linux - Read For Learn

Is there a way to see the execution tree of systemd?

systemd-analyze is your friend. For example systemd-analyze critical-chain outputs blocking tree of daemons. Mine for example: graphical.target @20.211s └─multi-user.target @20.211s └─nginx.service @19.348s +862ms └─network.target @19.347s └─NetworkManager.service @10.315s +9.031s └─basic.target @10.312s └─timers.target @10.311s └─systemd-tmpfiles-clean.timer @10.311s └─sysinit.target @10.295s └─systemd-update-utmp.service @10.167s +127ms └─systemd-tmpfiles-setup.service @10.124s +41ms └─local-fs.target @10.097s └─home-entd-Downloads.mount @10.093s +2ms └─home.mount @9.326s +672ms └─[email protected] @8.472s +696ms └─dev-sda6.device @8.471s … Read more

GPG does not have enough entropy

Have you had a look at RNG? Fedora/Rh/Centos types: sudo yum install rng-tools On deb types: sudo apt-get install rng-tools to set it up. Then run sudo rngd -r /dev/urandom before generating the keys. Reference: http://it.toolbox.com/blogs/lim/how-to-generate-enough-entropy-for-gpg-key-generation-process-on-fedora-linux-38022

How to disable everything in crontab -l?

crontab -e then comment out each line you don’t want to run with #.

How to do the port forwarding from one ip to another ip in same network?

These rules should work, assuming that iptables is running on server 192.168.12.87 : #!/bin/sh echo 1 > /proc/sys/net/ipv4/ip_forward iptables -F iptables -t nat -F iptables -X iptables -t nat -A PREROUTING -p tcp –dport 80 -j DNAT –to-destination 192.168.12.77:80 iptables -t nat -A POSTROUTING -p tcp -d 192.168.12.77 –dport 80 -j SNAT –to-source 192.168.12.87 You … Read more

I have a keypair. How do I determine the key length?

openssl rsa -in private.key -text -noout The top line of the output will display the key size. For example: Private-Key: (2048 bit) To view the key size from a certificate: $ openssl x509 -in public.pem -text -noout | grep “RSA Public Key” RSA Public Key: (2048 bit)

Should I install Linux applications in /var or /opt?

The standard for these issues is the Filesystem Hierarchy Standard. It’s a rather big document. Basically (and very roughly), the standard paths on Linux are: /bin & /sbin are for vital programs for the OS, sbin being for administrators only ; /usr/bin & /usr/sbin are for not vital programs, sbin being for administrators only ; … Read more

How can I get processor/RAM/disk specs from the Linux command Line? [duplicate]

CPU $ cat /proc/cpuinfo Memory : $ free $ cat /proc/meminfo HDD: $ df -h $ sudo fdisk -l $ hdparm -i /dev/device (for example sda1, hda3…)

Can I send some text to the STDIN of an active process running in a screen session?

This answer doesn’t solve the problem, but it’s left here because 30+ people found it useful, otherwise I would have deleted it long time ago. Write to /proc/*pid of the program*/fd/0. The fd subdirectory contains the descriptors of all the opened files and file descriptor 0 is the standard input (1 is stdout and 2 … Read more

How to force nginx to resolve DNS (of a dynamic hostname) everytime when doing proxy_pass?

It’s an intriguing question and AFAIK that’s not going to work well. You can try to use the upstream module and use the directives for failover to see if it works as a hack. 2018 edit: a lot of things changed. Check the answer by @ohaal to get real information about this.

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