Useful commands

useful-commands.sh

# Show free and used disk space
#   -a: Include all non-standard file systems
#   -h: Human readable sizes
df -ah

# Show disk usage of a file or directory (recursive)
#   -h: Human readable sizes
#   -s: Summarize results
du -hs /path/to/directory

# Show disk usage of a file or directory and its subdirectories (recursive)
#   -h: Human readable sizes
#   -d: Max depth N, d1 = max depth 1
#   -s: Summarize results
du -hd1s /path/to/directory

# Delete everything in a directory but not the directory itself
find /path/to/directory -mindepth 1 -delete

# Calculate a SHA256 checksum of the current directory contents (recursive)
# Ref: https://unix.stackexchange.com/a/35847
find | LC_ALL=C sort | tar -cf - --no-recursion -T - | sha256sum

# Run command in parallel
# Example runs `sh -c 'sleep 1 && echo {}'` 50 times in batches of 10
seq 1 50 | xargs -I {} --max-procs 10 sh -c 'sleep 1 && echo {}'

# Port forwarding
ssh -L <local_port>:localhost:<remote_port> [<user>@]<host>

# Port forwarding with Google Cloud SDK
# Example: gcloud compute ssh example -- -L 0.0.0.0:8080:localhost:80
gcloud compute ssh [<user>@]<instance> -- -L <local_bind_address>:<local_port>:<remote_host>:<remote_port>

# Port forwarding with Google Cloud SDK on Windows (the -- needs escaping with `)
# Example: gcloud compute ssh example `-- -L 0.0.0.0:8080:localhost:80
gcloud compute ssh [<user>@]<instance> `-- -L <local_bind_address>:<local_port>:<remote_host>:<remote_port>

# Count open files for a process
lsof -p $(pidof <process_name>) | wc -l

# Force line buffering in pipelines
# Ref: https://www.gnu.org/software/libc/manual/html_node/Buffering-Concepts.html
# Ref: https://mywiki.wooledge.org/BashFAQ/009
# Ref: https://unix.stackexchange.com/questions/97294/unbuffer-or-stdbuf-for-removing-stdout-buffering
{ echo "a"; sleep 10; } | grep a | grep a             # The first grep is not using line buffering for stdout as it's not writing to a tty. The second grep is.
{ echo "a"; sleep 10; } | stdbuf -oL grep a | grep a  # Force the first grep to use line buffering for stdout; or
{ echo "a"; sleep 10; } | unbuffer -p grep a | grep a # Use the unbuffer command to create a psuedo-tty, enabling line buffering for stdout. -p tells unbuffer to read from stdin.

# Infinite loop with randomized output
while true; do sleep 1; if test $(cat /dev/urandom | od -An -i -N1 | tr -d ' ') -ge 128; then echo '{ "some": { "json": "object" } }'; else echo 'some_string'; fi; done;

# Print 1024 lines of random bytes
n=$(printf '%d\n' '1024'); for i in $(seq 1 $n); do printf "%0$(printf $n | wc -c)d: %s\n" "$i" "$(cat /dev/urandom | xxd -p | head -c 32)"; done;

# Parallel processing example
seq 1 128 | parallel '{ test $(expr {} % 2) -eq 0 || printf "%-3d is odd\n" "{}"; }; { test $(expr {} % 2) -eq 0 && printf "%-3d is even\n" "{}"; };'

# Parallel processing example
# Run 2 background jobs and wait for them to finish
# -j0 runs infinite jobs so the sleeping jobs don't bottleneck parallel on the number of cores
seq 1 128 | parallel -j0 '{ test $(expr {} % 2) -eq 0 || printf "%-3d is odd\n" "{}"; } & { test $(expr {} % 2) -eq 0 && sleep 1 && printf "%-3d is even\n" "{}"; } & wait;'

# Validate JSON
jq --raw-input 'fromjson | empty' 2>/dev/null; test $? -eq 0; echo $?;

# Using file descriptors for redirections
# Ref: https://unix.stackexchange.com/questions/37660/order-of-redirections

# Named pipe to anonymous pipe
# Ref: https://superuser.com/questions/184307/bash-create-anonymous-fifo

# Fix Google Cloud SDK SSH
# -T to disable the forced pseudo TTY allocation through -t passed by default
gcloud compute ssh <instance> -- -T