Topic 2: More Shell Skills

Transcription

1 Topic 2: More Shell Skills Sub-topics: 1 quoting 2 shell variables 3 sub-shells 4 simple shell scripts (no ifs or loops yet) 5 bash initialization files 6 I/O redirection & pipes 7 aliases 8 text file formats 1

2 Sub-Topic 1: Quoting Useful command: echo Prints all its arguments to the screen Normal rules: spaces separate words in commands some characters have special meanings to bash Sometimes useful to quote strings or characters. Three quoting mechanisms: 'string': every character in the string taken literally "string": most characters taken literally \c: character taken literally 2

3 Difference Between Single & Double Quotes A few special characters are interpreted by double quotes but not single quotes 1.!: history expansion 2. $: shell variables 3. back quotes (`) next week 3

4 Sub-Topic 2: Shell Variables shell variables: a way to store information in bash To give a value to a variable: today=tuesday If no "today" variables exists, creates it. Sets value to the string "Tuesday" To refer to value of variable: use $. Example: echo $today 4

5 Referring to Shell Variables: More Awkward situation: $ object=computer $ echo "I have two $objects" I have two $ Why? Fix: $ echo "I have two ${object}s" I have two computers 5

6 Shell Variables and Quotes The $ character retains its special meaning inside double quotes. echo "Today is $today" Not inside single quotes. 6

7 Uses of Shell Variables store information for use by programs, bash also very useful in shell scripts (soon!) To see all variables with values: set 7

8 Shell Variables Are Temporary Changes & new variables only last for life of current shell. To make a permanent change: use an initialization file (soon!) 8

9 PATH $PATH Contains list of directories separated by ":"s. When you type ls Bash looks for a command named "ls". Searching order: 1. Is it a built-in shell command? 2. In each directory in $PATH: is there a program called "ls"? 9

10 PS1 $PS1 is your bash prompt. Can contain literal characters plus some special ones: \d: the current date \h: the name of the host machine \j: number of jobs you have running \s: the name of the shell \u: name of user \w: current working directory \!: history number of this command 10

11 Useful Variables Set By bash $HOME: your home directory $PWD: current directory $SHELL: path name of current shell $USER: your user name 11

12 Sub-Topic 3: Sub-Shells A shell is a program. Linux automatically runs the bash program when you log on. You can run it too. One reason: experiment with settings. Command to start a sub-shell: bash Exit back to login shell: exit Use Shell variable $SHLVL to see if you're in a sub-shell A sub-shell "inherits" many of your settings, including current folder, umask Does not automatically inherit variables or aliases export command: says variable should be inherited by sub-shells export myvar 12

13 Sub-Topic 4: Simple Shell Scripts shell script = file containing bash commands Comments in scripts: # means rest of line is a comment Special variable names: $0: name of command $#: number of arguments $1,$2,...: the individual arguments $*: all the arguments in one string, separated by spaces 13

14 Executing a Shell Script First step: must be executable (chmod) Type command name alone: bash looks in your PATH directories Type command name with directory: no lookup needed 14

15 Putting "." in Your Path Advantage: You can run scripts & programs in your current directory without specifying the directory Disadvantages: 1. Security: Have you downloaded a malicious program called "ls"? 2. Possibility of confusion: Have you created a program with the same name as another program earlier on your path? 15

16 Complication: scripts & sub-shells When you type the name of a script as a command: executed in a sub-shell. Consequence: can't use to create aliases or set variables. If you want script to affect current shell: source myscript Executes commands from myscript in the current shell. 16

17 Special Initial Comment #!/bin/bash At beginning of script, specifies this is a script to be run with bash. Why bother? Syntax of some commands different for every shell. Now your script will work for people using other shells. Not required for 220, but generally a good idea. 17

18 Sub-Topic 5: Bash Initialization Files In your home directory:.bash_profile: executed when you start a login shell.bashrc: executed when you start up any other shell Use these files for your own settings (variables, etc.) Advice: before you change, make a copy! don't remove calls to standard system files Sample initialization files in /cas/course/cisc220 18

19 Sub-Topic 6: I/O Redirection & Pipes Three standard files associated with any command or program: file 0: standard input: default is keyboard file 1: standard output: for regular output, default is screen file 2: standard error: for error messages, default is screen I/O Redirection can change the defaults for these three files. 19

20 Command For Examples Command to use as example: sort. Reads standard input and sorts to standard output. demo... (use ^d to end input) 20

21 Redirecting Standard Input command 0< file Reads input from file, instead of keyboard. Shorter form: command < file example: sort < myfile 21

22 Redirecting Standard Output command 1> file Sends standard output of command to file, instead of screen. Shorter form: command > file Example: sort > file Example of redirecting standard input and output: sort < input > output 22

23 Output Redirection: Existing Files Bash may object if you redirect to an existing file Depends on noclobber option. To view current options: set -o To set option: set -o noclobber To turn off option: set +o noclobber command > file: if file already exists, overwrite command >> file: if file already exists, append 23

24 Redirecting Standard Error command 2>filename example: ls file_thats_not_here 2>errs 24

25 Getting Rid of Output You Don't Want send to /dev/null Example: cmd produces error messages you want to throw away: cmd 2>/dev/null 25

26 Redirecting Both Output Files To 2 separate files: cmd 1>goodOutput 2>errOutput or: cmd >goodoutput 2>errOutput means file 2 is sent to same place as file 1 To the same file: cmd 1>outFile 2>&1 or: cmd 2>outFile 1>&2 26

27 Pipes cmd1 cmd2 Sends standard output of cmd1 to standard input of cmd2 Examples: command less command sort command sort > output command 2>&1 command2 cmd1 cmd2 cmd3 27

28 Sub-Topic 7: Aliases Alias = easy way to create synonym for a command provides shorthand for something you do often alias ll="ls l" Also a way to turn on certain options always. Example from standard.bashrc file: alias rm="rm -i" To execute original program (without alias): quote the name "rm" file1 file2 To cancel an alias: unalias rm To see all your aliases: alias (no arguments) 28

29 Interesting Aliases Set For You In CASLab rm="rm i" cp="cp i" mv="mv i" ls="ls F" interactive mode asks before overwriting file asks before overwriting file adds flags at end of file names 29

30 Sub-Topic 8: Text File Formats Basic text file format: ASCII or Unicode characters Problem: Different OSs, different character(s) for end of line! Unix/Linux: '\n' (newline) Windows/Dos: '\r' + '\n' (carriage return + newline) Mac: sometimes '\r', sometimes '\n'! Most programs can deal with any of these formats. Others are confused by some formats. Windows: try Notepad++ and Notepad Linux: try cat and emacs 30

31 Historical Note 31

32 Useful Tools od ("octal dump"): listing of bytes in a file default or -o: each pair of bytes as an octal number -c: each byte as a character -x: each pair of bytes as a hexadecimal number -d: each pair of bytes as a decimal number wc ("word count"): number of lines, words & characters in a file 32

33 Conversion Programs on Linux *** may not be available on all Linux systems! *** dos2unix, unix2dos: changes arguments (make a copy first!) no effect if file already in desired format 33

34 Fixing With FTP Proper settings in FTP program can avoid problems will translate text files into appropriate format while transferring 34