unix linux

cat command

I am a new Linux and Unix system user. How do I use cat command on Linux or Unix-like operating systems? Can you provide basic examples and syntax usage for cat command?

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I‘ve recently regenrated and uploaded a new public ssh-key using the ssh-copy-id command. Now I’m getting the following error:

ssh root@server3
Agent admitted failure to sign using the key.
root@server3’s password:

How do I fix this problem?

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How do I count and print particular character (say D or digit 7) in a string or variable under Bash UNIX / Linux shell?

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How do I stop a process under Ubuntu Linux using command line and GUI tools?

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How do I see if my friends or coworkers are logged into the same Linux or BSD or UNIX (AIX, HP-UX, Apple OS X) server / workstation as I am from a command line?

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How do I kill all users session under Linux or UNIX operating system using command prompt?

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How do I convert from octal to hex number using bc UNIX / Linux command line utility?

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