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Bash
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How to implement common bash idioms in Python?

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Python

Introduction

Many tasks commonly done in Bash — listing files, piping commands, reading environment variables, text processing — have cleaner and more portable equivalents in Python. Python's os, pathlib, subprocess, and shutil modules cover most of what Bash scripts do, with better error handling and cross-platform support. This article maps the most common Bash idioms to their Python equivalents.

Listing and Iterating Over Files

bash
1# Bash: loop over files matching a pattern
2for f in *.txt; do
3    echo "$f"
4done
python
1# Python: pathlib (recommended)
2from pathlib import Path
3
4for f in Path(".").glob("*.txt"):
5    print(f)
6
7# Python: os module
8import os
9for f in os.listdir("."):
10    if f.endswith(".txt"):
11        print(f)
12
13# Recursive glob
14for f in Path(".").rglob("*.txt"):
15    print(f)

Running Shell Commands

bash
# Bash: run a command and capture output
output=$(curl -s https://example.com)
python
1import subprocess
2
3# Run and capture output
4result = subprocess.run(
5    ["curl", "-s", "https://example.com"],
6    capture_output=True, text=True, check=True
7)
8output = result.stdout
9
10# Pipe between commands (equivalent to: ps aux | grep python)
11ps = subprocess.Popen(["ps", "aux"], stdout=subprocess.PIPE)
12grep = subprocess.Popen(["grep", "python"], stdin=ps.stdout, stdout=subprocess.PIPE, text=True)
13ps.stdout.close()
14output = grep.communicate()[0]

Environment Variables

bash
1# Bash
2export APP_ENV=production
3echo "$APP_ENV"
4echo "${APP_ENV:-default_value}"
python
1import os
2
3# Get with default
4env = os.environ.get("APP_ENV", "default_value")
5
6# Set
7os.environ["APP_ENV"] = "production"
8
9# Check existence
10if "APP_ENV" in os.environ:
11    print(os.environ["APP_ENV"])

File Tests (Existence, Type, Size)

bash
1# Bash
2[ -f "file.txt" ] && echo "File exists"
3[ -d "mydir" ] && echo "Directory exists"
4[ -s "file.txt" ] && echo "File is non-empty"
python
1from pathlib import Path
2
3p = Path("file.txt")
4
5if p.is_file():
6    print("File exists")
7
8if Path("mydir").is_dir():
9    print("Directory exists")
10
11if p.exists() and p.stat().st_size > 0:
12    print("File is non-empty")

Reading and Writing Files

bash
1# Bash: read file line by line
2while IFS= read -r line; do
3    echo "$line"
4done < file.txt
5
6# Bash: write to file
7echo "hello" > output.txt
8echo "world" >> output.txt
python
1# Python: read line by line
2with open("file.txt") as f:
3    for line in f:
4        print(line.rstrip())
5
6# Python: write (overwrite)
7with open("output.txt", "w") as f:
8    f.write("hello\n")
9
10# Python: append
11with open("output.txt", "a") as f:
12    f.write("world\n")

Text Processing (grep, sed, awk)

bash
1# Bash: grep
2grep "error" logfile.txt
3grep -c "error" logfile.txt   # count matches
4
5# Bash: sed substitution
6sed 's/old/new/g' file.txt
python
1import re
2
3# grep equivalent
4with open("logfile.txt") as f:
5    matches = [line for line in f if "error" in line]
6    print(f"Found {len(matches)} matches")
7
8# sed equivalent (regex substitution)
9with open("file.txt") as f:
10    content = f.read()
11modified = re.sub(r"old", "new", content)
12
13# awk: print specific columns
14with open("data.txt") as f:
15    for line in f:
16        cols = line.split()
17        if len(cols) >= 3:
18            print(cols[0], cols[2])  # print columns 1 and 3

Copying, Moving, and Deleting Files

bash
1cp source.txt dest.txt
2mv old.txt new.txt
3rm file.txt
4rm -rf mydir/
python
1import shutil
2from pathlib import Path
3
4shutil.copy2("source.txt", "dest.txt")   # cp (preserves metadata)
5shutil.move("old.txt", "new.txt")         # mv
6Path("file.txt").unlink(missing_ok=True)  # rm
7shutil.rmtree("mydir")                    # rm -rf

Command-Line Arguments

bash
# Bash: $1, $2, $@
echo "First arg: $1"
echo "All args: $@"
python
1import sys
2
3# sys.argv
4print(f"First arg: {sys.argv[1]}")
5print(f"All args: {sys.argv[1:]}")
6
7# argparse (for real scripts)
8import argparse
9parser = argparse.ArgumentParser()
10parser.add_argument("--name", required=True)
11parser.add_argument("--verbose", action="store_true")
12args = parser.parse_args()
13print(args.name)

Exit Codes and Error Handling

bash
# Bash
command || { echo "Failed"; exit 1; }
set -e  # Exit on any error
python
1import subprocess
2import sys
3
4try:
5    subprocess.run(["command"], check=True)
6except subprocess.CalledProcessError as e:
7    print(f"Failed with exit code {e.returncode}")
8    sys.exit(1)

Common Pitfalls

  • Using os.system() instead of subprocess.run(): os.system() does not capture output, does not raise on failure, and is vulnerable to shell injection. Always use subprocess.run() with a list of arguments.
  • Using shell=True unnecessarily: subprocess.run("ls -l", shell=True) invokes the shell and is a security risk if the command includes user input. Use subprocess.run(["ls", "-l"]) (list form) instead.
  • Forgetting text=True in subprocess: Without text=True, subprocess.run returns bytes (b"output") instead of strings. Add text=True (or encoding="utf-8") for string output.
  • Using string concatenation for file paths: base + "/" + name breaks on Windows. Use Path(base) / name or os.path.join(base, name) for portable paths.
  • Not handling file encoding: Bash reads files as bytes by default. Python 3 opens files as UTF-8 (on most systems). If the file uses a different encoding, pass encoding="latin-1" or the correct encoding to open().

Summary

  • Use pathlib.Path for file operations — it replaces ls, test -f, mkdir, and path manipulation
  • Use subprocess.run() with list arguments instead of os.system() for running commands
  • Use os.environ.get() with a default value instead of ${VAR:-default}
  • Use re.sub() for sed-like substitutions and list comprehensions for grep-like filtering
  • Use shutil for cp, mv, and rm -rf equivalents
  • Use argparse for command-line argument parsing instead of sys.argv for real scripts
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