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20 Linux Commands Interview Questions & Answers
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================================================================================
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**Q:1 How to check current run level of a linux server ?**
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Ans: ‘who -r’ & ‘runlevel’ commands are used to check the current runlevel of a linux box.
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**Q:2 How to check the default gatway in linux ?**
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Ans: Using the commands “route -n” and “netstat -nr” , we can check default gateway. Apart from the default gateway info , these commands also display the current routing tables .
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**Q:3 How to rebuild initrd image file on Linux ?**
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Ans: In case of CentOS 5.X / RHEL 5.X , mkinitrd command is used to create initrd file , example is shown below :
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# mkinitrd -f -v /boot/initrd-$(uname -r).img $(uname -r)
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If you want to create initrd for a specific kernel version , then replace ‘uname -r’ with desired kernel
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In Case of CentOS 6.X / RHEL 6.X , dracut command is used to create initrd file example is shown below :
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# dracut -f
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Above command will create the initrd file for the current version. To rebuild the initrd file for a specific kernel , use below command :
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# dracut -f initramfs-2.x.xx-xx.el6.x86_64.img 2.x.xx-xx.el6.x86_64
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**Q:4 What is cpio command ?**
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Ans: cpio stands for Copy in and copy out. Cpio copies files, lists and extract files to and from a archive ( or a single file).
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**Q:5 What is patch command and where to use it ?**
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Ans: As the name suggest patch command is used to apply changes ( or patches) to the text file. Patch command generally accept output from the diff and convert older version of files into newer versions. For example Linux kernel source code consists of number of files with millions of lines , so whenever any contributor contribute the changes , then he/she will be send the only changes instead of sending the whole source code. Then the receiver will apply the changes with patch command to its original source code.
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Create a diff file for use with patch,
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# diff -Naur old_file new_file > diff_file
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Where old_file and new_file are either single files or directories containing files. The r option supports recursion of a directory tree.
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Once the diff file has been created, we can apply it to patch the old file into the new file:
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# patch < diff_file
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**Q:6 What is use of aspell ?**
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Ans: As the name suggest aspell is an interactive spelling checker in linux operating system. The aspell command is the successor to an earlier program named ispell, and can be used, for the most part, as a drop-in replacement. While the aspell program is mostly used by other programs that require spell-checking capability, it can also be used very effectively as a stand-alone tool from the command line.
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**Q:7 How to check the SPF record of domain from command line ?**
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Ans: We can check SPF record of a domain using dig command. Example is shown below :
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linuxtechi@localhost:~$ dig -t TXT google.com
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**Q:8 How to identify which package the specified file (/etc/fstab) is associated with in linux ?**
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Ans: # rpm -qf /etc/fstab
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Above command will list the package which provides file “/etc/fstab”
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**Q:9 Which command is used to check the status of bond0 ?**
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Ans: cat /proc/net/bonding/bond0
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**Q:10 What is the use of /proc file system in linux ?**
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Ans: The /proc file system is a RAM based file system which maintains information about the current state of the running kernel including details on CPU, memory, partitioning, interrupts, I/O addresses, DMA channels, and running processes. This file system is represented by various files which do not actually store the information, they point to the information in the memory. The /proc file system is maintained automatically by the system.
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**Q:11 How to find files larger than 10MB in size in /usr directory ?**
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Ans: # find /usr -size +10M
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**Q:12 How to find files in the /home directory that were modified more than 120 days ago ?**
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Ans: # find /home -mtime +l20
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**Q:13 How to find files in the /var directory that have not been accessed in the last 90 days ?**
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Ans: # find /var -atime -90
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**Q:14 Search for core files in the entire directory tree and delete them as found without prompting for confirmation**
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Ans: # find / -name core -exec rm {} \;
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**Q:15 What is the purpose of strings command ?**
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Ans: The strings command is used to extract and display the legible contents of a non-text file.
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**Q:16 What is the use tee filter ?**
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Ans: The tee filter is used to send an output to more than one destination. It can send one copy of the output to a file and another to the screen (or some other program) if used with pipe.
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linuxtechi@localhost:~$ ll /etc | nl | tee /tmp/ll.out
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In the above example, the output from ll is numbered and captured in /tmp/ll.out file. The output is also displayed on the screen.
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**Q:17 What would the command export PS1 = ”$LOGNAME@`hostname`:\$PWD: do ?**
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Ans: The export command provided will change the login prompt to display username, hostname, and the current working directory.
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**Q:18 What would the command ll | awk ‘{print $3,”owns”,$9}’ do ?**
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Ans: The ll command provided will display file names and their owners.
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**Q:19 What is the use of at command in linux ?**
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Ans: The at command is used to schedule a one-time execution of a program in the future. All submitted jobs are spooled in the /var/spool/at directory and executed by the atd daemon when the scheduled time arrives.
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**Q:20 What is the role of lspci command in linux ?**
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Ans: The lspci command displays information about PCI buses and the devices attached to your system. Specify -v, -vv, or -vvv for detailed output. With the -m option, the command produces more legible output.
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--------------------------------------------------------------------------------
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via: http://www.linuxtechi.com/20-linux-commands-interview-questions-answers/
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作者：[Pradeep Kumar][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://www.linuxtechi.com/author/pradeep/
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20条Linux命令面试问答
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================================================================================
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**问:1 如何查看当前的Linux服务器的运行级别？**
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答: ‘who -r’ 和 ‘runlevel’ 命令可以用来查看当前的Linux服务器的运行级别。
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**问:2 如何查看Linux的默认网关？**
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答: 用 “route -n” 和 “netstat -nr” 命令，我们可以查看默认网关。除了默认的网关信息，这两个命令还可以显示当前的路由表。
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**问:3 如何在Linux上重建初始化内存盘影响文件？**
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答: 在CentOS 5.X / RHEL 5.X中，可以用mkinitrd命令来创建初始化内存盘文件，举例如下：
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# mkinitrd -f -v /boot/initrd-$(uname -r).img $(uname -r)
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如果你想要给特定的内核版本创建初始化内存盘，你就用所需的内核名替换掉 ‘uname -r’ 。
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在CentOS 6.X / RHEL 6.X中，则用dracut命令来创建初始化内存盘文件，举例如下：
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# dracut -f
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以上命令能给当前的系统版本创建初始化内存盘，给特定的内核版本重建初始化内存盘文件则使用以下命令：
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# dracut -f initramfs-2.x.xx-xx.el6.x86_64.img 2.x.xx-xx.el6.x86_64
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**问:4 cpio命令是什么？**
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答: cpio就是复制入和复制出的意思。cpio可以向一个归档文件（或单个文件）复制文件、列表，还可以从中提取文件。
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**问:5 patch命令是什么？如何使用？**
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答: 顾名思义，patch命令就是用来将修改（或补丁）写进文本文件里。Patch命令通常是接收diff的输出并把文件的旧版本转换为新版本。举个例子，Linux内核源代码由百万行代码文件构成，所以无论何时，任何代码贡献者贡献出代码，只需发送改动的部分而不是整个源代码，然后接收者用patch命令将改动写进原始的源代码里。
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创建一个diff文件给patch使用，
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# diff -Naur old_file new_file > diff_file
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旧文件和新文件要么都是单个的文件要么都是包含文件的目录，-r参数支持目录树递归。
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一旦diff文件创建好，我们就能在旧的文件上打上补丁，把它变成新文件：
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# patch < diff_file
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**问:6 aspell有什么用 ?**
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答: 顾名思义，aspell就是Linux操作系统上的一款交互式拼写检查器。aspell命令继任了更早的一个名为ispell的程序，并且作为一款嵌入式替代品 ，最重要的是它非常好用。当aspell程序主要被其它一些需要拼写检查能力的程序所使用的时候，在命令行中作为一个独立运行的工具的它也能十分有效。
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**问:7 如何从命令行查看域SPF记录？**
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答: 我们可以用dig命令来查看域SPF记录。举例如下：
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linuxtechi@localhost:~$ dig -t TXT google.com
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**问:8 如何识别Linux系统中指定文件(/etc/fstab)的关联包？**
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答: # rpm -qf /etc/fstab
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以上命令能列出供应给“/etc/fstab”文件的包。
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**问:9 哪条命令用来查看bond0的状态？**
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答: cat /proc/net/bonding/bond0
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**问:10 Linux系统中的/proc文件系统有什么用？**
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答: /proc文件系统是一个基于维护关于当前正在运行的内核状态信息的文件系统的随机存取存储器（RAM），其中包括CPU、内存、分区划分、I/O地址、直接内存访问通道和正在运行的进程。这个文件系统所代表的是各种不实际存储信息的文件，它们指向的是内存里的信息。/proc文件系统是由系统自动维护的。
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**问:11 如何在/usr目录下找出大小超过10MB的文件？**
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答: # find /usr -size +10M
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**问:12 如何在/home目录下找出120天之前被修改过的文件？**
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答: # find /home -mtime +120
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**问:13 如何在/var目录下找出90天之内未被访问过的文件？**
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答: # find /var \\! -atime -90
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**问:14 在整个目录树下查找核心文件，如发现则删除它们且不提示确认信息。**
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答: # find / -name core -exec rm {} \;
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**问:15 strings命令有什么作用？**
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答: strings命令用来提取和显示非文本文件的清晰内容。
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**问:16 tee filter有什么作用 ?**
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答: tee filter用来向多个目标发送输出内容。它可以向一个文件发送一份输出的拷贝并且如果使用管道的话可以在屏幕上（或一些其它程序）输出其它内容。
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linuxtechi@localhost:~$ ll /etc | nl | tee /tmp/ll.out
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在以上例子中，从ll输出的是在 /tmp/ll.out 文件中被捕获的，输出同样在屏幕上显示了出来。
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**问:17 export PS1 = ”$LOGNAME@`hostname`:\$PWD: 这条命令是在做什么？**
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答: 这条export命令会更改登录提示符来显示用户名、本机名和当前工作目录。
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**问:18 ll | awk ‘{print $3,”owns”,$9}’ 这条命令是在做什么？**
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答: 这条ll命令会显示这些文件的文件名和它们的拥有者。
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**问:19 :Linux中的at命令有什么用？**
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答: at命令用来安排一个程序在未来的做一次一次性执行。所有提交的任务都被放在 /var/spool/at 目录下并且到了执行时间的时候通过atd守护进程来执行。
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**问:20 linux中lspci命令的作用是什么？**
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答: lspci命令用来显示你的系统上PCI总线和附加设备的信息。指定-v，-vv或-vvv来获取详细输出，加上-r参数的话，命令的输出则会更具有易读性。
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--------------------------------------------------------------------------------
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via: http://www.linuxtechi.com/20-linux-commands-interview-questions-answers/
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作者：[Pradeep Kumar][a]
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译者：[ZTinoZ](https://github.com/ZTinoZ)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://www.linuxtechi.com/author/pradeep/
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