Topic 2: More Shell Skills Plan: about 3 lectures on this topic 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 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 exceptions later \c: character taken literally 1 2 Sub-Topic 2: Shell Variables Referring to Shell Variables: More 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 $today Awkward situation: malamb@linux5:~$ object=computer malamb@linux5:~$ echo "I have two $objects" I have two malamb@linux5:~$ Why? Fix: malamb@linux5:~$ echo "I have two ${object}s" I have two computers 3 4
Shell Variables and Quotes The $ character retains its special meaning inside double quotes. echo "Today is $today" Not inside single quotes. 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 5 6 Shell Variables Are Temporary Changes & new variables only last until you log out. To make a permanent change: use an initialization file (soon!) 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"? 7 8
PS1 Useful Variables Set By bash $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 $HOME: your home directory $PWD: current directory $SHELL: path name of current shell $USER: your user name 9 10 Sub-Topic 3: Sub-Shells Sub-Topic 4: Simple Shell Scripts 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 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 11 12
Executing a Shell Script Putting "." in Your Path First step: must be executable (chmod) Type command name alone: bash looks in your PATH directories Type command name with directory: no lookup needed 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? 13 14 Complication: scripts & sub-shells Special Initial Comment 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. #!/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. 15 16
Sub-Topic 5: Bash Initialization Files In your home directory:.bash_profile: executed when you log in (with source).bashrc: executed when you start up a sub-shell Use these files for your own settings (variables, etc.) 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. Advice: before you change, make a copy! don't remove calls to standard system files Sample initialization files in /cas/course/cisc220 17 18 Command For Examples Redirecting Standard Input Command to use as example: sort. Reads standard input and sorts to standard output. demo... (use ^d to end input) command 0< file Reads input from file, instead of keyboard. Shorter form: command < file example: sort < myfile 19 20
Redirecting Standard Output command 1> file Sends standard output of command to file, instead of screen. Shorter form: command > file Output Redirection: Existing Files If file already exists, bash will refuse command > file: if file already exists, overwrite command >> file: if file already exists, append Example: sort > file Example of redirecting standard input and output: sort < input > output 21 22 Redirecting Standard Error Getting Rid of Output You Don't Want command 2>filename example: ls file_thats_not_here 2>errs send to /dev/null Example: cmd produces error messages you want to throw away: cmd 2>/dev/null 23 24
Redirecting Both Output Files Pipes To 2 separate files: cmd 1>goodOutput 2>errOutput or: cmd >goodoutput 2>errOutput To the same file: cmd 1>outFile 2>&1 or: cmd 2>outFile 1>&2 means file 2 is sent to same place as file 1 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 25 26 Sub-Topic 7: Aliases Interesting Aliases Set For You Alias = easy way to create synonym for a command provides shorthand for something you do often alias long="ls l" rm="rm i" cp="cp i" mv="mv i" interactive mode asks before overwriting file asks before overwriting file Also a way to turn on certain options always. Example from standard.bashrc file: alias rm="rm -i" ls="ls F" adds characters at end of file names 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) 27 28
Extra Topic: 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'! Some programs can deal with any of these formats. Others are confused by some formats. Windows: try Notepad++ and Notepad Linux: try cat and emacs Two Useful Tools wc ("word count"): number of lines, words & characters in a file 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 29 30 Fixing With FTP Conversion Tools (in Linux) Proper settings in FTP program can avoid problems will translate text files into appropriate format while transferring unix2dos myfile: if myfile is in Unix/Linux text format, converts to Dos/Windows otherwise, does nothing dos2linux myfile: if myfile is in Unix/Linux text format, converts to Dos/Windows otherwise, does nothing Notice: overwrites original! 31 32