Shell Script Study Notes

• Shell Script Study Notes

Arithmetic operations

val=`expr $a + $b`

Operators

Special variables

POSIX program exit statuses

Input/output redirection

File include

Use . or source to include files.

. filename
source filename

Included files do not need execute permissions.

Using functions

Basic function structure

# Use the function name directly; optionally prefix with function
function_name(){

    # Use $#,$*,$@ and $0-$n to get the function name and other parameters
    val="$1"

    # Optional return value (must be an integer status code)
    # To return results, use global variables
    return $val
}

Function invocation

# Call the function name directly, space-separated arguments are allowed
function_name 1 2 3 4
# Use $? to get the return value
ret=$?

Common snippets

List all files in a directory

The code below lists all xlsx files in the downloads directory. Note that to list all files, use "$watch_dir"/*.

watch_dir="/Users/mylxsw/Downloads"
# Avoid returning a * when there is no match
shopt -s nullglob
for file in "$watch_dir"/*.xlsx
do
    echo $file
done

for file in "$watch_dir"/*/    # List all directories
do
    echo $file
done

See How to get the list of files in a directory in a shell script? and Bash Shell Loop Over Set of Files

Read lines from stdin

while read line
do
    if [ $line != 'EOF' ]
    then
        echo $line
    fi
done

Date and time

Get current date

# Output: 20161023
echo $(date +%Y%m%d)

Time comparison

Convert to UNIX timestamps, then compare directly.

time1=`date +%s`
time2=`date -d '2016-10-25 17:20:13' +%s`

if [ $time1 -gt $time2 ]; then
    echo "time1 > time2"
else
    echo "time1 < time2"
fi