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๐Ÿง  Parameter Expansion

Parameter expansion is one of the most powerful features of POSIX shells. It enables variable substitution, default values, substring extraction, pattern removal, and error handling โ€” all without invoking external tools.

๐ŸŽ“ Who This Is For

  • Engineers writing productionโ€‘grade shell scripts
  • DevOps/SRE teams optimizing automation performance
  • Developers replacing external tools (sed, cut, awk) with builtโ€‘in expansions
  • Anyone needing predictable, portable string manipulation in POSIX shells

๐Ÿงฉ Role in the Ecosystem

Parameter expansion is a core part of the POSIX shell execution model. It provides:

  • Fast, builtโ€‘in string manipulation
  • Portable alternatives to external commands
  • Error handling for required variables
  • Safe defaults for unset or empty parameters
  • A foundation for writing robust scripts

It is supported by all POSIX shells (dash, ash, ksh, bash in POSIX mode).

๐Ÿงฉ Key Concepts

1. Default and Fallback Values

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echo "${VAR:-fallback}"   # use fallback if VAR is unset or empty
echo "${VAR:=default}"    # assign default if VAR is unset or empty

2. Required Variables

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: "${TOKEN:?TOKEN is required}"

3. Substring Extraction

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name="robert"
echo "${name:0:3}"        # rob
echo "${name:2}"          # bert

4. String Length

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echo "${#name}"           # 6

5. Pattern Removal

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path="/var/log/nginx/access.log"
echo "${path%/*}"         # /var/log/nginx
echo "${path##*/}"        # access.log

6. Indirection (bash/ksh)

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var="TARGET"
TARGET="value"
echo "${!var}"            # value

๐Ÿ”ง Notable Techniques

Prefix/Suffix Removal

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file="backup_2025.tar.gz"
echo "${file#backup_}"    # 2025.tar.gz
echo "${file%.gz}"        # backup_2025.tar

Patternโ€‘Based Replacement (bash/ksh)

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text="a b c"
echo "${text// /_}"       # a_b_c

Safe Variable Expansion

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echo "${VAR:-}"           # expands to empty string if unset

Numeric Expansion (bash/ksh)

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num=5
echo "$(( num + 1 ))"     # 6

โš ๏ธ Limitations & Pitfalls

  • Some expansions are not POSIX (e.g., ${var//pattern/repl})
  • Dash/ash do not support substring syntax ${var:offset:length}
  • Indirection ${!var} is bash/kshโ€‘only
  • Pattern matching uses shell globs, not regex
  • Quoting rules can change expansion behavior
  • Expansion inside double quotes preserves whitespace; unquoted does not

๐Ÿง  When to Use Parameter Expansion

  • Replacing external tools for performance
  • Writing portable /bin/sh scripts
  • Validating required variables
  • Manipulating paths, filenames, and strings
  • Ensuring predictable behavior in CI/CD pipelines

โŒ When Not to Use Parameter Expansion

  • When complex parsing is required (use awk or a real language)
  • When portability across nonโ€‘POSIX shells is uncertain
  • When readability suffers due to overly dense expansions

โœ… Best Practices

  • Prefer parameter expansion over external tools for simple operations
  • Use ${var:?message} for required variables
  • Use ${var:-} to avoid unbound variable errors
  • Quote expansions unless intentional:
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echo "$var"
  • Avoid bashโ€‘only expansions in /bin/sh scripts
  • Test expansions under multiple shells (bash, dash, ash, ksh)

๐Ÿงช Testing Parameter Expansion

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set -x
VAR=""
echo "${VAR:-fallback}"
echo "${VAR:=default}"
echo "${#VAR}"

Test portability:

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dash script.sh
bash --posix script.sh
ksh script.sh

๐Ÿง  Summary

Parameter expansion is a fast, portable, and powerful mechanism for manipulating variables in POSIX shells. It replaces many external tools, improves performance, and enables robust error handling โ€” making it essential for productionโ€‘grade scripting.