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UNIX answers
Viewing and Editing files (cat/more/less/head/tail/vim/vi)
● cat o concatenate files and print on the standard output ● more o display files on a page-by-page basis (one screenful at a time) ● less o opposite of more o allows backward and forward movement o does not have to read the entire input file before starting, so with large input files it starts up faster than text editors like vi ● head o print the first 10(by default, can be changed with –n) lines of each FILE to standard output o with more than one FILE, precede each with a header givin the file name ● tail o print the last 10 lines of each FILE to standard output ● vim o is a text editor that is upwards compatible to Vi. It can be used to edit all kinds of plain text. o A lot of improvements compared to vi: ▪ Multi-level undo, syntax highlighting ● vi o screen oriented text editor
Which is best for viewing a large file? o Less ▪ Main advantage is that less command will not load whole file into memory,so it will be much faster than vi
How do you return the 20th^ -30th^ lines in a text file o Two ways ▪ sed –n –e ’20,30p’ input txt > output.txxt ▪ tail –n +10 input.txt | head –n 11 >output.txt
If you only have 3 lines in your file, which command would you use to view the contents of the file? ▪ Cat
Copying, Moving and Creating directories and files (cp/ scp/ mv
/mkdir/touch)
How can you move files from one directory to another? ▪ mv filename dir-name
How do you rename the file? ▪ mv oldfilename newfilename
How would I approach scenarios such as using “makefile”in different ways? ▪ mkdir, touch, scp (for file transfer)
Disk Space and Memory Usage ( free/ df/ du/ top)
▪ free o Displays amount of free and used memory in the system ▪ df o Report file system disk space usage ▪ du o Summarize disk usage of each FILE, recursively for directories ▪ top o display Linux tasks/ provides a dynamic real-time view of a running system.
● What are the columns of each command?
▪ OO..fuck off, if you need check manual, I will not memorize this. Bastards!
● How do you return the disk usage for a directory? ▪ du
● How can you find the memory usage? ▪ Free –m ▪ Cat /proc/meminfo - more detailed ▪ Top
● How do you check disk space?
▪ df/ df -h
● How do you check CPU usage? ▪ top
● How to view free disk space? ▪ df -h
● If a process/system is unresponsive what would you check?
▪ Not sure about this one
How do you see a list of processes? ●
How do you check what processes are currently running?
How do you stop a process that is running ● Use ps to find PID and then kill command (kill [signal] PID
Different ways to find PID? ● Pidof
Kill
● How do you kill a specific process? Kill pid1 pid2 pid Kill 3486 Kill -9 3486
● How to find PID to run kill on?
▪ Using lsof/pidof/ps and then combination with kill
● The different signals e.g. kill -9 (force kill) and kill -2 (interrupt) etc
▪ -9 nearly guarantee to kill the process
● What will kill do if you don’t pass a signal and the process you’re trying to kill is busy?
▪ I assume; wait until I am done without the flag
Ctrl Z / fg/ bg/ jobs/ nohup/ &
Ctrl Z - sends SIGTSTP to a foreground application, effectively putting it in the background, suspended. This is useful if you need to break out of something like an editor to go and grab some data you needed.
Fg– command continues a stopped job by running it in the foreground fg [%job_id]
Bg– control command that resumes suspended jobs while keeping them running in the background. This restart a stopped background process. bg[job]
Jobs– list background processes
Nohup– when using the command shell, prefixing a command with nohup prevents the command from being aborted if you log out or exit the shell
&- run the command in the background
String manipulation (sed/awk)
Sed – stream editor for filtering and transforming text
Awk – finds and replaces text, database sort/validate/index (awk ‘{print $1}’ input_file
● When do you use it and what's the syntax
● What is sed / What do you know about sed
▪ Cat example.txt | sed ‘d’ -
▪ to find all the files under /tmp directory with name xyz.txt.
▪ find /tmp -name xyz.txt
● How would you delete all files older than 5 days ▪ find /path/to/files* -mtime +5 -exec rm {} ;
The pipe command ( | ) and xargs
● What does the | command do
▪ Allow you to merge existing Unix commands in a new one
● Can you provide an example of how you would use it
▪ tail –n +10 input.txt | head –n 11 >output.txt
▪ ls -al | more ● Here's a simple pipeline command I use all the time, creating a long list of files and piping the output into the Linux more command
● What does the xargs command do
▪ Build and execute command lines from standard input
▪ $ xargs Hi, Welcome to TGS.Hi, Welcome to TGS.
shell scripts
make typescript of terminal session. Script makes a typescript of everything printed on your terminal.
● Different types of shell script
● Structure (What's the first line) o #!/bin/sh
● How to check test if it was a regular file in a bash script
● How to reload bash profile
● In a shell script, how do you check if a directory exists
Environment Variables ( export / echo )
● How do you set an environment variable
▪ $FOO=”bar”
▪ Export FOO (not be visible in childs processes unless you export it)
● How do you check an environment variable
▪ Echo $FOO (don’t forget the $)
wc
● How can you count the number of rows in a file ▪ wc -l
chmod
ability to change permissions on the file
● Describe and it's syntax (to change permissions)
▪ chmod 771 my_app
▪ chmod o+wx testfile
● How do you change permission on a file and its syntax
● If I try to save a table to disk and do not have permission how do I go about adding permissions to save/load
Symlink
Make a new name for a file (symbolic link)
● How to create a symlink
● What is a symbolic link
crontab
● How does it work
▪ Schedule a command to run at a later time
● How do you set up a command to be run periodically
▪ 0 5 * * * /home/myname/scripts/do-every-day.sh – run at 5 PM every day
● What do the following options mean: -e, -l, -r
▪ -r – remove the crontab
▪ -e –edit
