first PID number process.
Suspend: Terminates the current Shell's operation (invalid for the login shell).
Test: Check the file type and evaluate the conditional expression.
Times: Displays the cumulative user time and system time of the running of processes initiated by the current shell.
Trap [ARG] [n]: performs ARG when the shell receives a signal n (n is 0, 1, 2, or 15).
Type [command]: Displays the types of UNIX commands, such as: PWD is a built-in UNIX command for the shell.
Typeset: With declare. Set variables and give them properties.
Ulimit: Displays or sets the maximum amount of resources available for the process.
Umask [octal number]: User file creation pattern masks for owners, groups, and other users.
Unalias: Cancels all Unix command alias settings.
unset [name]: Cancels the definition of the value or function of the specified variable.
Wait [pid#n]: The background process waits for the PID number N to end and reports its end state.