Skip to content

Latest commit

 

History

History
146 lines (115 loc) · 6.01 KB

CommandsAndPrograms.md

File metadata and controls

146 lines (115 loc) · 6.01 KB

Command Basics

Commands are actually programs. They are not big programs like Photoshop or Word but they still are program, although called commands generally.

Commands are executable files reside in various places on the filesystem. Users can also go to the directory that command file resides and execute there. One of the common places is /bin. There are many executable files in /bin folder.

Let's see some commands that are used to have information about commands;

  • whereis <cmd_name>: Prints paths for binaries, sources and manuals
  • which <cmd_name> : Prints path for executable file(binary). Difference from whereis that which uses PATH to print which version of the command will be used.
  • whatis <cmd_name> : Prints simple information about the command
  • <cmd_name> [-v, --version]: Prints the version of the command
  • <cmd_name> [-h, --help] : Prints the help page(simplified manual)
  • q, ctrl q, ctrl x, escape: Exits the command
  • ctrl c: Interrupts the program, hard way...
  • ; : semicolon can be used to seperate commands in the same line

The PATH Variable

We know that commands are just executable files reside in various directories. But how does Unix know where are those files? The answer to this is PATH variable.

PATH variable simply holds a list of directories that Unix looks for when a command execution is needed. We can see current path with $PATH;

/home/cagri/bin:/home/cagri/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:
/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

When Unix looks for a command in these directories, it searches from the first one to the last one. You can set the path with PATH=<path_list>, without dollar sign before PATH. But changing this way

If you remove a command's installed directory from the PATH variable, UNIX will no longer be able find and execute them.

System Information Commands

  • date: Date which computer is set to, not UTC or anything.
  • uptime: Prints the current time, total time elapsed from the start of the system, number of users that logged into Unix currently and load averages.
  • who: Shows who is logged on, users may be duplicated(Not in Ubuntu, why?)
  • users: List of distinct users
  • uname[-mnrsvp]: Gives information about operating system. ** -s: kernel nameL -> Linux ** -n: network node hostnamec -> Cagri-UbuntuVB ** -r: kernel release -> 4.10.0-37-generic ** -v: kernel version -> # 41~16.04.1-Ubuntu SMP Fri Oct 6 22:42:59 UTC 2017 ** -m: machine hardware name -> x86_64 ** -p: processor -> x86_64 ** -i: hardware platform -> x86_64 ** -o: operating system -> GNU/Linux
  • hostname: host that we are on
  • domainname: domain that we are on(in a network environment)

Disk Information Commands

  • df: Shows storage analysis of currently mounted disks like size, used space, available space percentage and mounted path.
  • df -h: Shows size related information in 1024 version
  • df -H: Shows size related information in 1000 version
  • du: disk usage in the given path. Misbehaving in Ubuntu. Reports allocation for that file.

Viewing Processes

ps command is used to see process status.

When we execute commands by Shell, it passes this request to the Kernel. Kernel then allocates some memory and starts processes running on it.

  • ps: By default, it shows processes owned by user of the command
  • ps -a: Processes owned by all users
  • ps aux: All processes running on the Kernel. a means all processes, u means putting user column and x means showing background processes. Output of this command is snapshot of that time. First column is user, then process id(unique id given to processes), cpu percentage used, memory percentage used, virtual memory used and terminal.

Monitoring Processes

Sometimes we want to track processed dynamically as the memory, CPU usage and their existence changes. Command for that is top. top works differently in Unix and Mac OS X. Fill this section later with Ubuntu usage.

Stopping Processes

  • ctrl c: It assumes process runs in terminal, you are in the process and tells Unix to stop that process.
  • kill <process_id>: Unix will try to kill but for some processes, this will not kill process.
  • kill -9 <process_id>: Override Unix's careful approach, kill definitely.

Text File Helpers

  • wc: Prints the line, word and character count. For line, it counts line return.
  • sort: Sorts the lines of the file and prints it. Does not modify. Capital letters and lowercase letters are treated different than each other. ** -f: Ignores case of letters ** -r: Reverse sort ** -u: Sorted unique lines
  • uniq: Ignores consecutive duplicate lines. ** -d: Prints consecutive duplicate lines ** -u: Prints non-duplicate lines

Utility Programs

  • cal / ncal: Shows current month's calendar. ** [month_num] [year_num]: Prints the month's and year's calendar. ** -y : Prints current year's calendar.
  • nca: Rotates the day numbers and day names columns.
  • bc: bench calculator
  • expr: simpler calculator. expr 2 + 2
  • units: scientific unit converter

Author's note: Other than calendar and units, do not use these commands. Useless things.

Command History

We have seen up and down arrows are used to navigate between commands. There is also a utility of Unix that saves bash commands into a history file. The file is called .bash_history file in the home directory(~). This file is updated when bash is closed.

When you type history into bash, it will show you the list of commands with numbers before them. You can use those numbers to reuse the command. Type exclamation point before the number of the command. When we hit up arrow, we will see the actual command instead of the number.

You can type exclamation point before a part or full name of the command to return the most recent command starting with the argument. For example, !vi will bring last command executed starting with vi.

  • !3 : References history command 3
  • !-2 : References command which was 2 back
  • !cat: References most recent command starting with cat
  • !! : References previous command
  • !$ : References previous command's arguments