Shifting arguments (shift) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the shift command do in a bash script?

The shift command moves all the positional parameters to the left by one. This means $2 becomes $1, $3 becomes $2, and so on. It helps process arguments one by one.

intermediate

How do you shift by more than one argument at a time?

You can pass a number to shift, like shift 2, to move the positional parameters left by that many places at once.

intermediate

What happens if you shift more times than the number of arguments?

If you shift more than the number of arguments, the positional parameters become empty. Trying to access them will give empty values.

beginner

Why is shift useful in scripts that process command-line arguments?

It lets you handle each argument one at a time in a loop. After processing $1, you use shift to move to the next argument easily.

beginner

Example: What will be the output of this script?

#!/bin/bash
while [ "$#" -gt 0 ]; do
  echo "Argument: $1"
  shift
 done

The script prints each argument on its own line, one by one, until no arguments remain. For example, if run as ./script.sh apple banana cherry, it outputs:
Argument: apple Argument: banana Argument: cherry

What does shift do in a bash script?

A Prints all arguments

B Deletes all arguments

C Moves positional parameters left by one

D Reverses the order of arguments

shift moves all positional parameters left by one, so $2 becomes $1, etc.

How do you shift by two arguments at once?

A shift 2

B shift -2

C shift twice

D shift --two

You pass the number to shift, like shift 2, to move two arguments at once.

What happens if you shift more times than the number of arguments?

A Script crashes

B Positional parameters become empty

C Arguments repeat

D Nothing changes

Shifting beyond the number of arguments empties the positional parameters.

Why use shift in a loop processing arguments?

A To count arguments

B To skip arguments

C To print arguments backwards

D To process arguments one by one

shift helps move through arguments one at a time in a loop.

If a script starts with $1 = apple and runs shift, what is $1 now?

A The original $2

B apple

C empty

D The original $3

After shift, $1 becomes what was originally $2.

Explain how the shift command helps in processing command-line arguments in bash scripts.

Think about how you can handle each argument step-by-step.

Describe what happens when you use shift 3 in a script with five arguments.

Consider how the numbering of arguments changes after shifting.

Practice

1. What does the shift command do in a bash script?

easy

A. It moves all command-line arguments one position to the left, dropping the first argument.

B. It duplicates the first argument to the end of the list.

C. It reverses the order of all command-line arguments.

D. It prints all command-line arguments without changes.

Solution

1. Step 1: Understand the role of shift

   The shift command removes the first argument and shifts all others left.

2. Step 2: Effect on arguments

   After shift, the second argument becomes the first, the third becomes second, and so on.

3. Final Answer:

   It moves all command-line arguments one position to the left, dropping the first argument. -> Option A

4. Quick Check:

   shift drops first argument [OK]

Hint: Remember: shift drops first argument and moves others left [OK]

Common Mistakes:

• Thinking shift duplicates arguments
• Assuming shift reverses arguments
• Believing shift prints arguments unchanged

2. Which of the following is the correct syntax to shift two arguments in a bash script?

easy

A. shift --2

B. shift -2

C. shift 2

D. shift 1 1

Solution

1. Step 1: Recall shift syntax

   The correct syntax to shift by a number is shift N, where N is a positive integer.

2. Step 2: Identify valid option

   shift 2 shifts by two arguments; negative or multiple numbers are invalid.

3. Final Answer:

   shift 2 -> Option C

4. Quick Check:

   Use shift N with positive N [OK]

Hint: Use 'shift N' with positive number N to shift multiple args [OK]

Common Mistakes:

• Using negative numbers with shift
• Trying to pass multiple numbers
• Using invalid flags like --2

3. What will be the output of this script if run as ./script.sh apple banana cherry?

echo $1
shift
echo $1

medium

A. apple\napple

B. banana\ncherry

C. banana\napple

D. apple\nbanana

Solution

1. Step 1: Check initial $1 value

   Before shift, $1 is the first argument: 'apple'.

2. Step 2: Apply shift and check new $1

   After shift, $1 becomes the second argument: 'banana'.

3. Final Answer:

   apple\nbanana -> Option D

4. Quick Check:

   First echo prints first arg, second echo prints second arg after shift [OK]

Hint: First echo shows $1, shift moves args left, second echo shows new $1 [OK]

Common Mistakes:

• Assuming shift does not change $1
• Confusing argument positions after shift
• Expecting output to be 'banana\ncherry'

4. Identify the error in this script snippet:

shift 3
echo $1

Assuming the script is run with only two arguments.

medium

A. Error: shift 3 tries to shift more arguments than exist, causing an error.

B. No error; it prints the first argument unchanged.

C. Error: shift requires a negative number to work.

D. No error; it prints the third argument.

Solution

1. Step 1: Check number of arguments

   The script has only two arguments but tries to shift by three.

2. Step 2: Understand shift behavior on excess shift

   Shifting more than available arguments causes an error or empties all arguments.

3. Final Answer:

   Error: shift 3 tries to shift more arguments than exist, causing an error. -> Option A

4. Quick Check:

   Shifting beyond argument count causes error [OK]

Hint: Don't shift more than available arguments to avoid errors [OK]

Common Mistakes:

• Assuming shift works with negative numbers
• Ignoring argument count before shifting
• Expecting shift to print unchanged arguments

5. You want to process all command-line arguments one by one in a bash script using shift. Which script snippet correctly does this?

hard

A. for arg in "$@"; do shift; echo "$arg"; done

B. while [ "$#" -gt 0 ]; do echo "$1"; shift; done

C. shift; while [ "$#" -gt 0 ]; do echo "$1"; done

D. while [ "$#" -ge 1 ]; do echo "$2"; shift 2; done

Solution

1. Step 1: Understand argument count and shifting

   Use while [ "$#" -gt 0 ] to loop while arguments remain.

2. Step 2: Print first argument and shift each iteration

   Inside loop, print $1 then shift to move to next argument.

3. Final Answer:

   while [ "$#" -gt 0 ]; do echo "$1"; shift; done -> Option B

4. Quick Check:

   Loop with shift processes all args one by one [OK]

Hint: Use while loop with $# and shift to process args one by one [OK]

Common Mistakes:

• Using for loop with shift inside causing skipped args
• Not updating argument count in loop condition
• Shifting by 2 causing missed arguments