Implement cat, ls and wc in Python

Author: Konvaly

implement-shell-tools/cat/cat.py

@@ -0,0 +1,30 @@
+import argparse
+import sys
+
+parser = argparse.ArgumentParser(
+    prog="cat",
+    description="Concatenate files and print on the standard output",
+)
+parser.add_argument("-n", "--number", action="store_true",
+                    help="Number all output lines")
+parser.add_argument("-b", "--number-nonblank", action="store_true",
+                    help="Number non-empty output lines, overrides -n")
+parser.add_argument("files", nargs="+", help="The files to print")
+
+args = parser.parse_args()
+
+counter = 0
+for path in args.files:
+    with open(path, "r") as f:
+        for line in f:
+            if args.number_nonblank:
+                if line.strip("\n") == "":
+                    sys.stdout.write(line)
+                else:
+                    counter += 1
+                    sys.stdout.write(f"{counter:6}\t{line}")
+            elif args.number:
+                counter += 1
+                sys.stdout.write(f"{counter:6}\t{line}")
+            else:
+                sys.stdout.write(line)

implement-shell-tools/ls/ls.py

@@ -0,0 +1,53 @@
+import argparse
+import os
+import sys
+
+parser = argparse.ArgumentParser(prog="ls", description="List directory contents")
+parser.add_argument("-1", dest="one_per_line", action="store_true",
+                    help="List one file per line")
+parser.add_argument("-a", "--all", action="store_true",
+                    help="Do not ignore entries starting with .")
+parser.add_argument("paths", nargs="*", default=["."],
+                    help="The files or directories to list")
+
+args = parser.parse_args()
+
+
+def entries(directory):
+    names = os.listdir(directory)
+    if args.all:
+        names = names + [".", ".."]
+    else:
+        names = [name for name in names if not name.startswith(".")]
+    return sorted(names)
+
+
+files = []
+directories = []
+for path in args.paths:
+    if os.path.isdir(path):
+        directories.append(path)
+    elif os.path.exists(path):
+        files.append(path)
+    else:
+        print(f"ls: cannot access '{path}': No such file or directory",
+              file=sys.stderr)
+
+files.sort()
+directories.sort()
+
+show_headers = len(args.paths) > 1
+printed_anything = False
+
+for path in files:
+    print(path)
+    printed_anything = True
+
+for directory in directories:
+    if show_headers:
+        if printed_anything:
+            print()
+        print(f"{directory}:")
+    for name in entries(directory):
+        print(name)
+    printed_anything = True

implement-shell-tools/wc/wc.py

@@ -0,0 +1,48 @@
+import argparse
+import os
+
+parser = argparse.ArgumentParser(prog="wc", description="Print newline, word and byte counts")
+parser.add_argument("-l", "--lines", action="store_true", help="Print the newline counts")
+parser.add_argument("-w", "--words", action="store_true", help="Print the word counts")
+parser.add_argument("-c", "--bytes", action="store_true", help="Print the byte counts")
+parser.add_argument("files", nargs="+", help="The files to count")
+
+args = parser.parse_args()
+
+show_all = not (args.lines or args.words or args.bytes)
+show_lines = args.lines or show_all
+show_words = args.words or show_all
+show_bytes = args.bytes or show_all
+
+rows = []
+total = [0, 0, 0]
+
+for path in args.files:
+    with open(path, "rb") as f:
+        data = f.read()
+    counts = [data.count(b"\n"), len(data.split()), len(data)]
+    for i in range(3):
+        total[i] += counts[i]
+    rows.append((counts, path))
+
+if len(args.files) > 1:
+    rows.append((total, "total"))
+    width = len(str(sum(os.path.getsize(path) for path in args.files)))
+else:
+    width = 1
+
+
+def selected(counts):
+    chosen = []
+    if show_lines:
+        chosen.append(counts[0])
+    if show_words:
+        chosen.append(counts[1])
+    if show_bytes:
+        chosen.append(counts[2])
+    return chosen
+
+
+for counts, label in rows:
+    columns = " ".join(f"{value:{width}}" for value in selected(counts))
+    print(f"{columns} {label}")