user2650277 user2650277 - 4 months ago 16
Bash Question

How to hide command output in bash

I want to make my bash scripts more elegant for the end user.How do i hide the output when bash is executing commmands
For e.g
when bash execute

yum install nano


The following will show up to the user who executed the bash

Loaded plugins: fastestmirror
base | 3.7 kB 00:00
base/primary_db | 4.4 MB 00:03
extras | 3.4 kB 00:00
extras/primary_db | 18 kB 00:00
updates | 3.4 kB 00:00
updates/primary_db | 3.8 MB 00:02
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package nano.x86_64 0:2.0.9-7.el6 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
Package Arch Version Repository Size
================================================================================
Installing:
nano x86_64 2.0.9-7.el6 base 436 k

Transaction Summary
================================================================================
Install 1 Package(s)

Total download size: 436 k
Installed size: 1.5 M
Downloading Packages:
nano-2.0.9-7.el6.x86_64.rpm | 436 kB 00:00
warning: rpmts_HdrFromFdno: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
Importing GPG key 0xC105B9DE:
Userid : CentOS-6 Key (CentOS 6 Official Signing Key) <centos-6-key@centos.org>
Package: centos-release-6-4.el6.centos.10.x86_64 (@anaconda-CentOS-201303020151.x86_64/6.4)
From : /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
Installing : nano-2.0.9-7.el6.x86_64 1/1
Verifying : nano-2.0.9-7.el6.x86_64 1/1

Installed:
nano.x86_64 0:2.0.9-7.el6

Complete!


Now i want to hide this from the user and instead show

Installing nano ......


How can i accomplish this task.I will definitely help to make script more user friendly.In case an error occurs then it should be shown to the user.

Edit i would like to know how to show same message while a set of commands are being executed

Answer

Use this.

{
  /your/first/command
  /your/second/command
} &> /dev/null

Explanation

To eliminate output from commands, you have two options:

  • Close the output descriptor file, which keeps it from accepting any more input. That looks like this:

    your_command "Is anybody listening?" >&-
    

    Usually, output goes either to file descriptor 1 (stdout) or 2 (stderr). If you close a file descriptor, you'll have to do so for every numbered descriptor, as &> (below) is a special BASH syntax incompatible with >&-:

    /your/first/command >&- 2>&-
    

    Be careful to note the order: >&- closes stdout, which is what you want to do; &>- redirects stdout and stderr to a file named - (hyphen), which is not what what you want to do. It'll look the same at first, but the latter creates a stray file in your working directory. It's easy to remember: >&2 redirects stdout to descriptor 2 (stderr), >&3 redirects stdout to descriptor 3, and >&- redirects stdout to a dead end (i.e. it closes stdout).

    Also beware that some commands may not handle a closed file descriptor particularly well ("write error: Bad file descriptor"), which is why the better solution may be to...

  • Redirect output to /dev/null, which accepts all output and does nothing with it. It looks like this:

    your_command "Hello?" > /dev/null
    

    For output redirection to a file, you can direct both stdout and stderr to the same place very concisely, but only in bash:

    /your/first/command &> /dev/null
    

Finally, to do the same for a number of commands at once, surround the whole thing in curly braces. Bash treats this as a group of commands, aggregating the output file descriptors so you can redirect all at once. If you're familiar instead with subshells using ( command1; command2; ) syntax, you'll find the braces behave almost exactly the same way, except that unless you involve them in a pipe the braces will not create a subshell and thus will allow you to set variables inside.

{
  /your/first/command
  /your/second/command
} &> /dev/null

See the bash manual on redirections for more details, options, and syntax.

Comments