2>&1的含义解释

某次想将adb的help信息保存起来

F:\android系统\AndroidTool_Release_v2.\bin>adb >help.txt

结果

F:\android系统\AndroidTool_Release_v2.\bin>adb >help.txt
Android Debug Bridge version 1.0.

 -a                            - directs adb to listen on all interfaces for a c
onnection
 -d                            - directs command to the only connected USB devic
e
                                 returns an error if more than one USB device is
 present.
 -e                            - directs command to the only running emulator.
                                 returns an error if more than one emulator is r
unning.
 -s <specific device>          - directs command to the device or emulator with
the given
                                 serial number or qualifier. Overrides ANDROID_S
ERIAL
                                 environment variable.
 -p <product name or path>     - simple product name like 'sooner', or
                                 a relative/absolute path to a product
                                 out directory like 'out/target/product/sooner'.

                                 If -p is not specified, the ANDROID_PRODUCT_OUT

                                 environment variable is used, which must
                                 be an absolute path.
 -H                            - Name of adb server host (default: localhost)
 -P                            - Port of adb server (default: )
 devices [-l]                  - list all connected devices
                                 ('-l' will also list device qualifiers)
 connect <host>[:<port>]       - connect to a device via TCP/IP
                                 Port  is used by default if no port number
is specified.
 disconnect [<host>[:<port>]]  - disconnect from a TCP/IP device.
                                 Port  is used by default if no port number
is specified.
                                 Using this command with no additional arguments

                                 will disconnect from all connected TCP/IP devic
es.

device commands:
  adb push <local> <remote>    - copy file/dir to device
  adb pull <remote> [<local>]  - copy file/dir from device
  adb sync [ <directory> ]     - copy host->device only if changed
                                 (-l means list but don't copy)
                                 (see 'adb help all')
  adb shell                    - run remote shell interactively
  adb shell <command>          - run remote shell command
  adb emu <command>            - run emulator console command
  adb logcat [ <filter-spec> ] - View device log
  adb forward --list           - list all forward socket connections.
                                 the format is a list of lines with the followin
g format:
                                    <serial> " " <local> " " <remote> "\n"
  adb forward <local> <remote> - forward socket connections
                                 forward specs are one of:
                                   tcp:<port>
                                   localabstract:<unix domain socket name>
                                   localreserved:<unix domain socket name>
                                   localfilesystem:<unix domain socket name>
                                   dev:<character device name>
                                   jdwp:<process pid> (remote only)
  adb forward --no-rebind <local> <remote>
                               - same as 'adb forward <local> <remote>' but fail
s
                                 if <local> is already forwarded
  adb forward --remove <local> - remove a specific forward socket connection
  adb forward --remove-all     - remove all forward socket connections
  adb jdwp                     - list PIDs of processes hosting a JDWP transport

  adb install [-l] [-r] [-s] [--algo <algorithm name> --key <hex-encoded key> --
iv <hex-encoded iv>] <file>
                               - push this package file to the device and instal
l it
                                 ('-l' means forward-lock the app)
                                 ('-r' means reinstall the app, keeping its data
)
                                 ('-s' means install on SD card instead of inter
nal storage)
                                 ('--algo', '--key', and '--iv' mean the file is
 encrypted already)
  adb uninstall [-k] <package> - remove this app package from the device
                                 ('-k' means keep the data and cache directories
)
  adb bugreport                - return all information from the device
                                 that should be included in a bug report.

  adb backup [-f <file>] [-apk|-noapk] [-obb|-noobb] [-shared|-noshared] [-all]
[-system|-nosystem] [<packages...>]
                               - write an archive of the device's data to <file>
.
                                 If no -f option is supplied then the data is wr
itten
                                 to "backup.ab" in the current directory.
                                 (-apk|-noapk enable/disable backup of the .apks
 themselves
                                    in the archive; the default is noapk.)
                                 (-obb|-noobb enable/disable backup of any insta
lled apk expansion
                                    (aka .obb) files associated with each applic
ation; the default
                                    is noobb.)
                                 (-shared|-noshared enable/disable backup of the
 device's
                                    shared storage / SD card contents; the defau
lt is noshared.)
                                 (-all means to back up all installed applicatio
ns)
                                 (-system|-nosystem toggles whether -all automat
ically includes
                                    system applications; the default is to inclu
de system apps)
                                 (<packages...> is the list of applications to b
e backed up.  If
                                    the -all or -shared flags are passed, then t
he package
                                    list is optional.  Applications explicitly g
iven on the
                                    command line will be included even if -nosys
tem would
                                    ordinarily cause them to be omitted.)

  adb restore <file>           - restore device contents from the <file> backup
archive

  adb help                     - show this help message
  adb version                  - show version num

scripting:
  adb wait-for-device          - block until device is online
  adb start-server             - ensure that there is a server running
  adb kill-server              - kill the server if it is running
  adb get-state                - prints: offline | bootloader | device
  adb get-serialno             - prints: <serial-number>
  adb get-devpath              - prints: <device-path>
  adb status-window            - continuously print device status for a specifie
d device
  adb remount                  - remounts the /system partition on the device re
ad-write
  adb reboot [bootloader|recovery] - reboots the device, optionally into the boo
tloader or recovery program
  adb reboot-bootloader        - reboots the device into the bootloader
  adb root                     - restarts the adbd daemon with root permissions
  adb usb                      - restarts the adbd daemon listening on USB
  adb tcpip <port>             - restarts the adbd daemon listening on TCP on th
e specified port
networking:
  adb ppp <tty> [parameters]   - Run PPP over USB.
 Note: you should not automatically start a PPP connection.
 <tty> refers to the tty for PPP stream. Eg. dev:/dev/omap_csmi_tty1
 [parameters] - Eg. defaultroute debug dump local notty usepeerdns

adb sync notes: adb sync [ <directory> ]
  <localdir> can be interpreted in several ways:

  - If <directory> is not specified, both /system and /data partitions will be u
pdated.

  - If it is "system" or "data", only the corresponding partition
    is updated.

environmental variables:
  ADB_TRACE                    - Print debug information. A comma separated list
 of the following values
                                  or all, adb, sockets, packets, rwx, usb, sync
, sysdeps, transport, jdwp
  ANDROID_SERIAL               - The serial number to connect to. -s takes prior
ity over this if given.
  ANDROID_LOG_TAGS             - When used with the logcat option, only these de
bug tags are printed.

help.txt是空的。。。

那么我们怎么想在help.txt有输出呢

F:\android系统\AndroidTool_Release_v2.\bin>adb >help.txt >&

现在

我们的help.txt中有

Android Debug Bridge version 1.0.

 -a                            - directs adb to listen on all interfaces for a connection
 -d                            - directs command to the only connected USB device
                                 returns an error if more than one USB device is present.
 -e                            - directs command to the only running emulator.
                                 returns an error if more than one emulator is running.
 -s <specific device>          - directs command to the device or emulator with the given
                                 serial number or qualifier. Overrides ANDROID_SERIAL
                                 environment variable.
 -p <product name or path>     - simple product name like 'sooner', or
                                 a relative/absolute path to a product
                                 out directory like 'out/target/product/sooner'.
                                 If -p is not specified, the ANDROID_PRODUCT_OUT
                                 environment variable is used, which must
                                 be an absolute path.
 -H                            - Name of adb server host (default: localhost)
 -P                            - Port of adb server (default: )
 devices [-l]                  - list all connected devices
                                 ('-l' will also list device qualifiers)
 connect <host>[:<port>]       - connect to a device via TCP/IP
                                 Port  is used by default if no port number is specified.
 disconnect [<host>[:<port>]]  - disconnect from a TCP/IP device.
                                 Port  is used by default if no port number is specified.
                                 Using this command with no additional arguments
                                 will disconnect from all connected TCP/IP devices.

device commands:
  adb push <local> <remote>    - copy file/dir to device
  adb pull <remote> [<local>]  - copy file/dir from device
  adb sync [ <directory> ]     - copy host->device only if changed
                                 (-l means list but don't copy)
                                 (see 'adb help all')
  adb shell                    - run remote shell interactively
  adb shell <command>          - run remote shell command
  adb emu <command>            - run emulator console command
  adb logcat [ <filter-spec> ] - View device log
  adb forward --list           - list all forward socket connections.
                                 the format is a list of lines with the following format:
                                    <serial> " " <local> " " <remote> "\n"
  adb forward <local> <remote> - forward socket connections
                                 forward specs are one of:
                                   tcp:<port>
                                   localabstract:<unix domain socket name>
                                   localreserved:<unix domain socket name>
                                   localfilesystem:<unix domain socket name>
                                   dev:<character device name>
                                   jdwp:<process pid> (remote only)
  adb forward --no-rebind <local> <remote>
                               - same as 'adb forward <local> <remote>' but fails
                                 if <local> is already forwarded
  adb forward --remove <local> - remove a specific forward socket connection
  adb forward --remove-all     - remove all forward socket connections
  adb jdwp                     - list PIDs of processes hosting a JDWP transport
  adb install [-l] [-r] [-s] [--algo <algorithm name> --key <hex-encoded key> --iv <hex-encoded iv>] <file>
                               - push this package file to the device and install it
                                 ('-l' means forward-lock the app)
                                 ('-r' means reinstall the app, keeping its data)
                                 ('-s' means install on SD card instead of internal storage)
                                 ('--algo', '--key', and '--iv' mean the file is encrypted already)
  adb uninstall [-k] <package> - remove this app package from the device
                                 ('-k' means keep the data and cache directories)
  adb bugreport                - return all information from the device
                                 that should be included in a bug report.

  adb backup [-f <file>] [-apk|-noapk] [-obb|-noobb] [-shared|-noshared] [-all] [-system|-nosystem] [<packages...>]
                               - write an archive of the device's data to <file>.
                                 If no -f option is supplied then the data is written
                                 to "backup.ab" in the current directory.
                                 (-apk|-noapk enable/disable backup of the .apks themselves
                                    in the archive; the default is noapk.)
                                 (-obb|-noobb enable/disable backup of any installed apk expansion
                                    (aka .obb) files associated with each application; the default
                                    is noobb.)
                                 (-shared|-noshared enable/disable backup of the device's
                                    shared storage / SD card contents; the default is noshared.)
                                 (-all means to back up all installed applications)
                                 (-system|-nosystem toggles whether -all automatically includes
                                    system applications; the default is to include system apps)
                                 (<packages...> is the list of applications to be backed up.  If
                                    the -all or -shared flags are passed, then the package
                                    list is optional.  Applications explicitly given on the
                                    command line will be included even if -nosystem would
                                    ordinarily cause them to be omitted.)

  adb restore <file>           - restore device contents from the <file> backup archive

  adb help                     - show this help message
  adb version                  - show version num

scripting:
  adb wait-for-device          - block until device is online
  adb start-server             - ensure that there is a server running
  adb kill-server              - kill the server if it is running
  adb get-state                - prints: offline | bootloader | device
  adb get-serialno             - prints: <serial-number>
  adb get-devpath              - prints: <device-path>
  adb status-window            - continuously print device status for a specified device
  adb remount                  - remounts the /system partition on the device read-write
  adb reboot [bootloader|recovery] - reboots the device, optionally into the bootloader or recovery program
  adb reboot-bootloader        - reboots the device into the bootloader
  adb root                     - restarts the adbd daemon with root permissions
  adb usb                      - restarts the adbd daemon listening on USB
  adb tcpip <port>             - restarts the adbd daemon listening on TCP on the specified port
networking:
  adb ppp <tty> [parameters]   - Run PPP over USB.
 Note: you should not automatically start a PPP connection.
 <tty> refers to the tty for PPP stream. Eg. dev:/dev/omap_csmi_tty1
 [parameters] - Eg. defaultroute debug dump local notty usepeerdns

adb sync notes: adb sync [ <directory> ]
  <localdir> can be interpreted in several ways:

  - If <directory> is not specified, both /system and /data partitions will be updated.

  - If it is "system" or "data", only the corresponding partition
    is updated.

environmental variables:
  ADB_TRACE                    - Print debug information. A comma separated list of the following values
                                  or all, adb, sockets, packets, rwx, usb, sync, sysdeps, transport, jdwp
  ANDROID_SERIAL               - The serial number to connect to. -s takes priority over this if given.
  ANDROID_LOG_TAGS             - When used with the logcat option, only these debug tags are printed.

关于2>&1的含义：将标准错误输出重定向到标准输出

其他扩展阅读

https://blog.csdn.net/zhaominpro/article/details/82630528

https://yanbin.blog/linux-input-output-redirection/