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linux filenames

  • linux is flexible about filenames
  • file-system doesn’t require extensions to filenames
    • .sh: shell script file extension
  • filenames are case-sensitive

dot names

  • some linux directory names begin with a . (dot)
  • these are typically hidden
    • is not listed by the command ls
  • ls -a with the flag -a has to used to view these dirs
  • used for caching and configuration
    • other things that are not cared about

shell tools

  • commands are case-sensitive
    • so, even the command-line flags are case-sensitive

command history

  • the shell keeps a history of all the commands that were executed successfully
  • the up-arrow and down-arrow keys can be used to scroll through them
    • hitting enter runs them again
  • history command outputs a long list of previously executed commands
    • including from past shell sessions
  • Ctrl+R key-combo at the command-line brings up the ‘reverse-i-search’ function
    • handy tool to search previously executed commands
    • simply start typing in the text string and the command will be populated automatically

common commands

  • curl -o url/of-file/to-download.ext
    • downloads a file from the argument URL to the current dir
  • unzip things.zip
    • takes the filename to unzip as an argument
    • the zip files contents get extracted to current dir
    • verify with ls
  • cat file1.txt file2.txt
    • concatenates and outputs contents of two text files
    • use cat file.text to quickly read the contents of a text file in the prompt
  • tab-completion is a shell feature that automatically populates filenames in the command prompt
    • conflicts are listed when there are multiple filenames available for tab-completion
    • provide additional character and use tab-completion again
  • ls -l
    • along with list of names of elements in current dir
      • also shows the modification time
      • file size
      • dir or not status, and
      • permissions for user, groups and everyone
    • ls -al is used to see everything in the dir along with all their details

file analysis tools

  • wc file.txt
    • outputs the number of lines, number of words and bytes in a text file
  • diff file1.txt file2.txt
    • compares and outputs the differences between two files
    • helpful for version comparison and tracking changes between consecutive versions

command manual

  • man date
    • loads manual for the date command
    • all shell commands come with a manual
    • synopsis section lists syntax and all the flags/options and arguments for the command
    • the description section provides details about all the flags and arguments
    • use q to exit the manual page
researching commands
  • use search engines to research commands in addition to the man page
    • sometimes, the explanation is easier to understand on stack-overflow than interpret the manual entry for the command
  • rm -rf / is a dangerous command
    • erases all current user files
    • if admin, erases even the root files
    • rm: remove/delete things
      • -rf: recursively forcibly
      • /: everything in user root dir
      • doesn’t put it a trash dir
      • permanently removes deleted files

different types of command line programs

line based programs
  • Ctrl+C: interrupt signal

  • Ctrl+D: EOF (end-of-file) character

  • ping 8.8.8.8:
    • check if machine at IP address in argument is alive
    • program keep outputting until interrupt signal is sent
  • sort: a line based program example
    • keeps accepting input in new lines
    • read the input one line at a time until EOF is entered
      • doesn’t process input lines until EOF is sent
      • waits for next input line after current input is entered
    • when EOF is sent, processed output is displayed in terminal
    • the prompt is then ready for the next keyword
programs that wait for input
  • bc: simple calculator program
    • once started, mathematical expressions can be evaluated one after the other
    • the program waits for the next expression to evaluate
    • after evaluating an expression, it waits for the next expression
      • does not return the shell prompt for a new keyword
    • to exit the program and get shell back:
      • type quit
      • send EOF - Ctrl-D
      • Ctrl-C doesn’t work to return shell prompt
full-screen interactive programs
  • man: populates the entire shell screen with the program and hides the shell prompt
    • to get the shell back, the program has to be exited
    • exiting such programs have different keys combos
    • man command uses the built-in less program to display full-screen text
  • less file.txt:
    • opens the text file file.txt full screen in the shell
    • the contents of the text file can be read full-screen
  • less key combos:
    • use UP/DOWN arrow keys to scroll
    • D/Space: scroll down one page
    • U: scroll up one page
    • >: last line of the file
    • <: very first line of the file
    • enter line number at less prompt :
      • then hit ENTER to jump to input line number in text file
    • /: activates text search function
      • enter / and type the search keyword
      • use n to jump to next match
      • use N to jump to previous match
      • search is case-sensitive
      • reg-exps can be used for search
  • nano file.txt: shell text editor
    • handy to edit config files remotely for example
    • opens file.txt full-screen shell with text editing enabled
    • use the key combos listed at the bottom for the operation stated for
      • ^: Ctrl key
      • eg: ^X (Ctrl-X) exits nano with a save prompt

references