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title Linux Command Tutorial: tr
date 2026-09-12 00:00:00 +0000
categories
Technology
tags
Linux
GNU Coreutils
tr
Linux Command Tutorial
draft false
slug linux-tr-tutorial
description Authoritative reference tutorial for tr (GNU Coreutils), detailing character translation, deletion (-d), squeezing (-s), character classes, and POSIX portability.
upstream_suite gnu-coreutils
upstream_version GNU Coreutils 9.11
posix_standard POSIX.1-2024
research_date 2026-09-12

The Linux Command Tutorial series provides rigorous, upstream-verified references for essential system commands across Linux distributions and UNIX-like environments. Each article focuses on a single executable, combining exhaustive option documentation, verified real-world examples, security boundaries, and best practices directly derived from official source documentation and POSIX standards.

1. Introduction

Upstream: GNU Coreutils 9.11 | POSIX: POSIX.1-2024 (with GNU extensions) | Safety Tier: safe-read-only | Scope: text-processing

tr (translate) translates, squeezes, or deletes characters from standard input, writing the results to standard output. It operates strictly on character streams (not file path arguments) as a fast, byte-level transformation filter.

  • Upstream Project & Provenance: Distributed in GNU Coreutils (coreutils).
  • Portability & Standards Baseline: Standardized in IEEE Std 1003.1-2024 (POSIX.1-2024).
  • Target Research Implementation: Audited against GNU Coreutils 9.11 (tr(1)).
  • Applicability & Lifecycle: The standard filter for case conversion, stripping carriage returns, and collapsing whitespace.

2. Syntax and Command Model

2.1 Canonical Synopsis

tr [OPTION]... SET1 [SET2]

2.2 Execution Model

  • tr reads strictly from standard input (stdin). It does not take input file path operands.
  • Translation (SET1 -> SET2): Maps the N-th character in SET1 to the N-th character in SET2. If SET2 is shorter than SET1, SET2 is padded to match the length of SET1 with its last character.

3. Options

3.1 Primary Operational Flags

Short Flag Long Flag Description POSIX Defined
-c, -C --complement Use the complement of SET1. Yes
-d --delete Delete characters in SET1, do not translate. Yes
-s --squeeze-repeats Replace each sequence of a repeated character with a single occurrence. Yes
-t --truncate-set1 First truncate SET1 to length of SET2. Yes

3.2 Character Classes

POSIX character classes are specified enclosed in [: and :]:

  • [:alnum:], [:alpha:], [:digit:], [:lower:], [:upper:], [:space:], [:punct:], [:cntrl:].

4. Basic Usage

4.1 Quick-Reference Cheatsheet Card

Operation Command Notes
Lowercase to uppercase tr '[:lower:]' '[:upper:]' < file.txt Standard POSIX case translation
Uppercase to lowercase tr '[:upper:]' '[:lower:]' < file.txt Standard POSIX lowercase fold
Delete specific characters tr -d '\r' < dos.txt > unix.txt Strips DOS carriage return bytes
Squeeze repeated spaces tr -s ' ' < spaced.txt Collapses consecutive spaces into one
Replace character with newline tr ' ' '\n' < words.txt Splits space-separated list into lines
Keep only alphanumerics tr -cd '[:alnum:]' < input.txt Deletes complement of alphanumeric set
Translate delimiters tr ':' '\t' < /etc/passwd Converts colons to tabs

4.2 Lowercase to Uppercase Conversion

echo "linux command tutorial" | tr '[:lower:]' '[:upper:]'
LINUX COMMAND TUTORIAL

4.3 Deleting Specific Characters

echo "Phone: (555) 123-4567" | tr -d ' ()-'
Phone:5551234567

5. Practical Operations

5.1 Stripping Windows Carriage Returns (\r)

Cleaning up a DOS text file in a pipeline:

cat windows_file.txt | tr -d '\r' > unix_file.txt
  • Deletes every \r character without altering line feeds (\n).

5.2 Squeezing Repeated Whitespace

Normalizing inconsistent multi-space formatting into a single space:

echo "column1    column2        column3" | tr -s ' '
column1 column2 column3

5.3 Extracting Only Alphanumeric Characters (Complement -c)

Purging all non-alphanumeric characters, replacing them with newlines to generate a word frequency list:

tr -c '[:alnum:]' '\n' < article.txt | tr '[:upper:]' '[:lower:]' | sort | uniq -c | sort -nr | head -n 4
  • Uses -c to target everything that is not alphanumeric, mapping it to newlines.

6. Advanced Usage

6.1 Generating Random Cryptographic Passwords

Extracting printable characters from /dev/urandom:

tr -dc 'A-Za-z0-9!@#$%^&*' < /dev/urandom | head -c 24; echo
7b@Wq!9mK#1vL8zP$4xN&2aQ
  • Technical Analysis: -d (delete) combined with -c (complement) strips all bytes except those in the specified character set, producing a clean random token stream.

7. Exit Status, Environment, and Configuration

7.1 Exit Status Codes

Exit Code Meaning
0 Success: character translation completed.
>0 An error occurred (invalid character range, missing SET operand).

8. Safety, Security, and Portability

8.1 No Direct Filename Operands

Important

tr does not accept file paths as arguments. Running tr 'a' 'b' file.txt treats "file.txt" as part of SET2, causing catastrophic translation errors or infinite hangs waiting on stdin. Always supply input via shell redirection (< file.txt) or a pipeline (cat file.txt | tr ...).

9. Best Practices

  1. Always Use POSIX Classes Over Ranges for Case Folding:
    • Guidance: Use tr '[:lower:]' '[:upper:]' instead of tr 'a-z' 'A-Z'.
    • Authoritative Justification: POSIX documentation notes that character classes properly respect locale-specific casing rules.
  2. Use tr -d '\r' for Fast DOS to UNIX Conversion:
    • Guidance: Strip \r using tr -d '\r' rather than installing dos2unix.
    • Authoritative Justification: Standardized in all UNIX systems and operates directly on standard streams.
  3. Use -s Before Line Tokenization:
    • Guidance: Squeeze repeated delimiters (tr -s ' ') before piping into cut -d' '.
    • Authoritative Justification: Prevents empty field proliferation caused by consecutive spaces.

References

  1. GNU Coreutils tr Manual: https://www.gnu.org/software/coreutils/manual/html_node/tr-invocation.html
  2. POSIX.1-2024 tr Specification: The Open Group Base Specifications Issue 8. https://pubs.opengroup.org/onlinepubs/9799919799/utilities/tr.html