envShow the environment, or run a command with a changed one
env prints the environment variables the current process will pass to the programs it starts, or runs one command with variables added, removed or cleared. It's also the /usr/bin/env in '#!/usr/bin/env python3' shebangs.
env [-i] [-u NAME]... [NAME=VALUE]... [COMMAND [ARG]...]You've just SSHed into a server and want to see which variables your session will pass to programs you start.
env/usr/bin/env from coreutils. Without a command it prints its environment, one NAME=value per line.
Every process has an environment: an array of NAME=value strings that the kernel copies into the new process image when a program calls execve(path, argv, envp). It sits at the top of the process's stack and is visible later in /proc/<pid>/environ. env is a tiny program: with no command it walks its environ array and prints it, which shows exactly what your shell exported. With assignments, -u or -i it builds a modified array, then calls execvp so the command replaces env in the same process with that environment. execvp searches PATH, which is why '#!/usr/bin/env python3' finds python3 wherever it is installed. Changing a variable later in a running process (setenv) doesn't update /proc/<pid>/environ, and nothing a child does to its environment ever flows back to its parent. Shell variables that aren't exported aren't in the environment at all, so env doesn't show them.
(no args)Print the environment, one NAME=value per lineNAME=VALUE cmdRun cmd with NAME set or overridden-i, --ignore-environmentStart with an empty environment-u NAMERemove NAME from the command's environment-0End each line with NUL instead of newline (safe parsing)-S 'STRING'Split STRING into arguments (for shebangs with options)-C DIRChange directory before running the commandprintenv NAMEPrint one variable's value (exit 1 if unset)DB_HOST=x; ./app and expecting the app to see DB_HOSTA plain assignment is a shell variable. It isn't copied to child processes unless exported.
export DB_HOST=x; ./app # or for one run: DB_HOST=x ./appPassing secrets as environment variables and assuming they're privateThey're readable in /proc/PID/environ by root and the same user, and they leak into crash dumps, debug pages and child processes.
Read secrets from a 0600 file or a secrets manager at startupenv -i ./app to test a 'clean' runWithout PATH and HOME, the app (or the shell inside it) can't find commands and fails in confusing ways.
env -i HOME=$HOME PATH=/usr/bin:/bin ./appParsing env output to read values that contain newlinesA value with a newline looks like two variables.
env -0 | tr '\0' '\n' # or printenv NAMETry these on a Linux machine of your own. Each task has a hint and a solution.
Print all environment variables sorted by name and count them. (The sandbox shell is zsh, but env works the same.)
Run 'bash -c env' with an empty environment plus only PATH=/usr/bin:/bin and see what's left.
Show the environment your shell process started with by reading /proc.
Run the report script ~/app/worker/report.py with a different data file path without editing it. Its first argument defaults to ~/data/orders.csv; check how its shebang finds python.
envShow the environment, or run a command with a changed one
env prints the environment variables the current process will pass to the programs it starts, or runs one command with variables added, removed or cleared. It's also the /usr/bin/env in '#!/usr/bin/env python3' shebangs.
env [-i] [-u NAME]... [NAME=VALUE]... [COMMAND [ARG]...]You've just SSHed into a server and want to see which variables your session will pass to programs you start.
env/usr/bin/env from coreutils. Without a command it prints its environment, one NAME=value per line.
Every process has an environment: an array of NAME=value strings that the kernel copies into the new process image when a program calls execve(path, argv, envp). It sits at the top of the process's stack and is visible later in /proc/<pid>/environ. env is a tiny program: with no command it walks its environ array and prints it, which shows exactly what your shell exported. With assignments, -u or -i it builds a modified array, then calls execvp so the command replaces env in the same process with that environment. execvp searches PATH, which is why '#!/usr/bin/env python3' finds python3 wherever it is installed. Changing a variable later in a running process (setenv) doesn't update /proc/<pid>/environ, and nothing a child does to its environment ever flows back to its parent. Shell variables that aren't exported aren't in the environment at all, so env doesn't show them.
(no args)Print the environment, one NAME=value per lineNAME=VALUE cmdRun cmd with NAME set or overridden-i, --ignore-environmentStart with an empty environment-u NAMERemove NAME from the command's environment-0End each line with NUL instead of newline (safe parsing)-S 'STRING'Split STRING into arguments (for shebangs with options)-C DIRChange directory before running the commandprintenv NAMEPrint one variable's value (exit 1 if unset)DB_HOST=x; ./app and expecting the app to see DB_HOSTA plain assignment is a shell variable. It isn't copied to child processes unless exported.
export DB_HOST=x; ./app # or for one run: DB_HOST=x ./appPassing secrets as environment variables and assuming they're privateThey're readable in /proc/PID/environ by root and the same user, and they leak into crash dumps, debug pages and child processes.
Read secrets from a 0600 file or a secrets manager at startupenv -i ./app to test a 'clean' runWithout PATH and HOME, the app (or the shell inside it) can't find commands and fails in confusing ways.
env -i HOME=$HOME PATH=/usr/bin:/bin ./appParsing env output to read values that contain newlinesA value with a newline looks like two variables.
env -0 | tr '\0' '\n' # or printenv NAMETry these on a Linux machine of your own. Each task has a hint and a solution.
Print all environment variables sorted by name and count them. (The sandbox shell is zsh, but env works the same.)
Run 'bash -c env' with an empty environment plus only PATH=/usr/bin:/bin and see what's left.
Show the environment your shell process started with by reading /proc.
Run the report script ~/app/worker/report.py with a different data file path without editing it. Its first argument defaults to ~/data/orders.csv; check how its shebang finds python.