Jun 18·5 min· PV

Shell Programming

Shell startup and features, command forms, I/O redirection and pipes, shell variables and quoting, writing and running shell scripts, plus a brief intro to sed and awk.

Linux·Shell

Common Commands

read, expr, tput, and the like are external commands on Linux systems, and can be called from any shell script.

read

read reads one line from standard input and assigns it to variables. It supports several options to control how it reads.

Common options:

-p "prompt" # display a prompt-s          # silent mode, do not echo input-t seconds  # timeout-n chars    # return after reading the given number of characters

read var# assign all input data to var

read var1 var2 var3# the first argument goes to var1, the second to var2, and any remaining arguments to the last variable

read -p "Enter your name: " nameecho "Hello, $name"

If there is no data on standard input when read runs, the program waits for input or gets terminated.

Example:

$ read name age[stdin] yilan 23$ echo "student $name is $age years old"[stdout] student yilan is 23 years old

tput (terminal put) controls the terminal: it can position the cursor, set colors, clear the screen, and more. It connects to the terminal control code database terminfo and reads the control codes for the terminal indicated by the TERM environment variable.

Common uses:

tput clear       # clear the screentput cup row col # move the cursor to the given positiontput bold        # bold texttput smso        # start standout modetput rmso        # end standout modetput sgr0        # reset all attributestput cols        # print the number of terminal columnstput lines       # print the number of terminal lines

Bash Syntax

Common System Variables

$0    # name of the current shell program$1-$9 # the first through ninth command-line arguments$#    # number of command-line arguments (not counting $0)$*    # all command-line arguments$@    # all command-line arguments, each individually double-quoted$$    # process ID (PID) of the current process$?    # exit status of the last command$!    # process ID of the last background process

Bash Special Characters

Bash uses several kinds of brackets and special characters; understanding what they mean and how to use them matters a lot.

Command Substitution

$(command) # backticks, substitute the command's output$(command) # $() syntax, same as backticks and the recommended form

Condition Tests

[ string ]   # traditional condition test, spaces required on both sides[[ string ]] # Bash extended condition test, spaces on both sides, supports pattern matching

Arithmetic

((expr)) # arithmetic evaluation, no need to escape * and /

Grouping and Code Blocks

(command)    # run in a subshell{ command; } # run in the current shell; the semicolon and space after command are required

Conditionals

Note: the brackets in [ condition ] must have spaces on both sides, otherwise the condition test will not run correctly.

Basic syntax for if, else, and elif:

if [ condition ]; then  # runs when the condition is truefi

if [ condition ]; then  # runs when the condition is trueelse  # runs when the condition is falsefi

if [ condition1 ]; then  # runs when condition1 is trueelif [ condition2 ]; then  # runs when condition1 is false but condition2 is trueelse  # runs when all conditions are falsefi

String Tests

-z STRING          # true if the string is empty-n STRING          # true if the string is not emptySTRING1 = STRING2  # true if the two strings are equalSTRING1 != STRING2 # true if the two strings are not equal

Numeric Comparisons

NUM1 -eq NUM2          # true if the two numbers are equalNUM1 -ne NUM2          # true if the two numbers are not equalNUM1 -gt NUM2          # true if NUM1 is greater than NUM2NUM1 -ge NUM2          # true if NUM1 is greater than or equal to NUM2NUM1 -lt NUM2          # true if NUM1 is less than NUM2NUM1 -le NUM2          # true if NUM1 is less than or equal to NUM2[[ $string =~ regex ]] # true if the string matches the regular expression

File Tests

-e FILE # true if the file exists-f FILE # true if the file exists and is a regular file-d FILE # true if the file exists and is a directory-r FILE # true if the file exists and is readable-w FILE # true if the file exists and is writable-x FILE # true if the file exists and is executable

Logical Operators

&&   # logical AND||   # logical OR

Loops

for ((counter = 1; counter <= 5; counter++)); do  echo "Welcome, this is iteration number $counter."done

counter=0# loop until the user enters the number 5while [ $counter -ne 5 ]; do  echo "Enter a number (current count: $counter)"  read input_number  # check whether the input is a number  if ! [[ "$input_number" =~ ^[0-9]+$ ]]; then    echo "Error: Please enter a valid number."    continue # skip the rest of this iteration on invalid input  fi  # assign the input number to the counter  counter=$input_number  # print the current counter value  echo "You entered: $counter"done

echo "Enter a number between 1 and 3:"read numbercase $number in1) echo "You entered one." ;;2) echo "You entered two." ;;3) echo "You entered three." ;;*) echo "You did not enter a number between 1 and 3." ;;esac