Unix/Linux commands and shell programming презентация

Unix/Linux commands and shell programming Clemson University PARL Presented by Tim Shelling, UNIX guru

UNIX Overview Why UNIX? Control Commands often provide complete access to the system and its devices Most devices can be manipulated just like files Flexibility Commands are just programs Commands have common interface to allow interoperation with other commands The UNIX shells provide the “glue” for this ` Reliability Commands are typically lightweight since they typically do little more than invoke operating system calls Individual commands that are broken can easily be replaced Summary: All the above translate into… POWER

UNIX: The Command Line Accessing UNIX through a terminal telnet [hostname] [port] The omnipresent failsafe. Nowadays, turned off due to lack of adequate security. ssh [user@]hostname Secure. Data is encrypted over “the wire”. What we use. Not always available outside CU due to different versions, implementations, platform availability. Log in! 3 tries to get valid username and password right Show who is logged in w or who finger Logout! exit CTRL-D

UNIX: Accessing Documentation Commands are generally documented using the command man. man pages are subdivided into various sections Example: Documentation of the man command man man Example: Documentation of the time command man time Example: Documentation of the time C library function man 3 time man will present the manual page of the specified entry using more or less. In Linux, the default is less, but can be overridden less presents a screen-full at a time. ‘spacebar’ moves forward, ‘b’ moves backward, ‘$’ moves to end, ‘q’ quits, ‘?’ helps.

UNIX: Accessing Documentation A few commands (such as diff, gcc, awk) are doccumented using info. info is GNU-specific Uses its own hypertext ‘viewer’. arrow-keys select different links space pages forward ‘u’ goes back “up” a hyperlink level, like “back” in browsers Most commands have HTML references on the WWW. Don’t panic. Just e-mail me or Dan.

UNIX terminal management: screen Help CTRL-A ? Copy/Scrollback CTRL-A [ Paste CTRLA ] Lock CTRL-A x Detach CTRL-A d New Screen CTRL-A c Next/Previous CTRL-A n / CTRL-A p Reattach screen –D –A –r List active screen -ls

UNIX: Getting around the filesystems UNIX files are organized just like they are with PC’s and MAC’s Files are contained in collections of Directories. Directories may contain other Directories Different drives are “mounted” onto directories – there are no drive letters!! The “top level” directory is called the “root” directory and is referred to by “/” The current directory is referred to by “.” The directory one level up is referred to by “..” More dots don’t get you more levels up.  Shortcuts in Windows are called soft-links. Act just like normal files, directories, or whatever it is they refer to. Other filetypes include named pipes, character devices, block devices, sockets.

UNIX: Getting Around Commands to navigate the directories: pwd ls ls file; ls directory ; ; ls –a ; ls –l ; ls -R cd cd .. cd /home/tim/projects cd ~/projects cd ~tim/projects cd $HOME/projects mkdir rmdir mv mv oldfilename newfilename mv file1 file2 file3 newtargetdirectory cp -- syntax like mv cp –r dir1 dir1copy rm push pop find find . –ls find . –type d –print find . –type f –exec “echo” “{}” “;”

UNIX: More Standard Commands echo print out a string echo “$HOME is where I want to be” cat Output specified files in sequence cat file1 file2 file3 whereis Show where a file can be found printenv Display all environment variables grep Get Regular Expression and Print head first few lines of output head -5 tail last few lines of output tail -8

UNIX command line tricks Shell “glob” # mkdir /tmp/moved # mv * /tmp/moved # cp /tmp/moved/* . Filename Completion (tcsh, bash) # ls /tmp/m<TAB> Command line history (tcsh) history CTRL-P and CTRL-N, down/up arrows !previous – Runs the previous command beginning with the word previous.

UNIX: The SHells The “Shell” is simply another program which provides a basic human-OS interface. Shells can run interactively or as a shell script Two main ‘flavors’ of Shells: Bourne created what is now known as the standard shell: “sh”, or “bourne shell”. It’s syntax roughly resembles Pascal. It’s derivatives include “ksh” (“korn shell”) and now, the most widely used, “bash” (“bourne shell”). One of the creators of the C language implemented the shell to have a “C-programming” like syntax. This is called “csh” or “C-shell”. Today’s most widely used form is the very popular “tcsh”.

Unix: SH basics Modifying environment variables sh: PAGER=/usr/bin/less; export PAGER bash: export PAGER=/usr/bin/less tcsh: setenv PAGER /usr/bin/less Execute an external command (sh) # somecommand somecommand: command not found # echo $PATH /home/tim/bin:/usr/local/bin:/usr/bin:/bin # pwd /home/tim/bin/project1 # ./somecommand Hello world! # /home/tim/bin/project1/somecommand Hello world! # PATH=$PATH:`pwd`; export PATH # somecommand Hello world!

UNIX: Bourne SHell script syntax The first line of a sh script must start as follows: #!/bin/sh Any unquoted # is treated as the beginning of a comment until end-of-line Every line is first parsed for shell metacharacters. These include characters that the shell will do something with and include: # ‘ “ & > < $ % * [ ] ? ! ` ~ ; | , { } Distinct commands may be separated by end-of-line, semicolon, or comma Environment variables are $EXPANDED “Back-tick” subshells are executed and `expanded` Pipelines are created | joining the output of | one program | with the next Any commands left over must be builtins or external commands. An error will fail the pipeline, but the script will continue!

Unix Pipelines: Pipes are smokin’! Pipes take the output of the first program and feed that output into the input of the next program. Also sometimes known as “filters”. Examples: last | less last | grep ^root | less last | grep ^root | cut -d -f 2 | less grep “error” something.out | tail -1

Unix redirection: Lesser and Greater >&filename redirects the standard output and error to the file called filename: last | grep ^root >& root-logins.txt less root-logins.txt >filename redirects just standard output Don’t Clobber me! By default, > will overwrite existing files, but you can turn this off using shell settings and/or environment variables. Appendicitis! You can append to existing files this way: - sh: >>filename >&1 csh: >>&filename Use < to redirect a file to a command’s standard input # cat calculation.txt (3+2)*8 # bc < calculation.txt 40 Useful when a program does not already query the command line for files to read

Unix Shell Scripting: Conditional Execution program1 && program2 Program 2 will execute if and only if program1 exited with a 0 status Example: project1 && echo “Project1 Finished correctly!” program1 || program2 Program 2 will execute if and only if program1 exited with a non-0 status Example: project1 || echo “Project1 FAILED to complete!” Exit a script with an error: exit 1

UNIX commands for programmers man –k Search man pages by topic time How long your program took to run date print out current date/time test Compare values, existence of files, etc tee Replicate output to one or more files diff Report differences between two files sdiff Report differences side-by-side wc Show number of lines, words in a file sort Sort a file line by line gzip Compress a file gunzip Uncompress it strings Print out ASCII strings from a (binary) ldd Show DLLs/SOs program is linked to nm Show detailed info about a binary obj

Unix Shell scripting: foreach loops These are useful when you want to run the same program in sequence with different filenames. sh example: for VAR in test1 test5 test7b finaltest; do runmycode $VAR >$VAR.out done csh example: foreach VAR ( test1 test5 test7b finaltest ) runmycode $VAR >$VAR.out end

Unix job control Start a background process: program1 & program1 Hit CTRL-Z bg Where did it go? jobs ps Terminate the job: kill it kill %jobid kill pid Bring it back into the foreground fg %1 Start a job in the future at

Regular Expressions Powerful language for specifying strings of text to be searched and/or manipulated. Used by grep “Get Regular Expression and Print” – search files line by line sed Simple Editing tool, right from the command line awk Scripting language, executes “program” on matching lines perl Pathological Rubbish Lister. Powerful programming language Note: These are not “file-globs”. The syntax is similar, but the semantics are slightly different! Cannot be used to match nested structures

Regular Expressions: Summary Fundamentals: Match the specified character unless it is a ... . Match any character (except EOL) [character class] Match the characters in character class. [start-end] start to end [^character class] Match anything except the character class. $ Match the end of the line ^ Match the beginning of the line * Match the preceeding expression zero or more times ? Match the preceeding zero or one time | Match the lef hand side OR the right side (regexp) Group the regular expression \ Treat next character literally (not specially) Examples: Match a line beginning with a space-padded line number and colon. ^[ \t]*[0-9][0-9]*: Match my name (various spellings) (Tim Shelling)|(TJS)|(T\. Shelling)|(Timothy J\. Shelling) Match if the line ends in a vowel or a number: [0-9aeiou]$ Match if the line begins with anything but a vowel or a number: ^[^0-9aeiou]

Getting files from and to Unix