Exit Status and Chaining

Every command reports back

When a command finishes, it hands the shell a number from 0 to 255: its exit status. By convention 0 means success and anything else means failure. The shell keeps the last one in $? ¹:

me@linuxbox:~$ ls -d /usr/bin
/usr/bin
me@linuxbox:~$ echo $?
0
me@linuxbox:~$ ls -d /bin/usr
ls: cannot access '/bin/usr': No such file or directory
me@linuxbox:~$ echo $?
2

Many programs return 1 for any failure; others use different numbers for different problems, listed in the EXIT STATUS section of their man page ¹. grep, for instance, returns 0 if it found a match, 1 if it didn’t, and 2 if something went wrong, which makes it useful as a yes-or-no question.

$? is replaced by every command, including echo, so check it immediately. true and false are commands that do nothing but succeed and fail ¹.

Chaining commands

Four ways to put commands on one line ^{1 2}:

• ; runs the next command always: cd /tmp; ls
• && runs it only if the previous one succeeded: mkdir backup && cp notes.txt backup/
• || runs it only if the previous one failed: ping -c1 router || echo "network down"
• | runs both at the same time, feeding the first one’s output to the second: sort data.txt | uniq -c

&& is the one you’ll use most: “do this, and if it worked, do that.” It stops a sequence the moment something goes wrong, so you don’t copy files into a folder that failed to be created, or install from a list that failed to download:

me@linuxbox:~$ sudo apt update && sudo apt upgrade
me@linuxbox:~$ cd ~/projects/shed && make

|| supplies a fallback or a complaint:

me@linuxbox:~$ grep -q tornado notes.txt || echo "no mention of tornadoes"
me@linuxbox:~$ cd /mnt/backup || echo "backup drive isn't mounted"

The a && b || c trap

It’s tempting to read a && b || c as “if a, then b, else c.” It isn’t: c also runs if b fails ².

me@linuxbox:~$ test -d backup && cp big.iso backup/ || echo "no backup folder"
cp: error writing 'backup/big.iso': No space left on device
no backup folder

The message is wrong: the folder exists, but the copy failed. When you mean if-then-else, write if (below and in the scripting module).

Pipelines and pipefail

A pipeline’s exit status is the status of its last command ². Failures earlier in the pipe are hidden:

me@linuxbox:~$ cat /no/such/file | sort
cat: /no/such/file: No such file or directory
me@linuxbox:~$ echo $?
0

sort happily sorted nothing, and succeeded. Two fixes ²:

• set -o pipefail makes the pipeline fail if any stage fails. Scripts should almost always turn it on (module 8).
• ${PIPESTATUS[@]} lists the status of every stage of the last pipeline:

me@linuxbox:~$ cat /no/such/file | sort; echo "${PIPESTATUS[@]}"
1 0

Asking yes-or-no questions

Because success is 0, any command can be a question:

me@linuxbox:~$ grep -q "^me:" /etc/passwd && echo "user exists"
me@linuxbox:~$ test -f ~/.bashrc && echo "found it"
me@linuxbox:~$ [ -d ~/photos ] || mkdir ~/photos

• grep -q prints nothing, just sets the status ¹.
• test, and its other spelling [ ... ], checks files, strings, and numbers: -f (is a file), -d (is a directory), -e (exists), -z "$s" (string is empty), "$a" = "$b", "$n" -gt 10 ¹. The spaces inside the brackets are required.

if runs a command and branches on its exit status. That’s all if does ¹:

if grep -q tornado notes.txt; then
    echo "found a tornado note"
else
    echo "none yet"
fi

You’ll write plenty of these in module 8.

Your turn

So

Every command leaves an exit status in $?, 0 for success. Chain with ; (always), && (on success), and || (on failure), and use if rather than a && b || c for real either-or decisions. Pipelines report only their last stage unless pipefail is on. grep -q and test turn questions into exit statuses.