Shell Scripts. Jacob Morzinski Jan-23 1

Transcription

1 Shell Scripts Jacob Morzinski Jan-23 1

2 Why use shell scripts? They simplify your life. Store complicated commands in a script, to save effort and reduce typing errors. athrun ops rdesktop -ujmorzins -f N \ -a16 vash.mit.edu:4884 ldapsearch -u -LLL -h ldap.mit.edu \ -b dc=mit,dc=edu uid=jmorzins 2>&1 \ egrep -v '^(SASL[ /] objectclass:)' 2006-Jan-23 3

3 Basic: setting up a script 1. Edit the file. Emacs, vi, jedit, gedit, whatever you prefer. Run with: bash /path/to/file 2. (Make Unix happy.) Make sure the file begins with #!/bin/bash chmod a+rx file Make sure file is in your $PATH (or $path). 3. Run the script. file (or /path/to/file ) 2006-Jan-23 4

4 Basic: making Unix happy The #! line tells Unix what program to run the file through. We want to use bash. The chmod command tells Unix to make the file readable and executable. Your $PATH is a list of directories that Unix looks through, when trying to find a program whose name matches the word you just typed at the prompt Jan-23 5

5 Basic: setting the PATH If your shell is sh-based (like bash): PATH="$PATH":/mit/jmorzins/bin export PATH Sometimes you can combine these: export PATH="$PATH":/mit/jmorzins/bin If your shell is csh-based (like tcsh): set path = ( $path:q /mit/jmorzins/bin ) 2006-Jan-23 6

6 Basic: fixed-text example $ cat user1 #!/bin/sh vos exa user.jmorzins kvno jmorzins $./user1 user.jmorzins APHRODITE.MIT.EDU /vicepb Creation Wed Aug 30 09:50: jmorzins@athena.mit.edu: kvno = Jan-23 7

7 Basic: variables allow flexebility $ cat user2 #!/bin/sh vos exa user."$1" kvno "$1" $./user2 boojum user.boojum COCYTUS.MIT.EDU /vicepb Creation Sun Aug 10 00:08: boojum@athena.mit.edu: kvno = Jan-23 8

8 Review of basic knowledge At this point, you know: How to set up simple shell scripts How to pass one word into the script, to have the script run commands using that word ("$1") Jan-23 9

9 What the shell really does You: type a command The shell does: Variable substitution (interpolation)* File name substitution (globbing)* Parses the line into arguments* Finally, runs the command * Can be influenced by quoting Jan-23 10

10 Runs the command Searches through $PATH to find the executable file, if not an absolute path. You can run a sequence of commands if you separate them with semicolons ( ; ) Commands can return an exit status. Status = 0 means success Status!= 0 means some sort of failure You can use && and to chain commands in an and or or fashion Jan-23 11

11 Parses the line into arguments The shell splits the command line on white space. The first word is the command name The rest of the words are arguments, and are passed to the command Jan-23 12

12 File globs If files match your pattern, the file names are filled into the command line. Pattern *? [abc] [x-z] [!0-9] Meaning Match zero or more characters Matches just one character Match just one character, from a range Match one character, that is not in the range 2006-Jan-23 13

13 Variable substitution A variable can be a word, a number, or a special symbol: Some symbols: $$, $?, $*, $@ Numbers 0, and 1-9: $0, $1,, $9 Words: $PATH, $var, $longer_name Note that variable substitution happens before file substitution and before the commandline is split into words Jan-23 14

14 Quoting Single quote: '...' The shell ignores all enclosed characters Double quote: "..." The shell ignores most enclosed charactecters The shell does interpret $, `, and \ Backtick: `...` Run a command, insert the output Backslash: \ Special treatment for the following character Jan-23 15

15 Input/Output redirection Commands usually print to standard output, sometimes print to standard error. Commands read from standard input. Stdin is 0 Stdout is 1 Stderr is 2 Higher numbers are possible 2006-Jan-23 16

16 Table of I/O redirection cmd1 cmd2 commad > file command >> file command < file command 2>&1 command 1>&2 cmd1 `cmd2` pipe output from cmd1 into cmd2 put text into file, truncate file append text to file read from file send stderr to where stdout is send stdout to where stderr is capture output of cmd2 for command-line of cmd Jan-23 17

17 Setting Variables Setting variables var=value ENVVAR=value export ENVVAR Note: variables don t affect parent shells only environmental variables affect child shells 2006-Jan-23 18

18 Using variables $* - list of all command-line parameters. Bad. $@ Good list of all command-line parameters. $1, $2,, $9 individual command line parameters shift remove $1, shift all other variables down one set set new $1, $2, etc variables 2006-Jan-23 19

19 Utilities test test runs a test, returns true or false Good for if statements, or while loops also called [, but in that case it must be used with a matching ] -f -d -z -n =,!= -a, -o! -gt, -lt -ge, -le is a file is a directory is zero length is non-zero length compare strings and, or negate next part compare numbers 2006-Jan-23 20

20 Utilities grep Pattern searching Uses regular expressions Powerful, but hard to learn a. * [abc] [^abc] ^ $ \(z\) \1 letter a any one char zero or more char in a range, or inverted start of line end of line save a match, re-use a match 2006-Jan-23 21

21 Shell Loops - if if expr ; then cmd1 ; else cmd2 ; fi cmd1 and cmd2 can be complex the else can be omitted if [ x $var = x yes ] ; then echo good ; fi 2006-Jan-23 22

22 Shell Loops while, for while expr ; do cmd ; done for var in a b c d e ; do cmd ; done 2006-Jan-23 23

23 Shell functions name () { cmd1 ; cmd2 $@ ; } Act like mini-shell scripts. Can set variables in the current shell Jan-23 24