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Bash Shell Tips

For Lab 2 - Investigation 3 Part 3​

Data Input​

  • A shell can obtain data from a number of methods: reading input files, using arguments when issuing command (positional parameters), or prompting for data to store in a variable. The later method can be accomplished by using the read command.
  • Example:
read -p "Enter your name: " userName.

Mathematical Expressions​

  • In the bash shell, data is stored in variable as text, not other data types (ints, floats, chars, etc) like in compiled programs like C or Java. In order to have a shell perform mathematical operations, number or variable need to be surrounded by two sets of parenthesis ((..)) in order to convert a number stored as text to a binary number.
  • Examples
var1=5;var2=10
echo "$var1 + $var2 = $((var1+var2))"

Note: shell does not perform floating point calculations (like 5/10). Instead, other commands like awk or bc would be required for floating point calculations (decimals)

For Lab 3 - Investigation 3 Part 3​

Using sed to Manipulate Text​

  • The Linux command sed stands for Streaming Editor which is an effective way to manipulate a text file, output sent from a command, or from within a "here document". This command can manipulate matching text on a variety of criteria (such as line number(s), regular expression match, etc). Commands can then be used for manipulation such as omitting, printing, substituting, adding, and inserting text.
  • The sed option -n suppresses display of text so the print (p) command can be used; otherwise, the text will be displayed (with edits via the sed command instructions).
  • Results of text manipulation with sed can be stored in a variable using command substitution, or redirected to a file. NEVER redirect the stdout from a sed command to the same input file (or the input file will be destroyed)!
  • Examples
sed 's/|/ /g' <<+
I|like|weekends!
+
sed 's/$/\n/g' <<+
This text
should be
double-spaced!
+