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In this lesson, we’ll explore how the shebang (#!) directive affects script execution and portability. You will learn to:
  • Remove the shebang from a script and analyze its behavior.
  • Trace system calls with strace to observe kernel execution.
  • Demonstrate shebangs across different shells (e.g., Bash, C shell).
  • Adopt a modern, portable shebang and avoid common pitfalls.
The image is a slide titled "Shebang" with a checklist of three items related to modifying and recommending shebangs in scripts.

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The Shebang Analogy

Think of yourself as a polyglot translator facing an ancient manuscript. Each dialect (shell) has subtle differences. A shebang acts like a translator’s guide, ensuring your script is read by the intended interpreter. Without it, your login shell takes over, which may lead to unexpected behavior and portability issues. Example of a classic shebang: 
#!/bin/bash
The term shebang blends “hash” (#, 0x23) with “bang” (!, 0x21), sometimes pronounced “shbang.”

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What Happens Without a Shebang?

Create noshebang.sh without a shebang: 
# noshebang.sh
WORDS="I don't have a shebang and I still run"
for word in ${WORDS}; do
  if [[ 2 < 3 ]]; then
    echo "${word}"
  fi
done
Make it executable and run: 
chmod +x noshebang.sh
./noshebang.sh
# Each word prints because Bash (your login shell) interprets it
echo $SHELL
# /bin/bash
Relying on the parent shell works locally but isn’t portable—other environments may default to /bin/sh, which lacks Bash-specific features.

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Tracing Kernel Execution with strace

Compare a script without and with a shebang: 
# shebang.sh
#!/bin/bash
WORDS="I don't have a shebang and I still run"
for word in ${WORDS}; do
  if [[ 2 < 3 ]]; then
    echo "${word}"
  fi
done
Trace your shell’s PID ($$) in the background: 
sudo strace -Tfp $$ 2>&1 | grep -E 'execve' &
Run both scripts: 
./noshebang.sh
./shebang.sh
# [pid …] execve("./shebang.sh", ["./shebang.sh"], …) = 0
# Script runs successfully under /bin/bash
When the kernel detects #! followed by a valid interpreter, it invokes that program directly.
The image shows the symbols "#" and "!" with their hexadecimal values, 0x23 and 0x21, respectively, under the title "Shebang."

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Using a Different Shell: C Shell Example

C shell (csh) syntax is distinct. Running a C shell script under Bash without a shebang will fail: 
# is_csh.sh
set x = 'a'
if ($x == 'a') then
    echo "running on a c shell csh"
endif

chmod +x is_csh.sh
./is_csh.sh
# -bash: ./is_csh.sh: /bin/csh: syntax error: unexpected end of file
Add the correct shebang: 
#!/bin/csh
set x = 'a'
if ($x == 'a') then
    echo "running on a c shell csh"
endif

./is_csh.sh
# running on a c shell csh
The image illustrates a "Shebang" with a focus on the C shell (csh), featuring a triangle with a hash symbol and the text "CSH" inside, accompanied by a checkmark.

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Modern, Portable Shebang

Hardcoding interpreter paths can break across systems. Instead, use: 
#!/usr/bin/env bash
This invokes env to locate bash via your PATH, enhancing cross-platform compatibility.
Using #!/usr/bin/env bash avoids assumptions about interpreter locations, but it relies on env being in /usr/bin.
Shebang LineDescriptionPros & Cons
#!/bin/bashDirect path to BashFast invocation, but not portable if Bash is installed elsewhere.
#!/usr/bin/env bashFinds Bash in PATH via envPortable across environments, depends on a correct PATH.

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Demo: Bash Version Features

Create bash_versions.sh: 
#!/usr/bin/env bash
echo "Current Unix timestamp (integer): ${EPOCHSECONDS}"
echo "Current Unix timestamp (floating-point): ${EPOCHREALTIME}"
On macOS default Bash (v3): 
bash --version
./bash_versions.sh
# Current Unix timestamp (integer):
# Current Unix timestamp (floating-point):
After upgrading to Bash 5.2 (e.g., via Homebrew) and updating your PATH: 
bash --version
./bash_versions.sh
# Current Unix timestamp (integer): 1679282700
# Current Unix timestamp (floating-point): 1679282700.176761
By leveraging #!/usr/bin/env bash, you automatically use the most appropriate Bash installed on your system.

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Caveats & Recommendations

  • Minimal environments (e.g., BusyBox) may not include bash. Verify available interpreters in /etc/shells: 
    cat /etc/shells
# /bin/sh
# /bin/bash
# /sbin/nologin
…
If /usr/bin/env or your chosen shell isn’t available, scripts will fail. Always confirm interpreter paths before deployment.
  • Select a shebang that aligns with your target environment and installed shells.
For all remaining course examples, we’ll use: 
#!/usr/bin/env bash
Adjust this line if your system requires a different interpreter path.

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