Write Output of Bash Command to Log File – Example

Write output of a Bash command to a log file

To write the output of a Bash command to a log file, use redirection operators such as > and >>. Use > when you want to create or overwrite a file. Use >> when you want to append new output at the end of an existing log file.

This is useful in shell scripts where you want to keep a record of command output, timestamps, status messages, or errors for later checking.

Difference between > and >> for Bash log files

Right angle bracket symbol (>) writes the output of a Bash command to a file. If the file is not present, Bash creates it. If the file is already present, its existing content is overwritten.

Double right angle symbol (>>) appends output to a file. If the file is not present, Bash creates it. If the file is already present, new output is added after the existing content.

Operator	What it does	Common use in logging
>	Creates or overwrites a file	Start a fresh log file at the beginning of a script
>>	Appends to a file	Add more log lines without deleting previous lines
2>	Redirects error output only	Save command errors in a log file
&>	Redirects standard output and errors	Capture complete command output in Bash
2>&1	Sends errors to the same destination as output	Log both normal output and errors in portable shell style

When you are writing to the file for the first time and want no previous data to remain, use >. Later in the same script, use >> to add more lines to the same log file.

Bash script example: write command output to log_file.txt

Following is an example to demonstrate writing output of Bash Commands to Log File.

Bash Script File

#!/bin/bash

log=log_file.txt

# create log file or overrite if already present
printf "Log File - " > $log

# append date to log file
date >> $log

x=$(( 3 + 1 ))
# append some data to log file
echo value of x is $x >> $log

When you run the above bash script, following file would be created with the content specified.

log_file.txt

Log File - Fri Dec 15 11:47:03 IST 2017
value of x is 4

Append multiple Bash command outputs to the same log file

The following script starts with a new log file and then appends several lines. Quoting "$log" is a safer habit because it still works if the file path contains spaces.

#!/bin/bash

log="app.log"

echo "Script started" > "$log"
date >> "$log"
whoami >> "$log"
pwd >> "$log"
echo "Script finished" >> "$log"

If app.log already exists, the first echo command overwrites it. The later commands append to it.

Write both command output and errors to a Bash log file

Many commands produce two kinds of output: standard output and standard error. Normal redirection with > captures only standard output. To capture errors also, redirect standard error.

#!/bin/bash

log="command.log"

ls /tmp /path/that/does/not/exist > "$log" 2>&1

In the command above, > "$log" sends standard output to the file, and 2>&1 sends standard error to the same destination.

Bash also supports the shorter &> form for redirecting both standard output and standard error.

command &> "$log"

To append both standard output and errors instead of overwriting the file, use this form:

command >> "$log" 2>&1

Show Bash command output on console and save it to a log file with tee

Use tee when you want to see command output in the terminal and save the same output to a log file. By default, tee overwrites the file. Add -a to append.

#!/bin/bash

log="build.log"

echo "Build started" | tee "$log"
date | tee -a "$log"
echo "Build completed" | tee -a "$log"

To capture both output and errors while still printing them on the console, redirect errors before passing output to tee.

command 2>&1 | tee -a "$log"

Redirect all output from a Bash script to one log file

Instead of adding >> "$log" to every command, you can redirect all later standard output and errors once using exec. This is useful for longer scripts.

#!/bin/bash

log="script.log"

exec > "$log" 2>&1

echo "Script started"
date
ls /tmp
echo "Script finished"

After the exec line, output from later commands is written to script.log. If you want to append instead of overwrite, use exec >> "$log" 2>&1.

Add timestamps to each Bash log message

For script messages, a small logging function keeps the format consistent. The function below writes a timestamp and message to the same log file.

#!/bin/bash

log="app.log"

write_log() {
  printf '[%s] %s\n' "$(date '+%Y-%m-%d %H:%M:%S')" "$1" >> "$log"
}

write_log "Script started"
write_log "Processing input file"
write_log "Script completed"

This pattern is useful when you want each log entry to be easy to scan and sort by time.

Create a log directory before writing Bash output

If the log file is inside a directory that may not exist, create the directory before writing the file. Use mkdir -p so the command does not fail when the directory is already present.

#!/bin/bash

log_dir="logs"
log="$log_dir/app.log"

mkdir -p "$log_dir"

echo "Log file created" > "$log"

Common mistakes while writing Bash output to log files

• Using > when append is required: > overwrites the file. Use >> to keep previous log lines.
• Forgetting to capture errors: command > log.txt does not capture standard error. Use 2>&1 when you need both output and errors.
• Leaving log paths unquoted: prefer "$log" instead of $log.
• Writing to a missing directory: create the log directory first with mkdir -p.
• Overwriting logs during repeated runs: use a timestamped filename or append mode if old logs must be preserved.

Bash log file redirection FAQ

QA checklist for Bash command output logging tutorial

• Verify that examples clearly distinguish overwrite redirection > from append redirection >>.
• Confirm that examples capturing errors include 2>&1 or another explicit standard error redirection.
• Check that new Bash examples quote log file variables such as "$log".
• Confirm that tee examples mention whether they overwrite or append.
• Make sure output-only blocks use the output class, and syntax-only examples use language-bash syntax.

Conclusion: choosing the right Bash log file method

In this Bash Tutorial, we learned how to write the output of a Bash command to a file. Use > for a fresh log, >> for appending, 2>&1 for errors, tee for console plus file output, and exec when an entire script should write to a log file.