Shellscript Notes


Useful shellscript snippets

Commonly used

# Check if variable contains mystring
if [[ $var == *mystring*  ]]; then # The double closed brackets are essential for wildcard match
    echo "it does";

# Split on ":" and get 1st
echo "docker-image:0.1" | cut -d ":" -f1 # returns docker-image

# Find and replace all via pipe
echo "Hello I'm Dom" | sed 's/Dom/<void>/g' # "/" could be any char

# Redirect stdout and stderr to the void
./ >/dev/null 2>&1


# Read top lines of file
head -n 3 myfile

# Read bottom lines of file
tail -n 3 myfile

# Watch all updates to file
tail -f myfile

# Read file in window (not cat)
less myfile

# Make all directories if not exist
mkdir -p exist/not/dir

# Visualise dir structure
tree -L 2

# Remove env variable
unset myvar

# Alias command to short one
alias lc="ls -laht" # unalias to remove

# Piping output
echo "overwrite" > file # Same as 1>
echo "append" >> file


# See disk space usage (human readable)
df -h

# See file usage for dirs
du (--max-depth=2)

# Show running processes, a: show other user, u: show username, x: show non terminal
ps -aux

# Show usage


# Use command output as if it's file
diff <(grep string file1) <(grep string file2)

# Repeat last 3 command

# Repeat last arg of the last command

# Take all args from previous and use


# Secure input
read -s -p "Password: " password

# Default value
read -p "Address []: " ADDR


# Get value from object within array within object
jq -rec ".parent.children | .[] | .name" # raw:no quotes, e:null exit0, compact

# Get object attr and value and format "Key=$attr,Value=$val"
jq -r '.data | keys[] as $k | "Key=\($k),Value=\(.[$k])"'

# Use output in for loop
for object in $(echo "${JSON}" | jq -rec ".entries[] | .whole | @base64"); do
  echo "This is the value: $(echo ${val} | base64 -d | jq .attribute)"

# Args
curl localhost:8500/v1/catalog/nodes | \
    jq \
        --arg ip $(./ \
        '.[] | select(.Address==$ip)'
# Output mulitple entries in list
jq -rec '[.importItems[].itemId]'

# Create object
jq -rec '{} +{ Items:[.importItems[].itemId]}'


# Exit script if commands return nonzero
set -e

# Output command as they run (debug)
set -x

Enter your instance's address