The strace command in Linux lets you trace system calls and signals. Or in other words: it allows you to see what a program or process is doing.
Following is its syntax:
strace [OPTIONS] command
In the simplest case, strace runs the specified command until it exits. It intercepts and records the system calls which are called by a process and the signals which are received by a process. The name of each system call, its arguments and its return value are printed on standard error or to the file specified with the -o option. For example:
# strace ls
If a process is already running, you can trace it by simply passing its PID as follows; this will fill your screen with continued output that shows system calls being made by the process. To end it, press [Ctrl + C].
# strace -p 3569
Threads are a popular programming abstraction for parallel execution on modern operating systems. When threads are forked inside a program for multiple flows of execution, these threads share certain resources (e.g., memory address space, open files) among themselves to minimize forking overhead and avoid expensive IPC (inter-process communication) channel. These properties make threads an efficient mechanism for concurrent execution.
In Linux, threads (also called Lightweight Processes (LWP)) created within a program will have the same "thread group ID" as the program's PID. Each thread will then have its own thread ID (TID). To the Linux kernel's scheduler, threads are nothing more than standard processes which happen to share certain resources. Classic command-line tools such as ps or top, which display process-level information by default, can be instructed to display thread-level information.
Here are several ways to show threads for a process on Linux:
Using the ps command
The "-T" option for the ps command enables thread views. The following command list all threads created by a process with :
# ps -T -p <pid>
For example to list the threads for the following java process:
# ps -ef | grep 97947
deploy 97947 97942 1 00:51 ? 00:13:51 java
Run the following command:
# ps -T -p 97947
PID SPID TTY TIME CMD
97947 97947 ? 00:00:00 java
97947 97948 ? 00:00:00 java
97947 97949 ? 00:00:00 java
The "SID" column represents thread IDs, and "CMD" column shows thread names.
Using the top command
The top command can show a real-time view of individual threads. To enable thread views in the top output, invoke top with "-H" option. This will list all Linux threads. You can also toggle on or off thread view mode while top is running, by pressing 'H' key.
top - 14:43:25 up 6 days, 5:40, 2 users, load average: 0.87, 0.33, 0.22
Threads: 684 total, 1 running, 683 sleeping, 0 stopped, 0 zombie
%Cpu(s): 6.3 us, 4.0 sy, 0.0 ni, 89.6 id, 0.1 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 7910136 total, 384812 free, 1603096 used, 5922228 buff/cache
KiB Swap: 8388604 total, 8239100 free, 149504 used. 5514264 avail Mem
Note how in the example above the number of threads on the system is listed.
To restrict the top output to a particular process and check all threads running inside the process:
# top -H -p <pid>
Using htop
A more user-friendly way to view threads per process is via htop, an ncurses-based interactive process viewer. This program allows you to monitor individual threads in tree views.
To enable thread views in htop, launch htop, and press F2 to enter htop setup menu. Choose "Display option" under "Setup" column, and toggle on "Tree view" and "Show custom thread names" options. Presss F10 to exit the setup.
If you have a RHEL system that boots in emergency mode due to a corrupt file system, here are some steps to perform to resolve the issue.
Once booted in emergency mode, connect to the system on the console. Try running:
# journalctl -xe
To help determine why the system booted in emergency mode. For example, you may discover that the /var file system has an issue.
Once you do, shut down the system, and boot it from the boot ISO or DVD (depending if the system is a virtual or a physical system).
When using VMware, you'll have to edit the settings of the VM, and enable the "Force BIOS setup" setting, and mount/connect the boot ISO image. Then start it up, and open the console. Once in the BIOS, make sure CDROM is high in the boot order.
Once the system is booting RHEL, select "Troubleshooting", and "Rescue a Red Hat Enterprise Linux system". The system will boot up. Select "Continue" when prompted.
Run "df -h" to list the mounted directories. Since /var is the file system having the issue, unmount it:
# unmount /mnt/sysimage/var
Once /var is unmounted, run fsck to fix any issues:
# fsck /mnt/sysimage/var
Once completed, you can reboot the system. If you connected an ISO image earlier, make sure to disconnect the ISO image.
The "realm discover" command returns complete domain configuration and a list of packages that must be installed for the system to enrolled in the domain.
The "realm join" command then sets up the local machine for use with a specified domain by configuring both the local system services and the entries in the identity domain. the process run by "realm join" follows these steps:
- Running a discovery scan for the specified domain.
- Automatic installation of the packages required to join the system to the domain.
This includes SSSD and the PAM home directory job packages. Note that the automatic installation of packages requires the PackageKit suite to be running. If PackageKit is disabled, the system prompts you for the missing packages, and you will be required to install them manually using the "yum" utility.
- Joining the domain by creating an account entry for the system in the directory.
- Creating the /etc/krb5.keytab host keytab file.
- Configuring the domain in SSSD and restarting the service.
- Enabling domain users for the system services in PAM configuration and the /etc/nsswicht.conf file.
To install the software manually, run:
# yum install realmd oddjob oddjob-mkhomedir sssd
# yum install adcli krb5-workstation samba-common-tools
When run without any options, the "realm discover" command displays information about the default DNS domain. It is also possible to run a discovery for a specific domain, such as:
# realm discover mydomain.local
Before this works, make sure the system can access the domain controllers, such as the AD servers. You may have to add those to /etc/resolv.conf, as realmd will use DNS SRV lookups to find the domain controllers in the domain automatically.
To join the domain use the "realm join" command, for example:
# realm join -v - U user mydomain.local
By default, the join is performed as the domain administrator. For AD, the administrator is called "Administrator"; for IdM, it is called "admin". To connect as a different user, use the -U option, as was shown in the example above. When prompted for a password, type it in.
Once the join has been completed, use a separate user account to login to the system, to ensure that domain accounts work.
If you run into issue, make sure that the following ports are opened in the firewall, if present: 53, 389, 636, 88, 464, 3268, 3269 and 123.
For more details on joining a RHEL system to a domain, see:
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/windows_integration_guide/realmd-domain.
In case you've ever forgotten the root password of a RHEL 7 system, here's a good description of how to change it:
https://access.redhat.com/solutions/918283
If you need to remove a non-root LVM based file system, it can be removed using the following steps. If the file system contains any data, then please make sure to take a backup of that data before proceeding with the following steps.
In this example, we will be deleting "u01lv" from volume group "oracle". The logical volume is mounted on mount point /u01.
# df /u01 | grep u01
/dev/ampper/oracle/u01lv 209612800 80946812 128665988 39% /u01
# lvs | grep u01
u01lv oracle -wi-ao---- 200.00g
As you can see, mount point /u01 is define don logical volume u01lv, and is 200 GB.
Start off, by deleting the corresponding entry in /etc/fstab.
# grep u01 /etc/fstab
/dev/oracle/u01lv /u01 xfs defaults,noatime 0 0
Then unmount the file system:
# umount /u01
Disable the logical volume:
# lvchange -an /dev/oracle/u01lv
Delete the logical volume:
# lvremove /dev/oracle/u01lv
At this point, depending if this logical volume is defined on a disk within a volume group, you can, if the disk is now empty after removing the logical volume, also remove the disk from the volume group. First run the "pvs" command and check for an empty disk, such as:
# pvs
PV VG Fmt Attr PSize PFree
/dev/sdb oracle lvm2 a-- <5.00t <5.00t
If the disk is empty, as seen as disk /dev/sdb in the example above, you can then remove it from the volume group:
# vgreduce oracle /dev/sdb
Or, if the logical volume removed from the volume group was the very last logical volume to remove, and the volume group is empty, you can also remove the volume group. Do so by disabling the volume group and removing it:
# vgchange -an oracle
# vgremove oracle
And then finally remove any physical volumes used, such as /dev/sdb:
# pvremove /dev/sdb
XRDP is an Open Source Remote Desktop Protocol server, very similar to what is used on Windows Server systems, but XRDP is meant for Linux. Once installed on Linux, you can set up a RDP (or Remote Desktop Connection) session from a Windows system directly to a Linux system.
Here's how you install and configure it on RHEL or CentOS 7:
First of all, we need to install the EPEL repository and XRDP server:
# yum -y install https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
# yum -y install xrdp
Next, we need to start and enable the service:
# systemctl start xrdp.service
# systemctl enable xrdp.service
To check if its running, run:
# netstat -an | grep 3389
tcp 0 0 0.0.0.0:3389 0.0.0.0:* LISTEN
That's all. Now you can connect to your server from any Windows machine using RDP.
The first time a user logs into the default desktop (Gnome) for Red Hat version 7 based systems, they're prompted to set a language, add online accounts, and dropped into a help menu right from the start. While this might be nice for brand new users, it's certainly not ideal for everyone.
There is a very simple way to prevent this annoyance, by simple removing package gnome-initial-setup:
# yum -y erase gnome-inital-setup
If you've ever looked at the /tmp file system on a RHEL system, you may have noticed that it is, by default, simply a folder in the root directory.
For example:
# df -h /tmp
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/centos-root 100G 4.6G 96G 5% /
The risk of having this is, that anyone can fill up the root file system, by writing temporary data to the /tmp folder, which is risky for system stability.
Red Hat Enterprise Linux 7 offers the ability to use /tmp as a mount point for a temporary file storage system (tmpfs), but unfortunately, it is not enabled by default.
When enabled, this temporary storage appears as a mounted file system, but stores its content in volatile memory instead of on a persistent storage device. And when using this, no files in /tmp are stored on the hard drive except when memory is low, in which case swap space is used. This also means that the contents of /tmp are not persisted across a reboot.
To enable this feature, execute the following commands:
# systemctl enable tmp.mount
# systemctl start tmp.mount
RHEL uses a default size of half the memory size for the in-memory /tmp file system. For example on a system with 16 GB of memory, an 8 GB /tmp file system is set up after enabling the tmp.mount feature:
# df -h /tmp
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/centos-root 100G 53G 48G 53% /
# systemctl enable tmp.mount
# systemctl start tmp.mount
# df -h /tmp
Filesystem Size Used Avail Use% Mounted on
tmpfs 7.8G 0 7.8G 0% /tmp
By having this in place, it's no longer possible to fill up the root file system, when writing files and/or data to the /tmp file system. The downside, however, is that this uses memory, and when filling up the memory, may be using the swap space. As such, having a dedicated file system on disk for the /tmp folder is still the better solution.
The following command will generate a random password of 9 characters:
# openssl rand -base64 9
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