Extracting Substrings by Delimiter in Bash

The cut command is designed for simple field splitting, but it has a hard limitation: the delimiter must be a single character.

cut --delimiter="delim" -f 1,3
# Error: cut: the delimiter must be a single character

This is a deliberate design choice. cut prioritizes speed and simplicity for common use cases like splitting on tabs, spaces, or colons. If you need multi-character delimiters, you need a different tool.

Solution: Use awk for Multi-Character Delimiters

The most practical solution is awk, which treats the -F option as a full delimiter pattern rather than a single character.

awk -F 'delim' '{print $1, $3}' input.txt

This splits each line on the string delim and prints fields 1 and 3.

More Realistic Example

If you’re parsing a file with ::|:: as a delimiter:

awk -F '::\|::' '{print $1; print $3}' data.txt

Note the escaped pipe character — awk’s -F interprets the argument as an extended regular expression, so special regex characters need escaping.

Using FPAT for Complex Patterns

For more sophisticated field matching, use the FPAT variable to define what fields are rather than what separates them. This is cleaner when dealing with quoted fields or nested delimiters:

awk 'BEGIN {FPAT = "[a-zA-Z0-9_]+"} {print $1, $3}' input.txt

This extracts contiguous alphanumeric sequences as fields, ignoring any delimiter structure entirely.

FPAT is especially useful for CSV-like data with quoted fields:

awk 'BEGIN {FPAT = "([^,]+)|(\"[^\"]+\")"} {print $1, $3}' data.csv

sed as an Alternative

For simpler cases, sed can work too:

sed 's/delim/\n/g' input.txt | head -n 3 | tail -n 1

This converts the delimiter to newlines and extracts specific lines, but it’s less elegant for multiple fields.

perl for Maximum Flexibility

If you’re already using perl in your scripts, it handles multi-character delimiters natively:

perl -lane 'print $F[0], " ", $F[2]' -F 'delim' input.txt

The -a flag autosplits on the delimiter specified with -F, and @F contains the resulting fields.

Practical Comparison

Tool	Multi-char Delimiter	Speed	Complexity
cut	No	Fast	Low
awk	Yes (regex)	Good	Low-Medium
sed	Yes (via regex)	Good	Medium
perl	Yes	Good	Medium-High

For most cases, awk is the right choice: it’s universally available, handles complex delimiters, and the syntax is straightforward for field extraction.

Practical Tips and Common Gotchas

When working with programming languages on Linux, environment management is crucial. Use version managers like asdf, pyenv, or sdkman to handle multiple language versions without system-wide conflicts. Always pin dependency versions in production to prevent unexpected breakage from upstream changes.

For build automation, modern alternatives often outperform traditional tools. Consider using just or task instead of Make for simpler task definitions. Use containerized build environments to ensure reproducibility across different development machines.

Debugging Strategies

Start with the simplest debugging approach and escalate as needed. Print statements and logging often reveal the issue faster than attaching a debugger. For complex issues, use language-specific debuggers like gdb for C and C++, jdb for Java, or dlv for Go. Always check error messages carefully before diving into code.

Quick Verification

After applying the changes described above, verify that everything works as expected. Run the relevant commands to confirm the new configuration is active. Check system logs for any errors or warnings that might indicate problems. If something does not work as expected, review the steps carefully and consult the official documentation for your specific version.