Utilizing bash choices to alter the habits of scripts

Bash offers numerous choices that can be utilized to manage the habits of bash scripts. This submit examines a number of the extra helpful ones and explains easy methods to show which choices are in use and which aren’t.

Exiting when an error happens

If you need a bash script to exit quickly because it encounters an error—any error in any respect—in your scripts, you possibly can add the set -o errexit choice. In case your script incorporates a syntax error, tries to learn a file that doesn’t exist, makes an attempt to append to a file whenever you don’t have permission to take action, or misuses a command not directly, the script will finish abruptly. Right here is an easy instance:

#!/bin/bash

set -o errexit

tail NoSuchFile
echo -n “Enter textual content to be appended> “
learn txt
echo $txt >> NoSuchFile

Attempt to run this script, and also you’ll see this:

$ app2file
tail: NoSuchFile: No such file or listing

As a result of NoSuchFile doesn’t exist, the script exits, and the immediate for textual content to be appended to it’s by no means run. With out the errexit setting, the script would proceed operating after the “No such file” error and would accumulate any textual content entered after the immediate, create the file, and add the textual content to it.

Tracing a bash script

Utilizing the xtrace choice, each command in a script that’s run will likely be displayed. This feature is particularly helpful if you end up debugging a fancy script. You’ll be able to merely take away the xtrace choice or remark out that line whenever you now not want the command tracing.

#!/bin/bash

set -o xtrace

for day in Solar Mon Tue Wed Thu Fri Sat
do
    echo $day
    sleep 2
performed

The output from this script would start like what’s proven beneath. Traces starting with + indicators wouldn’t seem if the xtrace choice weren’t getting used.

$ loop-days-of-week
+ for day in Solar Mon Tue Wed Thu Fri Sat
+ echo Solar
Solar
+ sleep 2
+ for day in Solar Mon Tue Wed Thu Fri Sat
+ echo Mon
Mon
+ sleep 2
+ for day in Solar Mon Tue Wed Thu Fri Sat
+ echo Tue
Tue
…

Stopping at unbound variables

Scripts will typically ignore variables that haven’t been outlined. In case you use the nounset choice as proven within the instance beneath, they are going to exit moderately than proceed operating.

$ cat NoSuchVar
#!/bin/bash

set -o nounset

echo $1
echo $2
$
$ NoSuchVar 1
1
./NoSuchVar: line 6: $2: unbound variable

Utilizing the quick kind for choices

It’s typically extra useful to set choices in scripts utilizing their full names as this makes the scripts extra readable, however you possibly can elect to make use of their quick kinds. For instance, the xtrace choice might be invoked with the command set -o xtrace as within the instance proven earlier. It will possibly, nevertheless, even be set utilizing the easier command set -x. Lots of the bash choices have single-character names like this that can be utilized within the “set -?” format. This data might be discovered within the bash man web page, however the listing beneath ought to make this simpler.

Choice          Brief Type
======          ==========
allexport       -a
braceexpand     -B
errexit         -e
errtrace        -E
functrace       -T
hashall         -h
histexpand      -H
key phrase         -k
monitor         -m
noclobber       -C
noexec          -n
noglob          -f
notify          -b
nounset         -u
onecmd          -t
bodily        -P
privileged      -p
verbose         -v
xtrace          -x

Show the choices

To generate an inventory of choices accessible with bash and whether or not they’re energetic or not, you should utilize the set -o command with no arguments. I used the set -o | column command on the command line to generate the output beneath and adjusted it frivolously to make the columns line up so it will be simpler to learn for the reason that column alignment was thrown off by the one prolonged choice identify (interactive-comments).

allexport       off     ignoreeof               off     nounset         off
braceexpand     on      interactive-comments    on      onecmd          off
emacs           on      key phrase                 off     bodily        off
errexit         off     monitor                 on      pipefail        off
errtrace        off     noclobber               off     posix           off
functrace       off     noexec                  off     privileged      off
hashall         on      noglob                  off     verbose         off
histexpand      on      nolog                   off     vi              off
historical past         on      notify                  off     xtrace          off

Be aware that a lot of the bash choices are turned off.

To see which choices are enabled in a script, use the identical set -o command. The script proven beneath will listing the energetic choices as proven above, however can even show the energetic choices in a terse one-letter-per-option format.

#!/bin/bash

set -o
echo ===========================
printf %sn “$-“

Here’s a pattern of its output:

allexport       off
braceexpand     on
emacs           off
errexit         off
errtrace        off
functrace       off
hashall         on
histexpand      off
historical past         off
ignoreeof       off
interactive-comments    on
key phrase         off
monitor         off
noclobber       off
noexec          off
noglob          off
nolog           off
notify          off
nounset         off
onecmd          off
bodily        off
pipefail        off
posix           off
privileged      off
verbose         off
vi              off
xtrace          off
===========================
hB

Be aware that the “h” and “B” on the final line on this output mirror the standing of the hashall (h) and braceexpand (B) choices.

Wrap-Up

There’s much more to study easy methods to use the choices accessible in bash to manage how scripts run. I hope this submit will get you off to a superb begin.