bash Args, Signals, Functions Administrative Shell Scripting COMP2101 Fall 2018
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1 bash Args, Signals, Functions Administrative Shell Scripting COMP2101 Fall 2018
2 Error Output Failed commands often generate unwanted or irrelevant error messages That output can be saved as a log, sent to whoever should see it, or discarded Logs are usually kept in /var for programs we care about managing Redirect output using >&, >>&, &, or #>, or #>&# Use the logger command to send messages to the system logging daemon grapsnag >& /tmp/errormessage.txt if [ $?!= 0 ]; then logger -t $(basename "$0") -i -p user.warning -f /tmp/errormessage.txt && rm /tmp/errormessage.txt fi
3 Positional Arguments It is quite common to allow the user of a script to specify what the script is to operate on (e.g. a file, directory, host, interface, etc.) The method shown here requires the script writer to know or figure out how many command line arguments there are before working with them This script also demonstrates using exit status and stderr #!/bin/bash # identify files with setuid or setgid permissions in a # directory specified on the command line dirname="$1" if [ -z "$dirname" ]; then echo "You didn't give me a directory name on the command line" >&2 exit 1 fi [ -d "$dirname" ] (echo "$dirname is not a directory" >&2 ; exit 2) if [! -r "$dirname" ]; then echo "You don't have read permission for $dirname" >&2 exit 3 fi if [ -x "$dirname" ]; then echo "Setuid or setgid files:" find "$dirname" -type f -executable -perm ls 2>/dev/null sort -k 3 find "$dirname" -type f -executable -perm ls 2>/dev/null sort -k 4 exit 0 else echo "You don't have access permission for $dirname" >&2 exit 3 fi
4 Command Line Processing A loop can be used to cycle through the available command line arguments and interpret what is there We can use shift to renumber the command line variables each time through the loop Requiring arguments of the form -x or --option-name is known as using named arguments The case statement can be better than the if statement for this debug=0 while [ $# -gt 0 ]; do if [ "$1" == "-h" ]; then echo "Usage: $0 [-h --help]" exit 0 elif [ "$1" == "--help" ]; then echo "Usage: $0 [-h --help]" exit 0 else echo "Usage: $0 [-h --help]" echo "Argument '$1' not recognized" >&2 exit 2 fi shift done echo "This script will now do some useful task" #rest of script
5 Command Line Processing The case statement allows us to more clearly show what we are testing for Finding the commands that handle a specific named option is much easier when reading this script debug=0 while [ $# -gt 0 ]; do case "$1" in -h --help ) echo "Usage: $0 [-h --help]" exit 0 ;; * ) echo "Usage: $0 [-h --help]" echo "Argument '$1' not recognized" >&2 exit 2 ;; esac shift done echo "Command line processing complete." # rest of script...
6 Unnamed Arguments Sometimes you need one or more data items for a script and want it on the command line, but don't want the user to have to put option letters or names in front of it (e.g. fixmydir dirname1 dirname2) In your command line processing, assign things found on the command line without a leading dash to a variable which stores the list of data items from the command line Then you can examine that variable to see what the user gave you to work on declare -a stufftoprocess while [ $# -gt 0 ]; do case "$1" in -h --help ) echo "Usage: $0 [-h] [stuff...]" exit 0 ;; * ) stufftoprocess+=("$1") ;; esac shift done [ ${#stufftoprocess[@]} ] && echo "Will do work on ${stufftoprocess[@]} (${#stufftoprocess[@]} items)"
7 Practice Modify your cmdargs.sh script from lab2 to display all command line arguments by using a while loop with shift More detail for this is given in the first exercise of lab 4 on the course github site Create a script to use as a starting point for future scripts that have command line arguments See the second exercise of lab 4 o the course github site for more detail on how to do this
8 Functions A function is a named script block, it creates a command you can use elsewhere in your script Inside a function, the positional parameters hold the arguments given when invoking the function instead of the arguments given when the script itself was run Functions end with the status code of the last command to run in the function, data results can be passed back on stdout, or using an intermediary means such as storing data in a file or variable function myfunction { list }
9 Practice Create a function to display a message on stderr which was supplied on the function command line Create a function to display command syntax help for the generic command line processing script Implement the error message function and the command syntax help function in the generic command line processing script wherever they make sense
10 Signals Signal are a way of notifying a process you want it to do something Processes can catch and process or ignore most signals, see signal(7) KILL, STOP, CONT cannot be caught or ignored - STOP/CONT are used to pause/resume processes(jobs) Signals can be sent using the kill command $ kill -SIGNAL pid $ somecommand ^Z $ jobs $ fg %# $ bg %# The shell can send some signals (INT(^C), QUIT(^\), STOP(^Z)) based on keyboard input and manage processes using the jobs, fg, and bg commands
11 Trap In a shell script, catching signals is done with the trap command trap can run a command when a signal is caught, functions are often useful for this function cleanup { rm /tmp/mytemporaryfiles logger -t `basename "$0"` -i -p user.info -s Cleaning up and aborting exit 1 } trap cleanup SIGHUP trap cleanup SIGTERM trap cleanup SIGINT
12 Practice Create a script that starts a countdown, regularly displaying the remaining time See the lab 4 instructions on the course github site for more detail
13 Dialog boxes For more complex user interactions such as choosing files, selecting items from a list, or presenting graphics on text-only terminals, there is a the dialog command dialog can ask for input/decisions or display information dialog is useful when you are working on a terminal and want to present interactions in a more user-friendly way than just displaying text e.g. userpicked=$(dialog --menu "choose one" a 1 b 2 c 3 d 4 e 5 --output-fd 1) (for ((i=0;i<=100;i+=10)) do echo $i;sleep 1;done) dialog --gauge "progress" 7 60;clear foo=$(dialog --rangebox "Pick a value" output-fd 1);clear;echo "You chose $foo" dialog's command line can be inscrutable
14 Practice Modify the countdown script to display the remaining time using a progress bar using the dialog command
15 Script Organization It is helpful to organize nontrivial scripts in a consistent fashion Scripts can be divided into sections Sections may be placed in separate files if those sections are reusable (using the source command) by other scripts, e.g. function definitions Documentation VARIABLE definitions including inline data Aliases and functions Main script commands
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