P2-7.8 Supplementary Learning: Reading Shell Execution Flow¶

Section ID: P2-7.8 Version: v2026.09.15

In a shell, | connects one command’s output to the next command’s input. > and < connect output and input to files, while environment variables pass configuration values to programs. The shell commands below use Bash and assume Python is available as python.

Standard Input and Standard Output¶

Command-line programs can receive data through standard input and send results through standard output. In direct terminal use, these usually connect to the keyboard and screen, but can be redirected to files or other programs.

Save the following code as read_numbers.py. It reads one number per input line and prints the sum. Input 10, 20, 30 produces 60.

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import sys

# Read integers line by line from standard input and sum them.
numbers = [int(line) for line in sys.stdin if line.strip()]
print(sum(numbers))

In Bash, create the input file numbers.txt using the following command. printf converts \n into a newline.

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printf '10\n20\n30\n' > numbers.txt

Connecting Commands with a Pipe¶

cat sends file contents to standard output. Connecting it with | makes those contents the Python program’s standard input.

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cat numbers.txt | python read_numbers.py

The output is 60. The programs exchange data directly, without displaying the file for a person to retype. The receiving program must be written to read standard input for this connection to be useful.

A pipe does not simply mean execute commands in sequence. It connects the first command’s standard output to the next command’s standard input. Standard error, used for error messages, is not included by default.

Connecting Files to Input and Output¶

< connects the same file to Python’s standard input without using cat.

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python read_numbers.py < numbers.txt

Add > to save the result to total.txt instead of the screen.

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python read_numbers.py < numbers.txt > total.txt

On success, the sum is not displayed on screen; total.txt contains 60 and a newline. Check it with cat total.txt.

python train.py > train.log likewise saves only standard output. Do not assume all error messages also go into that file. File paths are interpreted relative to the working directory.

Saving Commands as a Shell Script¶

Save commands to repeat in run_summary.sh.

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python read_numbers.py < numbers.txt > total.txt
cat total.txt

From the folder containing the three files, run this command. Bash reads the script, saves the sum, and prints it.

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bash run_summary.sh

A shell script is a command file interpreted by a shell. The interpreting program differs from that used for a .py file containing Python code.

Passing Settings with Environment Variables¶

An environment variable is a name-value pair passed when a program runs. For example, it can specify a data folder outside the code.

Run this command in Bash.

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export BOOK_DATA_DIR="./data"

export passes the value to child processes subsequently started by this shell. It does not change the environment of other terminals already running.

Save this code in show_config.py and run python show_config.py in the same shell to print ./data.

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import os

# Print the fallback when the environment variable is not set.
print(os.environ.get("BOOK_DATA_DIR", "not set"))

In Windows PowerShell, set the environment variable as follows.

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$env:BOOK_DATA_DIR = "./data"
python show_config.py

Environment variable values are strings. Setting a folder path does not create the folder. Secrets such as API keys can also be passed this way, but must not be left in code or output logs.

Changing Numbers and Output Storage¶

Changing the last number in numbers.txt from 30 to 40 changes the sum from 60 to 70. Run this command to leave only the new sum 70 in total.txt.

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python read_numbers.py < numbers.txt > total.txt

With the same input, change > to >> and run again; the file now contains two lines of 70. The calculation code is unchanged; only the shell’s output connection differs.

Checking What a Command Changes¶

PowerShell pipelines can also pass objects between commands, and their syntax is not identical to Bash. In particular, do not copy the < input redirection above directly into PowerShell.

Checking Error Output and Exit Status¶

A nonnumeric line causes failure before a sum is produced. The following Bash commands create a separate input file and send results and errors to different files.

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printf '10\noops\n' > invalid-numbers.txt
python read_numbers.py < invalid-numbers.txt > invalid-total.txt 2> errors.log
printf 'exit=%s\n' "$?"
cat errors.log

Immediately after Python runs, $? holds the preceding command’s exit status: 1 in this example. errors.log contains a traceback with ValueError, and invalid-total.txt is empty. An empty result file is not a sum of zero. Another command changes $?, so check it immediately after the command being diagnosed.

The > redirection empties the output file before the program runs. Using the same file for input and output can therefore destroy the input; keep those paths separate. A Bash pipeline’s status normally comes from its final command, so success of that command does not establish success of earlier commands.

Checklist¶

• You can distinguish the programs that interpret shell scripts and Python scripts.
• You can explain that | connects standard output to standard input.
• You can distinguish file input, overwriting, and appending with <, >, and >>.
• You can explain that standard output and standard error are separate streams.
• You can read a setting passed through an environment variable in Python.
• You can check how file contents change when input numbers or output connections change.

Sources and References¶

• GNU Project, Bash Reference Manual, GNU Bash 5.3 manual, checked 2026-07-20. Used to confirm Bash's shell role, pipelines, redirection, and variable/environment-variable syntax.
• Microsoft Learn, about_Pipelines, PowerShell 7.6 documentation, checked 2026-07-20. Used to confirm that | is a pipeline operator that sends results from one command to the next in PowerShell.
• Microsoft Learn, about_Redirection, PowerShell 7.6 documentation, checked 2026-07-20. Used to confirm that redirection operators such as >, >>, and n> send or append output streams to files in PowerShell.
• Microsoft Learn, about_Environment_Variables, PowerShell 7.6 documentation, checked 2026-07-20. Used to confirm that environment variables are string configuration values used by the operating system and programs and can be inherited by child processes.