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OPS235 Lab 1 - CentOS7 - SSD

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{{Admon/note|Bash Shell Scripting Reference Guide:| '''She-bang Line'''<ul><li>Forces shell script to run in a specific Shell</li><li>Must be at beginning of first line: eg #!/bin/bash </li></ul>'''Variables'''~~ ~~<blockquote>'''Environment'''<ul><li>System-wide or "global" variable</li><li>Usually appear in UPPERCASE letters</li><li>Can view with command: '''set | more'''</li><li>'''$''' in front to expand variable to value<li>Examples: '''USER''', '''PATH''', '''HOME''', '''SHELL'''~~ ~~</li></ul></blockquote><blockquote>'''User-defined''' <ul><li>Variable created by user (command line, scripting)</li><li>Examples: ''myVar="my value"; readonly myVar; export myVar'' ''read -p "enter value: " myVar''~~ ~~</li></ul></blockquote><blockquote>'''Positional parameters'''<ul><li>Assign values with set command or shell script arguments</li><li>These variables are numbered (eg. $1, $2 ... $10}</li><li>Special parameters: $*, $@, $#, $$, $? </li></ul></blockquote>'''Command Substitution'''<ul><li>Useful method to expand output from a command to be used as an argument for another command.</li><li>Examples: ''file $(ls)'' ''set $(ls);echo $#;echo $*'' ''echo "hostname: $(hostname)"'' </li></ul>'''Logic Control Flow Statements'''<ul><li>'''$?''' variable true (0) if command runs; otherwise is false (non-zero)</li>Example: ''if echo $myVar | grep "match"'' ''then'' ''echo "Match"'' ''fi'' </li><li>The '''test''' command is used to test conditions. Square brackets '''[ ]''' is short-cut for test command (args contained inside with spaces). The '''exit''' command can be used to terminate the shell script with a false value. Example: ''if [ $USER = "root" ]'' ''then''  ''echo "You must be root" ''  ''exit1'' ''fi'' </li><li>For numberic comparison, use the following '''test options''': '''-gt''','''-ge''', '''-lt''', '''-le''', '''-eq''', '''-ne''' Examples: ''if test $age -gt 65 '' ''then''  ''echo "retire"'' ''else''  ''echo "don't retire"'' ''fi'' ''if [ $grade -gt 79 ]'' ''then''  ''echo "You get Good Mark"'' ''elif [ $grade -gt 49 ]'' ''then''  ''echo "You pass"'' ''else''  ''echo "You fail"'' ''fi'' </li><li>For testing for file information, you can use '''-d''' to test if directory pathname exists, and '''-f''' if the file pathname exists. You can use '''!''' for negation. Examples: ''if [ -d directory-pathname ]'' ''then'' ''echo "directory exists"'' ''fi'' ''if [ ! - f file-pathname ]'' ''then'' ''echo "File does not exist"'' ''fi''</li><li>'''Loops (iteration):''' Loops and logic are a very important elements of shell scripting (not to mention programming as well). Determinant loops (such as '''for''' loops) usually repeat for a preset number of times (eg. counts, positional parameters stored). In-determinant loops (such as '''while''' or '''until''' loops) may repeat based on unknown conditions (like waiting for user to enter correct data). Test conditions can be used with in-determinant loops, or even commands! If a command runs successfully (eg ls, cd, grep matching a pattern), zero (true) value is returned, otherwise a non-zero (false) value is returned. Command options or redirection to /'''dev/null''' can be used to just test if command runs, but not display stdout or stderr. Conditional statements "and" (&&) / "or" (||) can also be used when testing multiple conditions. '''Examples (try in a shell script)''' ''set ops235 is fun'' ''for x'' ''do''  ''echo "argument is $x"'' ''done'' ''for x in $(ls)'' ''do''  ''echo "Filename: $x"'' ''done'' ''read -p "enter a whole number: " num'' ''until echo $num | grep -q "^[0-9][0-9]*$"'' ''do''  ''read -p "Incorrect. Please enter WHOLE NUMBER: " num'' ''done'' ''read -p "pick a number between 1 and 10: " num'' ''while [ $num -lt 1 ] || [ $num -gt 10 ] ''do''  ''read -p "Incorrect. Please pick number between 1 and 10: " num'' ''done'' </li></ul>}}

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OPS235 Lab 1 - CentOS7 - SSD

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