Shell scripting

edos-sh is the shell, installed as /bin/sh. It is a real scripting language rather than a command dispatcher: functions, control flow, arithmetic, globbing, heredocs and job control.

One thing separates it from POSIX sh on sight. Every block ends with end — there is no fi, no done, no esac, and no then or do to open one either.

#!/bin/sh
for file in $(ls /bin)
    if [ -f /bin/$file ]
        echo "program: $file"
    end
end

Control flow works in scripts, not interactively.

Running a script

sh script.sh              # run a file
sh script.sh a b c        # with positional arguments
sh -c "echo hello"        # a single command
sh -e script.sh           # exit on the first failure
./script.sh               # needs a #!/bin/sh line

Variables

Assignment needs no keyword. export puts a variable in the environment children inherit.

NAME=value
export PATH=/bin
unset NAME

$?	exit status of the last command
$0	script name, or sh when interactive
$1–$9	positional arguments
$#	how many there are
$@	all of them

Double quotes expand, single quotes do not. ~ expands to $HOME.

echo "$HOME"     # /
echo '$HOME'     # $HOME

Substitution

$(...) captures a command’s output, and nests.

FILES=$(ls /bin)
echo "count: $(echo $(ls /bin | wc -w))"

$(( ... )) evaluates integer arithmetic: + - * / %, comparisons < <= > >= == !=, !, parentheses, and assignment. Variables inside need no $.

echo $(( (2 + 3) * 4 ))          # 20
i=0
i=$(( i + 1 ))

Conditions

test, also spelled [, sets the exit status. Any command works as a condition: zero is true.

if [ -f /bin/sh ]
    echo exists
elif [ -d /bin ]
    echo directory
else
    echo neither
end

Test	True when
-f PATH	a regular file
-d PATH	a directory
-e PATH	exists at all
-z STR / -n STR	empty / non-empty
STR = STR / STR != STR	strings match, or do not
A -eq B	also -ne, -lt, -gt, -le, -ge
! EXPR	negation

With [, the brackets are arguments and need spaces around them.

Loops

while [ -f /tmp/lock ]
    sleep 1
end

for f in one two three
    echo "item: $f"
end

break and continue do what they look like.

Functions

function greet
    echo "hello, $1"
    return 0
end

greet world

Arguments arrive as $1–$9 and $@, the same as a script’s. return sets the exit status and leaves the function.

Pipes, operators and redirection

cmd1 && cmd2        # only if cmd1 succeeded
cmd1 || cmd2        # only if it failed
cmd1 ; cmd2         # regardless
ls /bin | grep sh

Redirection takes an explicit descriptor and can duplicate one onto another, per pipeline stage.

echo hi > file           # truncate
echo more >> file        # append
cat < file               # read
cmd 2> errors            # stderr to a file
cmd 2>&1                 # stderr onto stdout
cmd &> everything        # both

Heredocs feed a block to stdin. Quoting the marker suppresses expansion.

cat > note << EOF
home is $HOME
EOF

cat << 'EOF'
this $HOME stays literal
EOF

Globbing

*, ? and [...] expand against the filesystem. A pattern matching nothing is passed through unchanged rather than erroring, and a quoted one is left alone.

ls /bin/e*
rm /tmp/*.log

Job control

Each pipeline gets its own process group, and the terminal is handed to whichever group is in the foreground — which is why Ctrl+C on sleep 30 | cat kills both stages instead of one.

sleep 60 &     # background
jobs           # what is running or suspended
fg 1           # bring it back
bg 1           # resume it in the background
wait           # block until children finish

Ctrl+Z suspends the foreground job.

Builtins

exit, help, pwd, cd, clear, echo, export, unset, env, history, test, [, break, continue, set, return, kill, jobs, fg, bg, wait, ifconfig, ip.

set -e exits on the first failure; set +e turns that back off.

A whole script

#!/bin/sh
# greet.sh -- greet everyone named on the command line

set -e

if [ $# -eq 0 ]
    echo "usage: greet.sh NAME..."
    exit 1
end

function greet
    echo "Hello, $1!"
end

count=0
for name in $@
    greet $name
    count=$(( count + 1 ))
end

echo "Greeted $count people."

$ sh greet.sh Alice Bob Charlie
Hello, Alice!
Hello, Bob!
Hello, Charlie!
Greeted 3 people.