Subshells

#!/usr/bin/env bash
file_count=$(find . -type f | wc -l)

#!/usr/bin/env bash
var="a"
subshell=$( var="b" )
echo "$var"        # still "a"
echo "$subshell"   # empty, because var="b" was in the subshell

#!/usr/bin/env bash
current_env="shell"
(
  echo "This is running in a ${current_env}"
)

$ ./subshell-v0.sh
This is running in a shell

#!/usr/bin/env bash
current_env="a"
(
  current_env="b"
  echo "Inside subshell: $current_env"
)
echo "Outside subshell: $current_env"

$ ./subshell-v1.sh
Inside subshell: b
Outside subshell: a

#!/usr/bin/env bash
current_env="a"

subsh_var=$(
  current_env="b"
  echo "$current_env"
)

echo "Parent environment: $current_env"
echo "Subshell output: $subsh_var"

$ ./subshell-v2.sh
Parent environment: a
Subshell output: b

#!/usr/bin/env bash
subsh_var=$(
  echo "Output"
  ls -fakeoption
)

$ ./subshell-v2-stderr.sh
ls: unrecognized option '--fakeoption'

# Separate lines
(
  command1
  command2
  command3
)

# Semicolons
(command1; command2; command3)

# Pipes
(command1 | command2 | command3)

# AND operator
(command1 && command2 && command3)

# OR operator
(command1 || command2)

#!/usr/bin/env bash
# Update and build inside /home/user/project
(cd /home/user/project && git pull && make)

$ pwd
/home/user/workspace
$ (cd /tmp && ls)
file_in_tmp
$ pwd
/home/user/workspace

#!/usr/bin/env bash
parent_pid=$$

(
  echo "Inside subshell: PID=$BASHPID"
)

echo "Outside subshell: PID=$parent_pid"

$ ./subshell-v5.sh
Inside subshell: PID=12345
Outside subshell: PID=12344

#!/usr/bin/env bash
tmpfile="/tmp/$$.tmp"
counter=1

# Initialize counter
echo "$counter" > "$tmpfile"

# Increment inside subshell
(
  new_count=$(( $(<"$tmpfile") + 1 ))
  echo "$new_count" > "$tmpfile"
)

# Read updated value
counter=$(<"$tmpfile")
echo "Counter after subshell: $counter"

# Clean up
rm "$tmpfile"

$ ./subshell-v6.sh
Counter after subshell: 2